Microstructure of Copolymers of Norbornene Based

polymers Article

Microstructure of Copolymers of Norbornene Based on Assignments of 13C NMR Spectra: Evolution of a Methodology Laura Boggioni, Simona Losio 1 and Incoronata Tritto *

ID

Istituto per lo Studio delle Macromolecole (ISMAC), Consiglio Nazionale delle Ricerche (CNR), Via E. Bassini 15, 20133 Milano, Italy; [email protected] (L.B.); [email protected] (S.L.) * Correspondence: [email protected]; Tel.: +39-022-369-9370





Received: 18 May 2018; Accepted: 7 June 2018; Published: 9 June 2018

Abstract: An overview of the methodologies to elucidate the microstructure of copolymers of ethylene and cyclic olefins through 13 C Nuclear magnetic resonance (NMR) analysis is given. 13 C NMR spectra of these copolymers are quite complex because of the presence of stereogenic carbons in the monomer unit and of the fact that chemical shifts of these copolymers do not obey straightforward additive rules. We illustrate how it is possible to assign 13 C NMR spectra of cyclic olefin-based copolymers by selecting the proper tools, which include synthesis of copolymers with different comonomer content and by catalysts with different symmetries, the use of one- or two-dimensional NMR techniques. The consideration of conformational characteristics of copolymer chain, as well as the exploitation of all the peak areas of the spectra by accounting for the stoichoimetric requirements of the copolymer chain and the best fitting of a set of linear equation was obtained. The examples presented include the assignments of the complex spectra of poly(ethylene-co-norbornene (E-co-N), poly(propylene-co-norbornene (P-co-N) copolymers, poly(ethylene-co-4-Me-cyclohexane)s, poly(ethylene-co-1-Me-cyclopentane)s, and poly(E-ter-N-ter-1,4-hexadiene) and the elucidation of their microstructures. Keywords: cyclic olefin copolymers; terpolymers; 13 C NMR; microstructure

1. Introduction Nuclear magnetic resonance (NMR) spectroscopy today is a powerful tool for the elucidation of polymer structure and dynamics. In particular, solution 13 C NMR is indispensable for assigning polymer microstructures. Significant advances have been made in NMR instrumentation and sophisticated pulse sequences, which allow us to solve interesting problems of polymer science [1]. However, it is not yet possible to predict NMR resonance frequencies accurately enough to completely characterize polymer microstructures despite the progress made in Quantum Mechanical theory and calculation methods. This is because the magnetic shielding of nuclei of flexible molecules like polymers are influenced by microstructures, and predictions must be averaged over all the conformations permitted to the chain with a specific microstructure, that is, each nucleus (i) experiences a local magnetic field Bloc(i) that depends on the local conformation of the vicinal carbons, which are related to the microstructure of the vicinal polymer segments. Microstructure → conformation → Bloc(i) → δ13 Ci [2] Empirical nuclear shielding effects [3–6], produced by substituents α, β, and γ to a carbon atom, were successfully used to understand polymer microstructure from their 13 C NMR spectra. Unrevealing the conformational origin of the nuclear shielding produced by γ substituents was fundamental, i.e., a γ substituent could only shield a 13 C nucleus if the central bond between them Polymers 2018, 10, 647; doi:10.3390/polym10060647

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fundamental, i.e., aa γγ substituent substituent could couldonly onlyshield shieldaa1313CCnucleus nucleusififthe thecentral centralbond bondbetween betweenthem them fundamental, i.e., produced a proximal arrangement by adopting a gauche conformation, that is, when the rotational produced a proximal arrangement by adopting a gauche conformation, that is, when the rotational produced a proximal arrangement by adopting a gauche conformation, that is, when the rotational internal —C(β) —C gauche, theresonance resonanceofof ofCC Cisis shifted by about ppm upfield internal C―C(α) (α)―C ―C (γ)γis gauche, the resonance isshifted shiftedby byabout about555ppm ppmupfield upfieldwith with (β) ) is internal C—C C―C (α)―C(β) ―C (γ)( is gauche, the with respect respect to trans trans [6]. [6]. respect to to trans [6]. α-, β-, and especially thethe conformationally sensitive γ-effects werewere used assign the NMR spectra α-, β-, and especially the conformationally sensitive γ-effects wereto used assign the NMR α-, β-, and especially conformationally sensitive γ-effects used totoassign the NMR and determine the microstructures of polymers in solutions and melts, where they are conformationally spectra and determine determine the the microstructures microstructures of of polymers polymersininsolutions solutionsand andmelts, melts,where wherethey theyare are spectra and flexible [2,6–11] .The first applications of the rotational state isomeric model for describing the conformationally flexible [2,6–11] .The first applications of the rotational state isomeric model conformationally flexible [2,6–11] .The first applications of the rotational state isomeric model forfor conformational date back to the late 70sback and early 80slate in the case ofearly polypropylene (PP) [6,9]. describing the statistics conformational statistics date back the late 70s and early80s 80sininthe the case describing the conformational statistics date toto the 70s and case ofof 13 13 Nowadays, PP resonances in high-resolution solution C NMR spectra exhibit a sensitivity to 13 polypropylene (PP) [6,9]. Nowadays, PP resonances in high-resolution solution C NMR spectra polypropylene (PP) [6,9]. Nowadays, PP resonances in high-resolution solution C NMR spectra stereosequences at the level. This that eleven repeat unit fragments of PP different exhibit sensitivity toundecad stereosequences the undecad undecad level. Thismeans means thateleven eleven repeat unit a sensitivity to stereosequences atatmeans the level. This that repeat unit only in one diad, which is meso (m) or racemic (r) (Scheme 1) may be detected, i.e. a 0.03 ppm difference fragments of PP PP different differentonly onlyin inone onediad, diad,which whichisismeso meso(m) (m)ororracemic racemic(r)(r)(Scheme (Scheme1)1) may detected, may bebe detected, in the resonance frequencies of methyl carbons in the repeat units of i.e. a 0.03 ppm difference difference in the thethe resonance frequencies themethyl methyl carbons thecentral centralrepeat repeat in resonance frequencies ofofcentral the carbons ininmmmmmmmrmr the 13 C NMR is usually only and mmmmmmmrmm PP undecads was observed [12]. More typically 13C 13C units of mmmmmmmrmr mmmmmmmrmrand andmmmmmmmrmm mmmmmmmrmmPP PPundecads undecadswas wasobserved observed[12]. [12].More More typically typically sensitive to tetrad and pentad stereosequences in homopolymers and triad comonomer sequences in NMR is usually usually only only sensitive sensitive to to tetrad tetrad and andpentad pentadstereosequences stereosequencesininhomopolymers homopolymersand andtriad triad copolymers. However, whenever a new polymer structure is synthesized, the structural sensitivity of comonomer sequences sequences in in copolymers. copolymers.However, However,whenever wheneveraanew newpolymer polymerstructure structureisissynthesized, synthesized, 13 C NMR allows us to observe a multitude of 13 C resonances, and it is challenging to assign them to 13C the structural sensitivity sensitivity of of 1313CC NMR NMR allows allowsus ustotoobserve observeaamultitude multitudeofof13C resonances,and andit itis is resonances, specific microstructures. challenging to assign assign them them to to specific specificmicrostructures. microstructures.

mmmm mmmm

rrrr rrrr

mrrr mrrr

Scheme 1. 1. Examples of of regioregular polypropylene sequences. Scheme Scheme 1.Examples Examples ofregioregular regioregularpolypropylene polypropylenesequences. sequences.

Here, we focus on our efforts to elucidate the microstructure ofofcyclic cyclic olefin-based copolymers Here, copolymers Here, we we focus focus on on our ourefforts effortsto toelucidate elucidatethe themicrostructure microstructureof cyclicolefin-based olefin-based copolymers 13 13 (COC), by by ethylene (E)-norbornene (N) copolymers. The spectra ofof (COC), C NMR NMR analysis, analysis, specifically of of (COC), by 13C C NMR analysis, specifically specificallyof ofethylene ethylene(E)-norbornene (E)-norbornene(N) (N)copolymers. copolymers.The Thespectra spectra E-co-N copolymers copolymers are quite complex because the norbornene unit in the polymer chain contains two E-co-N are quite complex because the norbornene unit in the polymer chain contains two copolymers are quite complex because the norbornene unit in the polymer chain contains two stereogenic carbons. carbons. addition, chemical shifts ofofthese follow simple stereogenic addition, thethe chemical shifts of these copolymers do not do follow simple additive stereogenic carbons.InIn In addition, the chemical shifts thesecopolymers copolymers donot not follow simple additive rules, due to the bicyclic nature of the norbornene structural units (see Schemes 2 and 3). rules, due to the bicyclic nature of the norbornene structural units (see Schemes 2 and 3). additive rules, due to the bicyclic nature of the norbornene structural units (see Schemes 2 and 3). Scheme illustrates some various possible ofofchain (alternating and Scheme some of of the various possible typestypes of chain fragments (alternating and blocks) Scheme 222illustrates illustrates some of the the various possible types chainfragments fragments (alternating and blocks) and also distinguishes between meso (m) and racemic (r) alternating units and between meso and alsoand distinguishes between meso (m) and racemic (r) alternating and between mesobetween and racemic blocks) also distinguishes between meso (m) and racemic (r) units alternating units and meso and racemic ENNE ENNNE sequences, where norbornene underwent 2,3 ENNE and ENNNE sequences, norbornene 2,3 cis-exo insertion. and racemic ENNE and and ENNNEwhere sequences, whereunderwent norbornene underwent 2,3cis-exo cis-exoinsertion. insertion.

Erythro di-isotactic Erythro di-isotactic alternating sequences alternating sequences

(meso) (meso)

(RS) (RS) (RS) (RS) (RS) (RS) (meso, meso) Triad

Erythro di-syndiotactic (racemic) Erythro di-syndiotactic (racemic) alternating sequences alternating sequences

meso Diad meso Diad

(RS) (SR) (RS) (RS) (SR) (RS)

(RS) (RS) (SR) (RS) (RS) (SR)

racemic Diad racemic Diad

(racemic, racemic) Triad (meso, racemic) Triad (meso, meso) Triad (racemic, racemic) Triad (meso, racemic) Triad Scheme 2. Segments of E-co-N copolymer chain with the adopted numbering. Scheme 2. 2. Segments Segmentsof ofE-co-N E-co-Ncopolymer copolymerchain chainwith withthe theadopted adoptednumbering. numbering. Scheme

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 + + + m  + m

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

m

m

 



r

r

Scheme 3.3.A E-co-N copolymer ofofsecondary (S) carbons Scheme 3. A random E-co-N copolymer chain with nomenclature of secondary (S) ethylene carbons Scheme Arandom random E-co-N copolymerchain chainwith withnomenclature nomenclature secondary (S)ethylene ethylene carbons (S is omitted for sake of simplicity. (S is (S omitted for for sake of of simplicity. is omitted sake simplicity.

Among COCs with various comonomercontents contentsand andmicrostructures microstructures E-co-N copolymers Among COCs various comonomer contents E-co-N copolymers areare the Among COCs with various comonomer and microstructures E-co-N copolymers are the the most versatile and interesting ones, andofone of thefamilies new families of olefinic copolymers made most versatile and interesting ones, and one the new of olefinic copolymers made available most versatile and interesting ones, and one of the new families of olefinic copolymers made available available by metallocenes. They were first synthesized by Kaminsky with ansa-zirconocenes in by metallocenes. They were firstsynthesized synthesized by Kaminsky in 1991 [13],[13], and by metallocenes. They were first by Kaminskywith withansa-zirconocenes ansa-zirconocenes in 1991 and 1991 [13], and aroused much interest because of their excellent thermal and optical properties [14–16]. aroused much interest because of their excellent thermal and optical properties [14–16]. Owing to aroused much interest because of their excellent thermal and optical properties [14–16]. Owing to Owing to theircombination unique combination of properties, they are engineering polymers that produced are produced their unique of properties, they are engineering polymers that are and their unique combination of are properties, they are and engineering polymers that are produced and and commercialized. They usually amorphous display a wide range of glass transition commercialized. They are usually amorphous and display a wide range glass transition commercialized. usually amorphous and◦°C, a wide range of composition glass transition temperatures room temperature to C,display largely by temperaturesTTgThey g,, from fromare room temperature toabout about220 220 largelydetermined determined bychain chain composition temperatures T g , from room temperature to about 220 °C, largely determined by chain composition and andstereochemistry. stereochemistry. and stereochemistry. The of of E-co-N copolymers hashas taken several years [17–32]. Thethorough thoroughmicrostructure microstructureinvestigation investigation E-co-N copolymers taken several years [17– 13 13 A32]. number of groups dedicated great efforts to assign thetheC NMR E-co-N copolymers. The investigation E-co-N copolymers hasofof taken several years [17– Athorough number ofmicrostructure groups dedicated great efforts toofassign C NMRspectra spectra E-co-N copolymers. 13 C NMR 13C NMR 13microstructure understanding E-co-N and P-co-N copolymer through Advances inofunderstanding E-co-N and P-co-N copolymer microstructure through signal 32].Advances A numberin groups dedicated great efforts to assign the C NMR spectra of E-co-N copolymers. signal assignments will be reviewed along with various methodologies utilized to achieve assignments will be reviewed along various methodologies utilized to achieve13them. Few Advances in understanding E-co-N andwith P-co-N copolymer microstructure through C NMR signal them. Few examples of recent novel assignment of poly(ethylene-co-4-Me-cyclohexane)s and examples of recent novel assignment of poly(ethylene-co-4-Me-cyclohexane)s and poly(ethylene-coassignments will be reviewed along with various methodologies utilized to achieve them. Few poly(ethylene-co-1-Me-cyclopentane)s and of evenspectra more complex of cyclic olefin terpolymers 1-Me-cyclopentane)s and of even more of cyclic spectra olefin terpolymers poly(E-ter-N-terexamples of recent novel assignment of complex poly(ethylene-co-4-Me-cyclohexane)s and poly(ethylene-copoly(E-ter-N-ter-1,4-hexadiene) 1,4-hexadiene) will be given. will be given. 1-Me-cyclopentane)s and of even more complex spectra of cyclic olefin terpolymers poly(E-ter-N-terIn 1 the1metallocene and half-titanocene precatalysts, used along with methylaluminoxane InFigure Figure the metallocene and half-titanocene precatalysts, used along with 1,4-hexadiene) will befor given. (MAO) cocatalyst the synthesis of copolymers whose microstructure will be elucidated, methylaluminoxane (MAO) cocatalyst for the synthesis of copolymers whose microstructure will be In Figure 1 the metallocene and half-titanocene precatalysts, used along with are displayed. elucidated, are displayed. methylaluminoxane (MAO) cocatalyst for the synthesis of copolymers whose microstructure will be elucidated, are displayed.

1

1

2

3

2

5

6

4

3

7

4

8

Figure1.1.Structures Structuresof ofthe themetallocenes metallocenesand andhalf-metallocenes half-metallocenesused usedfor forcopolymerization copolymerizationreported. reported. Figure

Results andDiscussion Discussion 5 and 2.2.Results

6

7

8

Figure 1. Structures of the Assignments metallocenes and half-metallocenes used for copolymerization reported. 2.1. Methodologies forSignal Signal 2.1. Methodologies for Assignments

The methodologies used to allow oneone to clarify the The methodologies to make make 1313CCNMR NMRsignal signalassignments assignmentsthat that allow to clarify 2. Results and Discussion used

microstructure of novel cyclic olefinolefin copolymers will be with the of E-co-N the microstructure of novel cyclic copolymers willillustrated be illustrated withexample the example of copolymers. copolymers. 2.1. E-co-N Methodologies for Signal Assignments Assigning 13C NMR spectra of norbornene-based copolymers includes synthesis of copolymers 13C NMR signal assignments that allow one to clarify the The different methodologies used to make with comonomer content (obtained by catalysts with different symmetries), synthesis of 13 copolymers with monomers selectively enriched, synthesis of model with compounds, use of monomicrostructure of novel cyclic Colefin copolymers will be illustrated the example of E-co-N

copolymers. Assigning 13C NMR spectra of norbornene-based copolymers includes synthesis of copolymers

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Assigning 13 C NMR spectra of norbornene-based copolymers includes synthesis of copolymers with different comonomer content (obtained by catalysts with different symmetries), synthesis of copolymers with monomers 13 C- selectively enriched, synthesis of model compounds, use of monoor two-dimensional NMR techniques, study of conformational characteristics of copolymer chain, and least-squares fitting of peak areas. 2.1.1. NMR Pulse Sequences Distortionless enhancement by polarization transfer (DEPT) 13 C spectra. allows one to distinguish between methyl or methine and methylene carbons; the methine and methyl signals appear positive, while the methylene signals are negative. Thus, DEPT experiments permit a first general assignment of methylene (C7) and methine (C1/C4 and C2/C3) carbon atoms. 2.1.2. Series of Copolymers with Different Comonomer Content The comparison of spectra of copolymers with different norbornene content, obtained by catalysts with different symmetries, has facilitated the assignment of a number of signals. Unfortunately, this approach when used alone is rather limited in assigning E-co-N copolymer spectra [23,24]. 2.1.3.

13 C

Enrichment

Comparison between 13 C NMR spectra of E-co-N copolymers of monomers with natural abundance of 13 C and those obtained with 13 C1 -enriched ethylene or 13 C5/6 -enriched norbornene allowed Fink to determine the number of C5/C6 or ethylene signals and to make important progress in their assignments [17]. 2.1.4. Conformational Characteristics of the Copolymer Chain Before our studies, no published paper considered the possibility of isotactic or syndiotactic types of regularity for alternating NENEN nor of meso/racemic norbornene diads (ENNE sequences). The elucidation of the conformational structure of the chain of E-co-N copolymers by molecular mechanics calculations and the correlation between conformation and 13 C NMR chemical shifts allowed us to make significant progress [19]. Conformer populations of the stereoregular and stereoirregular polynorbornene and alternating (N-E)x chains and of copolymer chains containing NN, EEE and NNN sequences were computed and allowed us to predict stereochemical shifts. For the first time, it was possible to distinguish between meso and racemic NEN sequences and to assign signals of the two methines C2, Cl of the cyclic unit and of the CH2 of ethylene in regularly alternating isotactic and syndiotactic copolymers, and the signals of the two methines of an N unit in a NEE... sequence [19,20]. 2.1.5. Model Compounds In principle, the synthesis of model compounds of a copolymer sequence offers the best approach to making certain assignments of the signals of the sequence and to successfully determine copolymer structure and tacticity. However, the synthesis of model compounds is not always accessible to the synthesis of hydrooligomerization followed by isolation of hydroisomers, which can be model compounds of a segment of the copolymer chain, used to investigate copolymer structure and tacticity. However, the higher the oligomerization number, the higher the number of possible stereisomers and more difficult the separation process. When available, these assignments are very valuable and useful for understanding the shifts. Although in the case of poly-norbornene and of its higher oligomers, due to strong steric interactions between non-adjacent units, which induce large deformations of the torsional angles and of the ring geometry, the results of molecular mechanics show that dimers and trimers are poor models [21].

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2.1.6. Two-Dimensional NMR Techniques Two-dimensional NMR techniques, including homonuclear 1 H-1 H, 13 C-13 C and heteronuclear experiments, have been helpful in extending signal assignments. INADEQUATE 13 C-13 C correlated NMR spectra have helped to attribute resonances of NN diads and to correct previous assignments. By using 13 C-1 H correlations, HMQC (Hetero Nuclear Multiple Quantum Coherence for one-bond correlations, and HMBC (Heteronuclear Multiple Bond Correlation) for two-or three-bond correlations, Bergstrom et al. identified C5/C6 and C2/C3 signals of norbornene diads [22]. A set of copolymers was extensively investigated by applying the heteronuclear 1 H-13 C experiments, namely gHSQC (gradient assisted Heteronuclear Single Quantum Coherence) and 1 H-13 C gHMBC experiment (gradient assisted Heteronuclear Multiple Bond Correlation) together with the not reported homonuclear 1 H-1 H data. The gHSQC experiment provides correlations between the resonances of 1 H and 13 C atoms having one-bond scalar couplings (1 JCH ), thus giving information on all the direct one bond proton-carbon correlations. Once assigned, all the direct 1 H-13 C connections, the 1 H-13 C gHMBC experiments allow for a complete and unambiguous resonance assignment. A set of copolymers was extensively investigated by applying gHSQC and gHMBC, allowing us to assign new resonances depending on the comonomer sequences at the triad level. 1 H-13 C

2.1.7. Ab Initio Theoretical 13 C NMR Chemical Shifts, Combined with R.I.S. (Rotational-Isomeric State) Statistics Ab initio computations can be a great tool for the elucidation of complex spectra and for the determination of polymer and copolymer microstructures. A proper model compound and a thorough conformational analysis of the models must be selected and associated with the quantomechanical study. On the basis of previous computations defining the main conformational characteristics of E-co-N copolymers, a thorough test of ab initio 13 C chemical shifts computations [gauge-invariant atomic orbitals (GIAO)] on known cases agreed well with experimental data, especially with the MPW1PW91 density functional theory (DFT), on properly energy-minimized structures. This method nicely confirmed signal assignment of ENNE sequences in spectra of E-co-N copolymers, where NN microblocks induce strong effects arising from ring distortions [30]. 2.1.8. Set of Equations and Least-Squares Fitting of Peak Areas A procedure for computing the molar fractions of the stereosequences that describe the E-co-N copolymer chain microstructure, has been conceived, which also allowed us by trial-and-error to extend the assignment of unknown signals of 13 C NMR spectra of E-co-N copolymers. The analysis of the spectra gives a certain number of peak integrals, each peak c being related to one or more signals. For each peak, it is possible to write a linear equation defining the normalized integral as a function of the unknown molar fractions. The 13 C NMR signals are associated with carbons of a stereosequence, thus generating a set of equations, whose best-fitting solution determines the microstructure of the copolymer chain and confirms or discards new assignments. This is based on the assumption that the area of a signal is proportional to the population of the carbons generating that signal. Thus, the normalized peak area NPA(i) of a signal due to carbon Ci contained ni times in the central monomer unit of sequence S represents the molar fraction f (Ci ) of carbon Ci. 2.2. Microstructural Analysis of E-co-N Copolymers The microstructural analysis by 13 C NMR of E-co-N copolymers with different microstructures was completely obtained at triad, tetrad and sometimes at pentad level by the methodology explained above that exploits all the peak areas of the spectra and accounts for the stoichoimetric requirements

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of the copolymer chain [29,30]. The procedure to analyze the 13 C NMR spectra of E-co-N copolymers is based on proper of peak the polymer into fragments, in number line withofthe assignment 13Clevel. assignment level. Thedivision observed areas ofchain the greatest possible attributed NMR 13 The observed peak areas of the greatest possible number of attributed C NMR signals assignable signals assignable are employed to generate a set of linear equations where the molar fractionsare are a setfor of linear equations where thepossible molar fractions variables. In summary, theemployed variables.toIngenerate summary, a given spectrum, it is to writeare a the linear equation from each for a given spectrum, it is possible to write a linear equation from each distinctly measured peak area: distinctly measured peak area:

𝑁𝑃𝐴 =NPA 𝑛𝑜𝑟𝑚𝑎𝑙𝑖𝑧𝑒𝑑 𝑝𝑒𝑎𝑘 peak 𝑎𝑟𝑒𝑎area = = ∑ 𝑐 𝑓ci f i = normalized i

where ci = coefficients and fi = unknown molar fractions = variables where ci = coefficients molar fractions = variables. i = unknown Least-squares fittingand of fthe set of equations so obtained gives the best solution for the molar Least-squares fitting of the set of equations so obtained gives the best solution for the molar fractions, which define the chain microstructure to achieve new signal assignments. The fractions, which define the chain microstructure to achieve new signal assignments. The microstructure microstructure of an E-co-N copolymer at tetrad or higher level, which distinguished between meso of an E-co-N copolymer at tetrad or higher level, which distinguished between meso and racemic and racemic contributions to alternating and block sequences, was obtained. This allowed us to contributions to alternating and block sequences, was obtained. This allowed us to determine the determine the reactivity ratios, to test the first-order and the second-order Markov statistics and thus reactivity ratios, to test the first-order and the second-order Markov statistics and thus to have to have information on the copolymerization mechanisms [30–33]. Examples of advancements information on the copolymerization mechanisms [30–33]. Examples of advancements achieved with achieved with this methodology are given below. this methodology are given below. 2.2.1. Microstructure Alternating NContent Content 2.2.1. Microstructure AlternatingE-co-N E-co-NCopolymers Copolymers with with Mid-Low Mid-Low N A A typical in alternating alternating sequences sequenceswith with typicalE-co-N E-co-Ncopolymer copolymerchain, chain, containing containing norbornene norbornene in differences stereochemistryor ornorbornene norbornene isolated isolated between blocks, is sketched in Scheme 4, differences in in stereochemistry betweenethylene ethylene blocks, is sketched in Scheme alongwith with the the adopted adopted carbon 4, along carbonnumbering numberingand anddenomination. denomination.

Scheme 4. A E-co-N in alternating alternatingsequences sequenceswith with Scheme 4. typical A typical E-co-Ncopolymer copolymerchain, chain,containing containing norbornene norbornene in differences stereochemistryorornorbornene norborneneisolated isolated between between ethylene differences in in stereochemistry ethyleneblocks. blocks.

C1-symmetric, bridged metallocenes and monocyclopentadienyl C1 -symmetric, bridged metallocenes and monocyclopentadienyltitanium titaniumamido amidocomplexes complexes(6 in Figure 1) are able to yield mainly alternating E-co-N copolymers. Figure 2. displays the NMR (6 in Figure 1) are able to yield mainly alternating E-co-N copolymers. Figure 2. displays the 1313 CCNMR spectrum of aofE-co-N copolymer about 43.6 43.6mol mol%%ofofN, N,along alongwith with spectrum a E-co-N copolymerprepared preparedwith with6/MAO, 6/MAO,containing containing about thethe signal assignment. signal assignment. Assignments indicated ininFigure the cyclic cyclicunit unitand andofofthe theethylene ethylene CH Assignments indicated Figure22ofoftwo twomethines methines C2 C2 of of the CH 2 2 signals in regularly alternatingisotactic isotacticand andsyndiotactic syndiotactic NEN ofof the Sαδ signals in regularly alternating NENsequences, sequences,asaswell wellasas the Sαδsignals signals shifted lowfieldwith withrespect respect to to S and S Sδε were respectively respectively achieved andand thethe SβγSβγ andand SγδSsignals shifted lowfield Sβε βε eeand achieved γδ signals δε, ,were through comparison of conformer populations [19]. Utilizing the observed peak areas of the assigned through comparison of conformer populations [19]. Utilizing the observed peak areas of the assigned 13 C signals and accounting for the stoichiometric requirements, it was possible to compute the molar 13C signals and accounting for the stoichiometric requirements, it was possible to compute the molar fractions of the stereosequences,which whichdefine definethe themicrostructure microstructure of fractions of the stereosequences, of an an alternating alternatingE-co-N E-co-Ncopolymer copolymer containing meso (m) and racemic (r) alternating sequences [26]. containing meso (m) and racemic (r) alternating sequences [26]. Catalytic systems composed i-Pr[(3-R-Cp)(Flu)]ZrCl = Me 1 symmetric 2 (with Catalytic systems composed of of C1 Csymmetric i-Pr[(3-R-Cp)(Flu)]ZrCl 2 (with R =R Me (4) (4) or or i-Pr i-Pr (5)) and methylaluminoxane are able to produce isotactic alternating copolymers with norbornene (5)) and methylaluminoxane are able to produce isotactic alternating copolymers with norbornene incorporation up to 40 mol % (Scheme 5). In Figure 3, the quantitative analysis at pentad level of incorporation up to 40 mol % (Scheme 5). In Figure 3, the quantitative analysis at pentad level of the the spectrum of a sample of an E-co-N copolymer prepared with metallocene 4 with 20.4 mol % of spectrum of a sample of an E-co-N copolymer prepared with metallocene 4 with 20.4 mol % of norbornene content is reported. norbornene content is reported. This analysis was achieved by writing for each peak area one linear equation, which relates This analysis was achieved by writing for each peak area one linear equation, which relates the the NPA to the variables chosen to describe the microstructure in terms of sequence distribution. NPA to the variables chosen to describe the microstructure in terms of sequence distribution. LeastLeast-squares fitting of the set of equations so obtained provided the best solution for the molar fraction squares fitting of theInset equations so obtained provided the best solution the molar fraction of of each sequence. theofabsence of NN diads, only six variables are needed to for represent quantitatively each sequence. In the absence of NN diads, only six variables are needed to represent quantitatively the areas of the signals observed in the spectra of the copolymers investigated. The variables chosen were: f(m) = (NEN) = (ENENE); f0 = (NEEN) = 1/2(NEENE);

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the areas of the signals observed in the spectra of the copolymers investigated. The variables chosen were: f (m) = (NEN) = (ENENE);

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f 1 =2018, (NEEEN); Polymers 10, x FOR PEER REVIEW

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f(isl)f (isl) = 1/2(NEE) = 1/2(ENEE); = 1/2(NEE) = 1/2(ENEE); fE(isl) = the total amount ofofisolated f(isl) = 1/2(NEE) = 1/2(ENEE); f E(isl) = the total amount isolatedE; E; fN(isot) = (NENEN). fE(isl) = the total amount of isolated E; f N(isot) = (NENEN). fN(isot) = (NENEN).

Figure 2. 13C NMR spectrum of an alternating atactic E-co-N copolymer prepared with 6 catalyst. [34]. Figure C NMR spectrum alternating atactic atactic E-co-N prepared withwith 6 catalyst. [34]. [34]. Figure 2. 132. C 13 NMR spectrum ofof ananalternating E-co-Ncopolymer copolymer prepared 6 catalyst.

The full exploitation of information contained in the E−co-N copolymer 13C NMR spectra allowed 13 C13NMR us toThe make assignments pentad level contained (see Figureinin 3). Such a level of assignments usallowed to select The full exploitation ofat the E−co-N copolymer C allowed NMR spectra allowed full exploitation ofinformation information contained the E− co-N copolymer spectra the best statistical model describing E-co-N copolymerization with C 1 symmetric catalysts, to study to make assignments pentadlevel level(see (see Figure Figure 3). ofof assignments allowed us tous select us tousmake assignments atat pentad 3).Such Sucha alevel level assignments allowed to select thebest influence of ligand substituents on the polymerization mechanism and to establish the importance the statistical model describing E-co-N copolymerization with C symmetric catalysts, to study the best statistical model describing E-co-N copolymerization with1C1 symmetric catalysts, to study of chain migration mechanism versuson chain mechanism [32]. the influence of ligand substituents theretention polymerization mechanism and to establish the importance

the influence of ligand substituents on the polymerization mechanism and to establish the importance of chain migration mechanism versus chain retention mechanism [32]. of chain migration mechanism versus chain retention mechanism [32].

EENEE

NENEE

NENEN

Scheme5.5:Catalytic Catalyticsystems systemscomposed composedof ofC1 C1symmetric symmetrici-Pr[(3-R-Cp)(Flu)]ZrCl2 i-Pr[(3-R-Cp)(Flu)]ZrCl2(with (withRR==Me Me(4) (4)or or Scheme i-Pr(5)) (5))and andmethylaluminoxane. methylaluminoxane. EENEE NENEE NENEN i-Pr

Scheme 5: Catalytic systems composed of C1 symmetric i-Pr[(3-R-Cp)(Flu)]ZrCl2 (with R = Me (4) or i-Pr (5)) and methylaluminoxane.

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Figure Quantitativeanalysis analysisof ofthe thespectrum spectrum of of a a sample with 4 4 Figure 3. 3. Quantitative sample of ofan anE-co-N E-co-Ncopolymer copolymerprepared prepared with Figure 3. Quantitative analysis of the spectrum of a sample of an E-co-N copolymer prepared with 4 (N = 20.4 mol % [34]). (N = 20.4 mol % [34]). (N = 20.4 mol % [34]).

2.2.2. Microstructure at Tetrad Level of a Random E-co-N Copolymers with Mid N Content 2.2.2. Microstructure 13at Tetrad Level of a Random E-co-N Copolymers with Mid N Content In Figure 4, the NMR spectrum of a poly(E-co-N) with 50.8 mol % of norbornene produced 13 C C In In Figure 4, 4, thethe NMR spectrum ofofa apoly(E-co-N) 13C Figure NMR spectrum poly(E-co-N)with with50.8 50.8mol mol%%ofofnorbornene norborneneproduced produced by by catalyst rac-Et(Indenyl) 2ZrCl 2 (1) is shown. catalyst rac-Et(Indenyl) ZrCl (1) is shown. 2 2 by catalyst rac-Et(Indenyl) 2ZrCl 2 (1) is shown.

2.2.2. Microstructure at Tetrad Level of a Random E-co-N Copolymers with Mid N Content

Figure 4. 13C NMR spectrum of a poly(E-co-N) with 50.8 mol % of N produced by rac-Et(Indenyl)2ZrCl2 13 C NMR Figure 4.Figure of a poly(E-co-N) withwith 50.850.8 molmol % of byby rac-Et(Indenyl) 4. 13C spectrum NMR spectrum of a poly(E-co-N) %N of produced N produced rac-Et(Indenyl) 2ZrCl22 (1) [34]. (1) [34]. 2 ZrCl (1) [34].

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Progress in chemical shift assignments of these copolymer spectra has included the discrimination of meso/racemic relationships between norbornene units in alternating NEN and in Polymers 2018, 10, 647 9 of 24 ENNE sequences. In Figure 5, the 13C NMR spectra of a poly(E-co-N) with 58 mol % of norbornene Polymers 2018, 10, x FOR PEER REVIEW 9 of 24 produced by catalyst i-Pr[(Cp)(Flu)]ZrCl2 (3) (a) and a poly(E-co-N) with 33 mol % of norbornene Progress chemical shift assignments copolymer spectra has included the discrimination produced by in catalyst ZrCl2of(1)these (b) of are showncopolymer [25]. Progress in rac-Et(Indenyl) chemical shift 2assignments these spectra has included the of meso/racemic relationships between norbornene units in alternating NEN and in ENNE sequences. discrimination of meso/racemic relationships between norbornene units in alternating NEN and in 13 C NMR spectra of a poly(E-co-N) with 58 mol % of norbornene produced by catalyst In Figure 5, the ENNE sequences. In Figure 5, the 13C NMR spectra of a poly(E-co-N) with 58 mol % of norbornene i-Pr[(Cp)(Flu)]ZrCl (3) (a) and a poly(E-co-N) with mol % of norbornene by catalyst produced by catalyst i-Pr[(Cp)(Flu)]ZrCl 2 (3) (a) and33 a poly(E-co-N) with 33 molproduced % of norbornene 2 rac-Et(Indenyl) ZrCl (1) (b) are shown [25]. produced by rac-Et(Indenyl)2ZrCl2 (1) (b) are shown [25]. 2 catalyst 2

Figure 5. 13C NMR spectra of a poly(E-co-N) produced by catalyst i-Pr[(Cp)(Flu)]ZrCl2 (3) with 58 mol 13 C13NMR spectra of a poly(E-co-N) produced by catalyst i-Pr[(Cp)(Flu)]ZrCl (3) with Figure C by NMR spectrarac-Et(Indenyl) of a poly(E-co-N) produced by catalyst 2 mol % of Figure N)5.(a);5.and catalyst 2ZrCl 2 (1) with 33 moli-Pr[(Cp)(Flu)]ZrCl % of N (b) [34]. 2 (3) with 58 %% of of N)N) (a);(a); andand by catalyst rac-Et(Indenyl) 2ZrCl2 2 (1) with 33 mol of mol N (b)%[34]. 58 mol by catalyst rac-Et(Indenyl) ZrCl with%33 of N (b) [34]. 2 (1)

2.2.3. Microstructure at Tetrad Level of a Random E-co-N Copolymers with High N Content Microstructure at Tetrad Level a RandomE-co-N E-co-N Copolymers Copolymers with N Content 2.2.3. 2.2.3. Microstructure at Tetrad Level ofof a Random withHigh High N Content The microstructure of a series of E-co-N copolymers synthesized in the presence of rac-Me2Si(2The microstructure of a series of E-co-N copolymers synthesized in the presence of rac-Me2Si(2The microstructure of adominated series of by E-co-N copolymers in thesequences presenceasofis Me-[e]-Indenyl) 2ZrCl2 (2) was the high amount ofsynthesized meso-meso NNN Me-[e]-Indenyl) 2ZrCl2 (2) was dominated by the high amount of meso-meso NNN sequences as is rac-Me Si(2-Me-[e]-Indenyl) ZrCl6.26.The (2) was dominated bystereosequences the high amount meso-meso NNN visible the of Figure possible stereosequences as of the norbornene 2in visible in spectrum the spectrum of 2Figure Theincrease increase of of possible as the norbornene blockblock sequences as is visible in the spectrum of Figure 6. The increase of possible stereosequences length increases and the presence of internal and external norbornene units cause a large number of length increases and the presence of internal and external norbornene units cause a large number of as the norbornene block length increases and the presence of internal and external norbornene units signals. Some assignments of of signals triadsare arelisted listed Table signals. Some assignments signalsofofnorbornene norbornene triads in in Table 1. 1. cause a large number of signals. Some assignments of signals of norbornene triads are listed in Table 1.

Figure 6. Cont.

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Figure 6. 13C NMR spectrum of an E-co-N copolymer prepared with 2 containing 40.2 mol % of N [34]. Table 1. Assignments of 13C NMR of spectra of random E-co-N copolymers with high N content. Figure 6. 13 C NMR spectrum of an E-co-N copolymer prepared with 2 containing 40.2 mol % of N [34]. Figure 6. 13 C NMR spectrum of an E-co-N copolymer prepared with 2 containing 40.2 mol % of N [34]. Chemical Chemical Chemical Chemical Carbon Carbon Carbon Carbon 13 shift shift shift shift Table 1. Assignments of C NMR of spectra of random E-co-N copolymers with high N content. Table 1. Assignments of 13 C NMR of spectra of random E-co-N copolymers with high N content. C1/C4 C5′ m,m 26.59 C7 m,m 33.07 34.92 C2/C3 T 45.42/47.13 Chemical Chemical Chemical Chemical m,m Carbon Carbon Carbon Carbon Chemical Chemical Chemical Chemical shift shift shift shift C5′ r,m 27.20–27.70 C7 T 34.34 C1/C4 T 35.18 C2/C3 T 49.34 Carbon Carbon Carbon Carbon shift shift shift shift C1/C4 C2/C3 C5′ m,m 26.59 C7 m,m 33.07 34.92 C2/C3 T 45.42/47.13 C5/C6 T 29.37 C7 T 35.70 C1/C4 T 36.94 49.80 0 m,mm,m C5 m,m 26.59 C7 m,m 33.07 C1/C4 34.92 C2/C3 45.42/47.13 m,m T 0 r,m 27.20–27.70 C7 34.34 C1/C4 T 35.18 C2/C3 T 49.34 C5r,m C5′ 27.20–27.70 C7 TT 34.34 C1/C4TT 35.18 C2/C3TT 49.34 C5/C6 T 29.49 C7 m,m 36.74 C1/C4 37.87 C2/C3 50.00 C5/C6 T 29.37 C7 T 35.70 C1/C4 T 36.94 C2/C3 m,m 49.80 C2/C3 C2/C3 C5/C6 TTT 29.37 C7 T 35.70 C1/C4 TT 36.94 49.80 C5/C6 30.50 C7 36.94 C1/C4 39.28–39.39 52.70–52.84 C5/C6 29.49 C7m,m m,m 36.74 C1/C4 T 37.87 C2/C3 T 50.00 m,m m,m C5/C6 T 30.50 C7 m,m 36.94 C1/C4 T 39.28–39.39 C2/C3 m,m 52.70–52.84 C5/C6 29.49 C7 m,m 36.74 C1/C4 T 37.87 C2/C3 T 50.00 C5/C6Tm,m C5/C6 30.58 C1/C4 40.80 C2/C3 53.44 30.58 C1/C4 TT 40.80 C2/C3 TT 53.44 C2/C3 m,m C1/C4 T 41.32 C5/C6 T 30.50 C7 m,m 36.94 C1/C4 T 39.28–39.39 52.70–52.84 m,m C1/C4 41.32 C1/C4TT 41.45 C1/C4TT 41.56 C5/C6 C1/C4 41.45 30.58 C1/C4 T 40.80 C2/C3 T 53.44 m,m C1/C4 T 41.56 41.32 2.3. Microstructural Analysis of P-co-N Copolymers C1/C4 T C1/C4 T 41.45 2.3. Microstructural Analysis of P-co-N Copolymers The insertion of norbornene into the isotactic polypropylene chain was expected to give P-co-N C1/C4 T 41.56

The insertion norbornene into the isotactic polypropylene chain was expected to give P-co-N copolymers with Tof g values higher than those of E-co-N copolymers with the same N content and copolymers withpolypropylene TAnalysis g values higher than those of E-co-N copolymers with the same N content and molar mass since has TCopolymers 2.3. Microstructural of P-co-N g higher than polyethylene. However, differences in stereo- and molar mass since polypropylene haswell Tg higher polyethylene. However, in stereo-of regioregularity of propylene units as as in thethan comonomer distribution anddifferences the stereoregularity The insertion of norbornene intounits the isotactic polypropylene chain was expected to give P-co-N and regioregularity of propylene as well as in the comonomer distribution the norbornene, result in complex microstructures of the polymer chain and spectra even moreand complex copolymers with T g values higher than those of E-co-N copolymers with the same N content and stereoregularity of norbornene, result in complex microstructures of the polymer chain and spectra than those of E-co-N copolymers (Scheme 6). Thus, a detailed interpretation of P-co-N copolymer molar masscomplex since polypropylene has Tg copolymers higher than(Scheme polyethylene. However, in stereoeven more than those of E-co-N 6). Thus, a detaileddifferences interpretation of Pspectra was more difficult to achieve. For simplicity characteristics of P-co-N copolymers synthesized and of propylene as to well as inFor thesimplicity comonomer distributionof and the co-N regioregularity copolymer spectra was more units difficult achieve. characteristics P-co-N with ansa-metallocenes of C2 symmetry, proven effective for producing prevailingly isotactic and stereoregularity of norbornene, result in complex microstructures of the polymer chain and spectra copolymers synthesized with ansa-metallocenes of C2 symmetry, proven effective for producing regioregular polypropylene are discussed. even more complex than of E-co-N copolymers (Scheme 6). Thus, a detailed interpretation of Pprevailingly isotactic andthose regioregular polypropylene are discussed. co-N copolymer spectra was more difficult to achieve. For simplicity characteristics of P-co-N copolymers synthesized with ansa-metallocenes of C2 symmetry, proven effective for producing prevailingly isotactic and regioregular polypropylene are discussed. Scheme 6. A typical random P-co-N copolymer chain. Scheme 6. A typical random P-co-N copolymer chain.

2.3.1. P-co-N Copolymers with Isolated N Units 2.3.1. P-co-N Copolymers with Isolated N Units Scheme 6. A typical random P-co-N copolymer chain. In Scheme 7, a schematic representation of a regular P-co-N copolymer chain (PPNPP) along Scheme 7, a schematic of a regular P-co-N copolymer chain (PPNPP) along withInthe numbering of carbonrepresentation atoms used is shown. Norbornene has been inserted cis-2,3-exo into with the 2.3.1. P-co-N Copolymers with Isolated N Units the numbering of carbon atoms used is shown. Norbornene has been inserted cis-2,3-exo into the

In Scheme 7, a schematic representation of a regular P-co-N copolymer chain (PPNPP) along with the numbering of carbon atoms used is shown. Norbornene has been inserted cis-2,3-exo into the

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metal-carbon bond. The methyls of the two propylene consecutive monomer units are in erythro as in an isotactic polypropylene chain as well as in erythro relationship with the norbornene unit. Polymers 2018, 10,bond. x FOR The PEERmethyls REVIEW 11as of in 24 13C NMR metal-carbon of thespectrum two propylene consecutive monomer units with are in1,erythro Figure 7 displays the of a P-co-N copolymer prepared containing an isotactic polypropylene in erythro relationship with the norbornene unit. about 40 mol % of N, alongchain with as thewell finalassignal assignment. metal-carbon bond. The methyls of the two propylene consecutive monomer units are in erythro as in an isotactic polypropylene chain as well as in erythro relationship with the norbornene unit. 6 5 Figure 7 displays the 13C NMR spectrum of a P-co-N copolymer prepared with 1, containing about 40 mol % of N, along with the final 1 signal assignment. 4 P S 7 T 6 5

2

1 S  P  TP  

3

7 2

T S P 4 

3

Scheme 7. 7. Schematic chain the used used  S  Tcopolymer Scheme Schematic representation representation of of aa regular regular P-co-N P-co-N copolymer chain along along with with the numbering of of carbons. carbons. numbering

P

P

13 C NMR spectrum of a P-co-N copolymer prepared with 1, containing Figure therepresentation Scheme77displays . Schematic of a regular P-co-N copolymer chain along with the used aboutnumbering 40 mol % of ofcarbons. N, along with the final signal assignment.

Figure 7. The 13C NMR spectrum of a P-co-N copolymer obtained by 1 containing 41 mol % of N [34].

2.3.2. Microstructure at Triad Level P-co-N Copolymers with Mid-Low N Content Synthesized with C2 Symmetric Metallocenes 13C NMR spectrum of a P-co-N copolymer obtained by 1 containing 41 mol % of N [34]. Figure 7. 7. The The 13 Figure C NMR spectrum of a P-co-N copolymer obtained by 1 containing 41 mol % of N [34].

The spectrum in Figure 7 shows seven groups of signals with similar areas due to carbons of norbornene inserted at as in theLevel structure depicted in Scheme 8.Mid-Low DEPT experiments and comparison 2.3.2. Microstructure atTriad Triad Level P-co-N Copolymers withMid-Low Content Synthesized with 2.3.2. Microstructure P-co-N Copolymers with NN Content Synthesized with Cof2 the chemical shifts of these signals with those of E-co-N copolymers allowed us to assign the main C2 Symmetric Metallocenes Symmetric Metallocenes signals of these copolymers. Then, ab initio theoretical 13C NMR chemical shifts, combined with R.I.S. The spectrum spectrum in in Figure Figure 77 shows shows seven seven groups groups of of signals signals with with similar similar areas areas due due to to carbons carbons of of The statistics of the P-co-N polymer chain, gave detailed indications for the final 13C NMR assignment norbornene inserted inserted as as in in the the structure structure depicted depicted in in Scheme Scheme 8. 8. DEPT DEPT experiments experiments and and comparison comparison of of norbornene spectra of copolymers with N isolated units [35–37]. In detail, C6 and C5 methylenes resonate at 30.10 the chemical shifts shifts of of these these signals signals with with those of of E-co-N E-co-N copolymers copolymers allowed allowed us to to assign assign the the main main the andchemical 27.34 ppm, respectively, while the C7those methylene appears at 31.91 ppm. Theus C1 and C4 methynes 13 13 signals of these copolymers. copolymers. Then, Then, ab C NMR chemical shifts, combined with R.I.S. signals ab initio initio theoretical theoretical NMR combined R.I.S. resonateofatthese 37.17 and 41.32 ppm, respectively, while C3 Cand C2 chemical methynesshifts, appear at 45.40 with and 53.32 statistics of of the the P-co-N P-co-N polymer polymer chain, chain, gave gave detailed detailed indications indications for for the the final final 1313C C NMR NMR assignment assignment statistics ppm, respectively. In addition to signals of polypropylene, the methyl Pβ carbon atom appears at spectra of units [35–37]. In detail, C6 and C5 methylenes resonate at 30.10 spectra of copolymers copolymerswith withNNisolated isolated units [35–37]. In detail, C6 and C5 methylenes resonate at 21.24 ppm. and 27.34 ppm, respectively, while the C7 methylene appearsappears at 31.91 ppm. Theppm. C1 and C4C1 methynes 30.10 and 27.34 ppm, respectively, while the C7 methylene at 31.91 The and C4 An apparently great difference between the values of comonomer content obtained from the resonate atresonate 37.17 and 41.32and ppm, respectively, while C3 and C3 C2and methynes appearappear at 45.40 methynes at signals 37.17 41.32 ppm, respectively, C2 methyl methynes at and 45.4053.32 and areas of norbornene and those calculated fromwhile the propylene signals suggested the ppm, respectively. In addition to signals of polypropylene, the methyl P β carbon atom appears at 53.32 ppm, respectively. In addition to signals of polypropylene, the methyl P carbon atom appears β existence of propylene 1,3 misinsertions in the Mt-N bond. 21.24 ppm. at 21.24 ppm. scheme, based on the above assignments and on the peak area measurements of their A general An apparentlygreat great difference between values of comonomer content obtained from the An apparently thethe values ofatcomonomer content fromwith the areas 13C NMR spectra, was set difference to describebetween the microstructure triad level of P-co-Nobtained copolymers midareas of norbornene signals andcalculated those calculated from the propylene methylsuggested signals suggested the of norbornene and those from the propylene methyl signals existence low N content,signals synthesized with C2 symmetric metallocenes and thus giving mainly the isospecific P existence of propylene 1,3 misinsertions in bond. the Mt-N bond. of propylene 1,3 misinsertions in the Mt-N homosequences. The scheme, based on the above assignments and the peak area measurements of A general scheme, based on the above assignments and on the peak area measurements of their 13C NMR spectra, was set to describe the microstructure at triad level of P-co-N copolymers with midlow N content, synthesized with C2 symmetric metallocenes and thus giving mainly isospecific P homosequences. The scheme, based on the above assignments and the peak area measurements of

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A general scheme, based on the above assignments and on the peak area measurements of their NMR spectra, was set to describe the microstructure at triad level of P-co-N copolymers with mid-low N content, synthesized with C2 symmetric metallocenes and thus giving mainly isospecific P homosequences. The scheme, based on the above assignments and the peak area measurements of their 13 C NMR spectra, includes: (i) definition of the possible triads composing the copolymer chain; (ii) use of NMR techniques to assign new signals; and (iii) a best-fitting procedure to determine the copolymer microstructure [38]. Triad definitions: The P-co-N copolymer chain sketched in Scheme 7 has a typical random copolymer sequence distribution that we have described at triad level (Chart 1), initially ignoring differences in tacticity. Since propylene P may be inserted in the copolymer chain 1,2 (P12 ), 1,3 (P13 ), and 2,1 (P21 ), a copolymer chain with four monomer units (M) P12 , N, P13 , and P21 is possible. On the basis of previous works on P-co-N copolymerization [36], it was assumed that units P13 and P21 may be inserted only after N and that N can be inserted only after P12 . Therefore, only nine diads (P12 P12, P12 N, NP12 , NN, NP13 , NP21 , P13 P12 , P13 N and P21 P12 ) and 23 triads, depicted in Chart 1, are possible. In addition, because of the asymmetry of the bonds between N and P12 (or P21 ), in general two diads M1 M2 and M2 M1 are not equivalent. In the chart, each triad M1 M2 M3 is represented as M3 M2 M1 , i.e., running from right to left (the catalyst metal being bound to the left side of the chain). For each copolymer sample, 13 C NMR signals were associated with the chemical environment of the carbons originating the signals, given in Chart 1, and used to generate a set of equations whose best-fitting solution determines the microstructure of the copolymer chain:

13 C

For example, NPA(CH3 ) =

[ f ( P12 ) + f ( P21 )] 4 f (N) + 3

On the basis of the assignments available, each atom of the central monomer M2 was given a code number representing the signal (or group of signals) assigned to that atom in the environment of triad M1 M2 M3 (Chart 1). Since assignments were far from being at the complete triad level, the same code could involve two or more triads. Two-dimensional NMR techniques, including homonuclear 1 H-1 H and heteronuclear 1 H-13 C experiments have also been helpful in extending assignments [39,40]. The main novel assignments obtained and shown in Figure 8 are summarized below: (i)

the resonances at 16.22 and 16.34 ppm due to the methyl carbon atom of central P in P21 P12 N (S8), and NP21 P12 (S23), respectively; (ii) the signal at 20.05 ppm due to the methyl carbon atom (Pγ) of the P in P12 P12 N (S2); (iii) the signal at 21.26 ppm of the Pβγ of alternating triad NP12 N (S4); (iv) the signals from 21.05 to 21.94 ppm to Pβ methyls in triad NP12 P12 (S3) adjacent to a variable number of P12 units all in isotactic relationship; (v) the signal at 20.25 ppm to the methyl of triad NP12 P12 (S3) adjacent to a variable number of P12 units having different tacticity; (vi) (the signal at 25.45 ppm to the C5 of central N in P12 NN (S10); (vii) the signal centered at 32.21 ppm to the CH of central P in NP12 N (S4); (viii) the signals at 33.60 and 33.90 ppm to CH2 of P in the triads NP21 P12 (S23) and P21 P12 N (S8), respectively; (ix) the signal at 35.70 ppm to the Sαγ methylene of a 1,3 propylene inserted units in NP13 P12 triad (S21). 2.4. Microstructural Analysis of Poly(Ethylene-co-4-MechCHE) and Poly(Ethylene-co-MeCPE) Recently, Nomura developed catalysts that enable incorporations (especially with 1,2- or 2,1-insertion) of di-, tri-substituted olefins, traditionally inactive monomers in transition metal catalyzed coordination polymerization. [41–45] In particular, 4-methylcyclohexene (4-MeCHE) and

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1-methylcyclopentene (1-MeCPE) copolymers have been synthesized recently with nonbridged half-titanocenes containing anionic donor ligands of type, Cp’TiX2(Y) (Cp’ = cyclopentadienyl group, Y = aryloxo, ketimide, phosphinimide etc.) [46]. Here, elucidation of microstructure of poly(E-co-4-MeCHE)s and poly(E-co-1-MeCPE)s is reported. Figure 9 shows the 13 C NMR spectra for poly(ethylene-co-4-methylcyclohexene) (poly(E-co-4Polymers 2018, 10, x FOR PEER REVIEW 13 of 24 MeCHE) with different comonomer contents prepared with (t BuC5 H4 )TiCl2 (O-2,6-Cl2 C6 H3 ) (8)-MAO catalyst system, and a DEPT spectrum [46]. In Chart 2, a poly(E-co-4-MeCHE) chain, with isolated catalyst system, and a DEPT spectrum [46]. In Chart 2, a poly(E-co-4-MeCHE) chain, with isolated comonomer incorporation, showing two possible stereoisomers, is displayed. comonomer incorporation, showing two possible stereoisomers, is displayed.

Chart 1. Possible triads of a random P-co-N copolymer chain. Chart 1. Possible triads of a random P-co-N copolymer chain.

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b)

Figure 8. Complete (a) and expanded regions (b) of a 13C spectrum of a P-co-N copolymer prepared Figure 8. Complete (a) and expanded regions (b) of a 13 C spectrum of a P-co-N copolymer prepared with 1 and containing 34% of N [34]. with 1 and containing 34% of N [34].

In order to assign the spectra of Figure 9, it was necessary to consider that, after one or more 4In order to assign the spectra Figure 9, it wasinnecessary that,originate after onedifferent or more MeCHE insertions, assumed to beofcis, the methyl position 4toofconsider CHE may 4-MeCHE insertions, cis, the methyl position 4 of CHE may originate different stereoisomers. Indeed,assumed as showntoinbe Scheme 8, there areinfour possible types of insertion for 4-MeCHE: stereoisomers. as shown in Scheme 8, there are four possible types of insertion for 4-MeCHE: cis-1,2 or cis-2,1Indeed, insertions with methyl position cis/trans to Ti-alkyls. cis-1,2In orcopolymers cis-2,1 insertions methyl of position cis/trans to Ti-alkyls. with awith low content 4-MeCHE, as those synthesized, the four possible 4-MeCHE In copolymers a low 4-MeCHE, asof those synthesized, the four possible 4-MeCHE additions will givewith rise to twocontent possibleofstereoisomers the sequence EE(4-MeCHE)EE, that we call additions will give rise on to two possible stereoisomers sequence EE(4-MeCHE)EE, that we chain call cis cis or trans depending whether Me in 4-position isof cisthe or trans to the first CH2 of the polymer or(see trans depending whether Me 4-position is the cis or trans chemical to the firstshifts. CH2 of the polymer chain (see Chart 2). Suchon a difference caningreatly affect carbon Chart 2). Such a difference can greatly affect the carbon chemical shifts.

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Chart 2. A poly(E-co-4-MeCHE) chain, with isolated comonomer incorporation, showing two possible Chart poly(E-co-4-MeCHE) chain, with isolated comonomer incorporation, showing two Chart 2. A poly(E-co-4-MeCHE) chain, isolated comonomer incorporation, showing two Chart 2. 2. AA poly(E-co-4-MeCHE) chain, withwith isolated comonomer incorporation, showing two possible possible stereoisomers. stereoisomers. possible stereoisomers. stereoisomers.

Figure 9. 13C NMR spectra (a–c) and (d) the DEPT spectrum for poly(E-co-4-MeCHE)s prepared by (8) 13C NMR spectra (a–c) and (d) the DEPT spectrum for poly(E-co-4-MeCHE)s prepared by (8) Figure Figure 9.C13NMR C NMR spectra (a–c) and DEPT spectrum poly(E-co-4-MeCHE)s prepared [34]. Figure 9. 139. spectra (a–c) and (d)(d) thethe DEPT spectrum forfor poly(E-co-4-MeCHE)s prepared byby (8)(8) [34]. [34]. [34].

Scheme 8. Possible in in copolymerization of of ethylene with 4Scheme 8. Possibleciscis(1,2(1,2-and and2,1-) 2,1-)insertion insertionmodes modes copolymerization ethylene with Scheme 8. Possible cis (1,2and 2,1-) insertion modes in copolymerization of ethylene with 4Scheme 8. Possible cis (1,2and 2,1-) insertion modes in copolymerization of ethylene with 4methylcyclohexene (4-MeCHE) [34]. 4-methylcyclohexene (4-MeCHE) [34]. methylcyclohexene methylcyclohexene (4-MeCHE) (4-MeCHE) [34]. [34].

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The The two two chair chair forms, forms, the the most most stable stable conformations conformations of of the the cyclohexane cyclohexane ring ringof ofeach eachstereoisomer, stereoisomer, have been considered considered in in aa qualitative qualitative way. way. The conformer and thus the the average average have been The stability stability of of the the conformer and thus properties of each isomer depend on the axial or equatorial positions assumed by the Me, CH and, properties of each isomer depend on the axial or equatorial positions assumed by the Me, 2(12) CH, 2(12) CH 2(21) substituents and on their interactions (named Cα21 and Cα12 in Chart 2). The methyl in position and CH2(21) substituents and on their interactions (named Cα21 and Cα12 in Chart 2). The methyl in 4 causes steric interactions when in the axial the When methylthe of methyl the cis isomer is axial with position 4 causes steric interactions when in position. the axial When position. of the cis isomer is the axial CH 2, this conformer can be ignored. Vice versa, when the methyl of the trans isomer is axial, axial with the axial CH2 , this conformer can be ignored. Vice versa, when the methyl of the trans isomer it less-stable gauche interactions with thewith ring,the which omitted. is only axial,shows it onlythe shows the less-stable gauche interactions ring,cannot which be cannot be omitted. Taking into account these conformational considerations, the two methyls at 22.69 and 23.01 Taking into account these conformational considerations, the two methyls at 22.69 and 23.01 ppm 13C spectrum of poly(E-co-4-MeCHE) in Figure 9, were assigned to trans and cis 13 ppm observed in the observed in the C spectrum of poly(E-co-4-MeCHE) in Figure 9, were assigned to trans and cis stereoisomers, respectively.2D 2Ddata dataallowed allowedus ustotoassign assignthe theother other signals: from correlations stereoisomers, respectively. signals: (i)(i) from thethe correlations of of the two methyls in the HMBC spectrum, along the proton dimension, it was possible to assign the the two methyls in the HMBC spectrum, along the proton dimension, it was possible to assign the closest carbon atoms atoms C C3,, C 4, and C5; (ii) the methyl protons at 0.82 ppm, (CH3 at 22.69 ppm) correlate closest carbon 3 C4 , and C5 ; (ii) the methyl protons at 0.82 ppm, (CH3 at 22.69 ppm) correlate with three three carbons carbons at at 26.77, 26.77, 35.65, 35.65, and and 39.34 and allowed allowed us us to to assign assign the the C C4 atom, with 39.34 ppm, ppm, respectively, respectively, and 4 atom, C 5 and C3 of trans stereoisomer; (iii) from HMBC spectrum, the resonances of the cis stereoisomer C5 and C3 of trans stereoisomer; (iii) from HMBC spectrum, the resonances of the cis stereoisomer positioning at at 30.36, 30.36, 33.59, 33.59, and and 37.56 37.56 ppm ppm were were assigned assignedto toCC5,, C C4,, and C3, respectively; (iv) the other positioning 5 4 and C3 , respectively; (iv) the other signals assigned as as follows: follows: 42.18 ppm to to C C1;; 37.14 ppm to C2; 33.73 ppm to C6 of the trans signals were were assigned 42.18 ppm 1 37.14 ppm to C2 ; 33.73 ppm to C6 of the trans stereoisomer; 41.58 ppm to C 1; 37.89 ppm to C2; 34.64 ppm to C6 of the cis stereoisomer. stereoisomer; 41.58 ppm to C1 ; 37.89 ppm to C2 ; 34.64 ppm to C6 of the cis stereoisomer. 13 Figure forfor poly(ethylene-co-1-MeCPE) prepared by Figure 10 10 shows shows 13CCNMR NMRspectra spectraand anda DEPT a DEPT poly(ethylene-co-1-MeCPE) prepared t t (by BuC 5H4)TiCl2(O-2,6-Cl2C6H3)-MAO (8) and (1,2,4-Me3C5H2)TiCl2(O-2,6-Cl2C6H3) (7)-MAO catalyst ( BuC5 H4 )TiCl2 (O-2,6-Cl2 C6 H3 )-MAO (8) and (1,2,4-Me3 C5 H2 )TiCl2 (O-2,6-Cl2 C6 H3 ) (7)-MAO systems. Chart 3 Chart describes possiblepossible insertion patterns in incorporation of 1-MeCPE in the catalyst systems. 3 describes insertion patterns in incorporation of 1-MeCPE in copolymerization. the copolymerization.

Chart 3. 3. A A poly(ethylene-co-1-MeCPE) poly(ethylene-co-1-MeCPE) chain, chain, showing showing two two possible possible MeCPE MeCPE incorporations. Chart incorporations.

Most resonances were basis of of DEPT DEPT spectra spectra and and comparison comparison with with those those of of Most resonances were assigned assigned on on the the basis poly(ethylene-co-CPE) [47–53]. poly(ethylene-co-CPE) [47–53]. The the copolymer copolymer prepared rather The spectrum spectrum of of the prepared with with (8)–MAO (8)–MAO catalyst catalyst system system in in Figure Figure 10a 10a is is rather simple. There are nine peaks in addition to the strong (CH 2)n peak at about 30 ppm of PE, arising simple. There are nine peaks in addition to the strong (CH2 )n peak at about 30 ppm of PE, arising from from the different environment 1,2 insertion of 1-MeCPE. The methyl onresonates CPE resonates 25.7 the different environment of cis of 1,2cis insertion of 1-MeCPE. The methyl on CPE at 25.7atppm, ppm, while from DEPT spectrum and by comparison with spectra of poly(E-co-CPE), resonances at while from DEPT spectrum and by comparison with spectra of poly(E-co-CPE), resonances at 51.42 51.42 and 31.3 ppm are easily assigned to C 6′ and C7′, respectively. From 2D NMR spectroscopy and and 31.3 ppm are easily assigned to C60 and C70 , respectively. From 2D NMR spectroscopy and from from additive by adding the methyl to the chemical of a poly(ethylene-co-CPE) additive rules, rules, by adding the methyl effect toeffect the chemical shifts of shifts a poly(ethylene-co-CPE) taken as taken as a model, it was possible to assign the other carbon atoms. From the HMBC and a model, it was possible to assign the other carbon atoms. From the HMBC spectrum andspectrum the long-term the long-term correlations of the methyl protons, it was possible to assign C 2′ at 43.77 ppm; C3′ at 38.17 correlations of the methyl protons, it was possible to assign C20 at 43.77 ppm; C30 at 38.17 ppm; and C10 ppm; and C1′ at 34.18 ppm. at 34.18 ppm. The other forfor comparison to spectra of poly(ethylene-co-CPE). The The other signals signalsare areeasily easilyassigned assigned comparison to spectra of poly(ethylene-co-CPE). spectra of copolymer prepared with 7MAO catalyst system (Figure 10b,c) show resonances The spectra of copolymer prepared with 7- MAO catalyst system (Figure 10b,c) show resonances assignable to 1,21,2- and (see Chart Chart 3). 3). The assignable to and 1,3-incorporations 1,3-incorporations (see The spectra spectra of of copolymer copolymer prepared prepared by by 8-MAO 8-MAO catalyst system show signals of only 1,2(or 2,1-) insertion. catalyst system show signals of only 1,2- (or 2,1-) insertion. 2.5. of Poly(E-ter-N-ter-HED) Poly(E-ter-N-ter-HED) 2.5. Microstructural Microstructural Analysis Analysis of In this last last section, section, attempts attempts to to determine determine the the microstructure microstructure ofofethylene/norbornene/1,5ethylene/norbornene/1,5In this hexadiene hexadiene terpolymers terpolymers poly(E-ter-N-ter-HED) poly(E-ter-N-ter-HED) synthesized synthesized by by an an ansa ansa metallocene metallocene precursor precursor (X) (X) activated by MAO are shown [51]. HED can be inserted as a linear α-olefin; however, after insertion,

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cyclization can occur. In this case, it is necessary to verify the selectivity for the formation of cis or Polymers 2018, 10, x FOR PEER REVIEW 17 of 24 trans cyclopentane structures polymer (Schemes 9 and 10). however, after insertion, activated by MAO are shown into [51]. the HED can be chain inserted as a linear α-olefin; cyclization can occur. In this case,case, it isitnecessary to verify thethe selectivity forfor thethe formation ofof ciscis oror trans cyclization can occur. In this is necessary to verify selectivity formation trans cyclopentane structures the polymer chain (Schemes and 10). cyclopentane structures into theinto polymer chain (Schemes 9 and910).

Figure 10. C NMR spectra and thedept deptspectrum spectrum for for poly(ethylene-co-1-MeCPE)s prepared by: by: (a) (a) 13 Figure 10. 13 C and the poly(ethylene-co-1-MeCPE)s prepared Figure 10. C NMR NMR spectra spectra and the dept spectrum for poly(ethylene-co-1-MeCPE)s prepared by: (a) tBuC5H4)TiCl2(O-2,6-Cl2C6H3)-MAO catalyst system (8), (b,c) (1,2,4-Me3C5H2)TiCl2(O-2,6-Cl2C6H3) ( t t BuC55H 3)-MAO catalyst system (8), (b,c) (1,2,4-Me3C5H2)TiCl2(O-2,6-Cl2C6H3) (( BuC H44)TiCl )TiCl22(O-2,6-Cl (O-2,6-Cl22CC6H 6 H3 )-MAO catalyst system (8), (b,c) (1,2,4-Me3 C5 H2 )TiCl2 (O-2,6-Cl2 C6 H3 ) (7)-MAO catalyst system [34]. (7)-MAO catalyst catalystsystem system[34]. [34]. (7)-MAO 13

Scheme 9. Ethylene/Norbornene/1,5-hexadiene terpolymerization performed by the ansa-metallocene precursor (X) activated by MAO. Scheme 9. Ethylene/Norbornene/1,5-hexadiene Ethylene/Norbornene/1,5-hexadiene terpolymerization Scheme 9. terpolymerization performed performed by by the the ansa-metallocene ansa-metallocene precursor (X) (X) activated activatedby byMAO. MAO. precursor In order to assign the 13C NMR spectra of poly(E-ter-N-ter-HED), terpolymers with different norbornene synthesized. The poly(E-ter-N-ter-HED) spectra were comparedwith to those of 13C NMR spectra In order order to tocontent assignwere the 13 of poly(E-ter-N-ter-HED), poly(E-ter-N-ter-HED), terpolymers different In assign the C NMR spectra of terpolymers with different poly(HED), and of poly(N-co-HED), synthesized with the same catalyst. norbornene content content were were synthesized. synthesized. The The poly(E-ter-N-ter-HED) poly(E-ter-N-ter-HED) spectra spectra were were compared compared to to those those of of norbornene The 13C NMR spectrum of poly(HED) depicted in Figure 11, assigned for comparison to poly(HED), and and of of poly(N-co-HED), poly(N-co-HED), synthesized synthesized withthe thesame samecatalyst. catalyst. poly(HED), literature data [52–54], proved clearly that HEDwith was mainly incorporated as 1,3-cyclopentane (CP) 13C NMR spectrum of poly(HED) depicted in Figure 11, assigned for comparison to The 13 The This C NMR spectrum of poly(HED)cyclization depicted inofFigure 11, assigned for comparison units. means that intramolecular 1,2-inserted HED occurred before to theliterature next literature data [52–54], proved clearly that HED was mainly incorporated as 1,3-cyclopentane (CP) data monomer [52–54], proved that HED fraction was mainly as 1,3-cyclopentane (CP) units. insertion,clearly and only a small of the incorporated incorporated diene formed vinyl terminated units. This that means that the intramolecular cyclization of 1,2-inserted HEDwere occurred the next branches (Vy)intramolecular along polymer backbone. cyclopentane structures assigned as follows: This means cyclization ofThe 1,2-inserted HED occurred before thebefore next monomer monomer insertion, and only a small fraction of the incorporated diene formed vinyl terminated cis rings, 30.19 ppm (4′,5′-c), 37.43 ppm (1′,3′-c), 39.5 ppm (2′-c); rings,vinyl 31.43terminated ppm (4′,5′-t),branches 36.02 insertion, and only a small fraction of the incorporated dienetrans formed branches (Vy) along the polymer backbone. The cyclopentane structures were assigned as follows: ppm (1′,3′-t), 37.62 ppm (2′-t); methylene bridge carbon, 42.02 ppm (αα). Polymers were characterized (Vy) along the polymer backbone. The cyclopentane structures were assigned as follows: cis rings, cis rings, 30.19 ppm (4′,5′-c), 37.43 ppm (1′,3′-c), 39.5 ppm (2′-c); trans rings, 31.43 ppm (4′,5′-t), 36.02 by a predominance of trans rings, determined by diastereoselectivity of the intramolecular cyclization 0 0 0 0 0 0 0 30.19 ppm (4 ,5 -c), 37.43 ppm (1 ,3 -c), 39.5 ppm (2 -c); trans rings, 31.43 ppm (4 ,5 -t), 36.02 ppm ppm (1′,3′-t), 37.62 ppm (2′-t); methylene bridge carbon, 42.02 ppm (αα). Polymers were characterized step. 0 0 0

(1 ,3 -t), 37.62 ppm (2 -t); methylene bridge carbon, 42.02 ppm (αα). Polymers were characterized by a predominance of trans rings, determined by diastereoselectivity the intramolecular cyclization aby predominance of trans rings, determined by diastereoselectivity of theofintramolecular cyclization step. step.

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2B 4 ' 3B brB4 4 1B4 4B 2B 4 4 3B4 4B4

43

' Scheme 10. General molecular structures obtained in poly(E-ter-N-ter-HED) and relative assignment

Scheme 10.B)General molecular structures obtained in poly(E-ter-N-ter-HED) and relative assignment 5 4 of carbon atoms. (A) cis-1,3-cyclopentane unit; (B) trans-1,3-cyclopentane unit; (C) 1-butenyl branch. of carbon atoms. (A) cis-1,3-cyclopentane unit; (B) trans-1,3-cyclopentane unit; (C) 1-butenyl branch. Scheme 10. General molecular structures obtained in poly(E-ter-N-ter-HED) and relative assignment of carbon atoms. (A) cis-1,3-cyclopentane unit; (B) trans-1,3-cyclopentane unit; (C) 1-butenyl branch.

Figure 11. 13C NMR spectrum (108.58 MHz, C2D2Cl4, 103 °C) of poly(HED) sample prepared by 5/MAO system. Figurecatalytic 11. 13C NMR spectrum (108.58 MHz, C2D2Cl4, 103 °C) of poly(HED) sample prepared by Figure 11. 13 C NMR spectrum (108.58 MHz, C2 D2 Cl4 , 103 ◦ C) of poly(HED) sample prepared by 5/MAO catalytic system.

5/MAO catalytic12, system. In Figure the expansion in the region between 32 and 41 ppm of the 13C NMR spectra of 13C NMR spectra of In Figure 12, the expansion in region between 32 and 41 ppm(c) of the poly(HED) (a); poly(N-co-HED) (b);theand of poly(E-ter-N-ter-HED) prepared under similar 13 C NMR poly(HED) (a); poly(N-co-HED) (b); and of poly(E-ter-N-ter-HED) (c) prepared similarspectra polymerization conditions are compared. In Figure 12, the expansion in the region between 32 and 41 ppm of theunder polymerization conditions are compared. The spectra clearly demonstrate thatand resonance positions of signals ascribed to 1,3-cyclopentane of poly(HED) (a); poly(N-co-HED) (b); of poly(E-ter-N-ter-HED) (c) prepared under similar The spectra clearlysensitive demonstrate that resonanceand positions of signalseffects. ascribedItto 1,3-cyclopentane units of HED are highly to compositional stereochemical appears from Figure polymerization conditions are compared. arefrequencies highly sensitive to compositional and and stereochemical effects. It appears from Figure in 12units that of theHED NMR of carbon atoms 1′,3′-cis 1′,3′-trans of poly(N-co-HED) depicted The spectra clearly demonstrate that resonance positions of signals ascribed to depicted 1,3-cyclopentane that the NMR frequencies of carbon andto 1′,3′-trans poly(N-co-HED) in (b)12are slightly shifted upfield by c.a. 0.05atoms ppm 1′,3′-cis compared those ofof poly(HED) depicted in (a). This units of(b) HED are highly sensitive to compositional and stereochemical effects. It appears from Figure 12 are slightly shifted upfield by c.a. 0.05 ppm compared to those of poly(HED) depicted in (a). This effect is certainly related to the placements of norbornene units close to C1 and C3 positions of the effect certainly related to the placements norbornene units close to C1 and C3 positions of the that the NMRis frequencies atomsresonances 10 ,30of -cis and 10 ,30 -trans of 1′,3′-cis poly(N-co-HED) depicted in (b) are cyclopentane ring. On of thecarbon other hand, of carbon atoms and 1′,3′-trans are shifted cyclopentane ring. by Onc.a. the other hand,compared resonances to of those carbonof atoms 1′,3′-cis and 1′,3′-trans are shifted slightly shifted upfield 0.05 ppm poly(HED) depicted in (a). This effect is downfield by over 1 ppm in the case of poly(E-ter-N-ter-HED), depicted in Figure 12c. This large downfield by over 1 ppm in the case of poly(E-ter-N-ter-HED), depicted in Figure 12c. This large certainly of different norbornene units close to1 C C3 positions of cyclopentane the cyclopentane effectrelated is due to to the the placements placements of substituents at C and C3 positions of the 1 and effect is due to the placements of different substituents at C 1 and C3 positions of the cyclopentane 0 -cis which are likely units in present situation. thisand regard, between ring. ring, On the other hand,ethylene resonances ofthe carbon atoms 10 ,3In 10 ,30signals -trans appearing are shifted downfield ring, which are likely ethylene units in the present situation. In this regard, signals appearing between 34.81and 35in ppm, indicated in Figure 12c with αβγ, depicted were ascribed to methylene carbons from the by over ppm the case of poly(E-ter-N-ter-HED), in Figure 12c. This large effect 34.8 and 35 ppm, indicated in Figure 12c with αβγ, were ascribed to methylene carbons from the is due main chain immediately adjacent to cyclopentane units, and in proximity to norbornene. to the placements different substituents at C1 and Cunits, the cyclopentane ring, which main chain of immediately adjacent to cyclopentane and inofproximity to norbornene. 3 positions Interestingly, the in Figure Figure12c, 12c,the thepresence presenceofofseveral several weak signals Interestingly, thecase case ofpoly(E-ter-N-ter-HED) poly(E-ter-N-ter-HED) in weak signals are likely ethyleneinin units in of the present situation. In this regard, signals appearing between 34.8 and in in thethe region between 36 and 39 ppm is most likely associated with the shifting of the resonance of region between 36 and 39 ppm is most likely associated with the shifting of the resonance of 35 ppm, indicated in Figure 12c with αβγ, were ascribed to methylene carbons from the main chain cyclopentane units depending on the microstructure of the terpolymer. cyclopentane units depending on the microstructure of the terpolymer.

immediately adjacent to cyclopentane units, and in proximity to norbornene. Interestingly, in the case of poly(E-ter-N-ter-HED) in Figure 12c, the presence of several weak signals in the region between 36 and 39 ppm is most likely associated with the shifting of the resonance of cyclopentane units depending on the microstructure of the terpolymer.

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Figure 12. Expansions of the region between 32 and 41 ppm of 13C NMR spectra (108.58 MHz, C2D2Cl4, 13 C NMR spectra (108.58 13C Figure 12. Expansions the region 32 ppm and ppm Figure Expansions ofprepared theofregion between 32 and 41 NMRofspectra (108.58 MHz, C2D2Cl4, 103 °C) of 12. polymers by between X/MAO: (a) of41poly(HED); (b) poly(N-co-HED); (c) ◦ MHz,°C) C2 Dof Cl4polymers , 103 C) ofprepared polymers prepared by X/MAO: (a) poly(HED); (b)poly(N-co-HED); poly(N-co-HED); (c) 103 by X/MAO: (a) poly(HED); (b) (c) 2 poly(E-ter-N-ter-HED). poly(E-ter-N-ter-HED). poly(E-ter-N-ter-HED).

However, it is difficult to assign each carbon signalarising arising from the 1,3-cyclopentane units. The However, each carbon signal from the the 1,3-cyclopentane units. The However,ititisisdifficult difficulttotoassign assign each carbon signal arising from 1,3-cyclopentane units. peak intensities are often weak because ofofthe low level ofofCP the terpolymers, and peak intensities are often weak because the relatively low level of CP units in the and The peak intensities are often weak because ofrelatively the relatively low level CP units units ininterpolymers, the terpolymers, several several signals are signals completely hidden bybyadjacent resonances norbornene Thevarious various signals are completely hidden adjacent resonances of of norbornene units.units. The various types types and several are completely hidden by adjacent resonances of norbornene units. The of possible chain fragments that may form in the terpolymer are summarized in Scheme 11. types of possible chain fragments that may form in the terpolymer are summarized in Scheme 11. of possible chain fragments that may form in the terpolymer are summarized in Scheme 11. '

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Scheme 11. Possible chain fragments of a poly(E-ter-N-ter-HED) chain containing low amount of ' ' 1,5-hexadiene incorporated.

Scheme Scheme 11. Possible chainchain fragments ofofa apoly(E-ter-N-ter-HED) chain containing low amount of 11. Possible fragments poly(E-ter-N-ter-HED) chain containing low amount of The whole 13C NMR spectrum of a sample, prepared by X/MAO catalytic system at N/E/HED = 1,5-hexadiene incorporated. 1,5-hexadiene incorporated.

4/1/4, is shown in Figure 13 with the general assignment for each of the resolved groups of peaks. The signal that appeared at 38.36 ppm was identified as the methyne carbons 1′,3′-cis of the The whole 13C unit. NMR of asignal sample, prepared by X/MAO catalytic system at N/E/HED = cyclopentane Thespectrum adjacent weak at 38.18 ppm should correspond to the methylene carbon

4/1/4, is shown in Figure 13 with the general assignment for each of the resolved groups of peaks. The signal that appeared at 38.36 ppm was identified as the methyne carbons 1′,3′-cis of the

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The whole 13 C NMR spectrum of a sample, prepared by X/MAO catalytic system at N/E/HED = 4/1/4, is shown in Figure 13 with the general assignment for each of the resolved groups of 0 0 peaks. The 10, signal Polymers 2018, x FORthat PEERappeared REVIEW at 38.36 ppm was identified as the methyne carbons 1 ,3 -cis 20 ofof 24 the cyclopentane unit. The adjacent weak signal at 38.18 ppm should correspond to the methylene 0 -trans, while the 20 -cis carbon signal was probably located around 39.3 ppm, hidden by the carbon 2′-trans,2 while the 2′-cis carbon signal was probably located around 39.3 ppm, hidden by resonances norbornene carbon carbon atoms. atoms. resonances ascribed ascribed to to the the C1/C4 C1/C4 norbornene

13C NMR spectrum (108.58 MHz, C2D2Cl4, 103 ◦ Figure 13. 13. 13 of poly(E-ter-N-ter-HED) poly(E-ter-N-ter-HED) sample sample Figure C NMR spectrum (108.58 MHz, C2 D2 Cl4 , 103 °C) C) of prepared by by X/MAO X/MAO catalytic catalytic system system at atN/E/HED N/E/HED ==4/1/4. prepared 4/1/4.

The signal assigned to carbons 1′,3′-trans appearing at 37.03 ppm was located in the cluster of The signal assigned to carbons 10 ,30 -trans appearing at 37.03 ppm was located in the cluster of peaks between 36.5 and 37.4 ppm, relative to C1/C4 and C7 norbornene NNN triads. A weaker signal peaks between 36.5 and 37.4 ppm, relative to C1/C4 and C7 norbornene NNN triads. A weaker signal around 36.5 ppm was also noted. In this region, a separate integration of peaks was not feasible; thus, around 36.5 ppm was also noted. In this region, a separate integration of peaks was not feasible; thus, the intensity of the resonance associated with the 1′,3′-trans carbon atoms was impossible to estimate. the intensity of the resonance associated with the 10 ,30 -trans carbon atoms was impossible to estimate. The methylene resonance of 4′,5′-trans carbon atoms was expected to fall in the range between 31.5 The methylene resonance of 40 ,50 -trans carbon atoms was expected to fall in the range between 31.5 and and 30.5 ppm, within the same interval of the C7 carbon resonances of norbornene, while the 30.5 ppm, within the same interval of the C7 carbon resonances of norbornene, while the resonance resonance of carbons 4′,5′-cis was expected to fall around 29.8 ppm, completely obscured by the E-N of carbons 40 ,50 -cis was expected to fall around 29.8 ppm, completely obscured by the E-N backbone backbone resonances. A new group of signals, indicated with symbol αβγ, was seen in the range resonances. A new group of signals, indicated with symbol αβγ, was seen in the range between between 34.81 and 35.4 ppm, which are probably related to carbons from the main chain (α’β, αβ’, 34.81 and 35.4 ppm, which are probably related to carbons from the main chain (α’β, αβ’, α’γ, α’δ, α’γ, α’δ, ββ’, and β’γ) immediately adjacent to cyclopentane and in proximity to the norbornene unit; ββ’, and β’γ) immediately adjacent to cyclopentane and in proximity to the norbornene unit; in the in the same interval, a weak signal arising from C1/C4 NNN triads was expected. Resonances same interval, a weak signal arising from C1/C4 NNN triads was expected. Resonances assignable to assignable to pendant double bonds (Vy), which are due to HED polymerization without cyclization, pendant double bonds (Vy), which are due to HED polymerization without cyclization, were assigned were assigned to unsaturated carbon atoms appearing in the region between 110 and 150 ppm. Thus, to unsaturated carbon atoms appearing in the region between 110 and 150 ppm. Thus, signals at 112.1 signals at 112.1 and 137.6 ppm were attributed to the side chain double bond carbon atoms 4B4 and and 137.6 ppm were attributed to the side chain double bond carbon atoms 4B4 and 3B4 sketched in 3B4 sketched in Scheme 10. These assignments allow us to conclude that 1,5-hexadiene can be Scheme 10. These assignments allow us to conclude that 1,5-hexadiene can be incorporated into the incorporated into the polymer chain as 1-butenyl branches (Vy) or in the form of cyclopentane polymer chain as 1-butenyl branches (Vy) or in the form of cyclopentane structures (CP) connected at structures (CP) connected at C1 and C3 positions to the polymer backbone and to calculate C1 and C3 positions to the polymer backbone and to calculate compositions of poly(E-ter-N-ter-HED) compositions of poly(E-ter-N-ter-HED) on the basis of the peak integrals of carbons in the 13C NMR on the basis of the peak integrals of carbons in the 13 C NMR spectra. spectra. 3. Conclusions Despite the unique versatility of 13C NMR spectroscopy whenever a new polymer structure is synthesized, it is challenging to assign the specific polymer microstructure from 13C NMR experiments. Here, an overview is given of the efforts in elucidating the microstructure of cyclic olefin

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3. Conclusions Despite the unique versatility of 13 C NMR spectroscopy whenever a new polymer structure is synthesized, it is challenging to assign the specific polymer microstructure from 13 C NMR experiments. Here, an overview is given of the efforts in elucidating the microstructure of cyclic olefin based copolymers through 13 C NMR analysis. Their spectra are quite complex due to the presence in the polymer chain of stereogenic carbons per monomer unit and because the chemical shifts of these copolymers do not obey simple additive rules. The examples presented illustrate how it is possible to assign 13 C NMR spectra of these copolymers through a methodology, which includes synthesis of copolymers with different comonomer content and by catalysts with different symmetries, the use of mono- or bidimensional NMR techniques, the consideration of conformational characteristics of the copolymer chain, the exploitation of all the peak areas of the spectra by accounting for the stoichoimetric requirements of the copolymer chain and the best fitting of a set of linear equations obtained. In particular, the focus is on the elucidation of E-co-N and P-co-N copolymer microstructures through 13 C NMR analysis. Advances in signal assignments of the complex spectra of these copolymers are reviewed along with the various methodologies utilized to achieve them. In addition, the methods used for understanding the microstructures of poly(ethylene-co-4-MeCHE)s, poly(ethylene-co-1-MeCPE)s, and of poly(E-ter-N-ter-HED) from 13 C NMR are reported. The microstructural description of the copolymer chain, attained from such assignments, is important for acquiring information on the copolymerization mechanisms and on understanding relationship between polymer microstructures and polymer properties. Author Contributions: I.T. conceived and designed the experiments and wrote the paper; L.B. conceived some of the methodologies, conducted investigation and analyzed the data, S.L. conducted investigation. Funding: This research received no external funding. Acknowledgments: We are indepted to D. R. Ferro for his pioneering conformational studies, without whom this work would have not been possible. Conflicts of Interest: The authors declare no conflict of interest.

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