Graft Copolymers

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Encyclopedia of Polymer Science and Technology

Graft Copolymers

Standard Article

N. Hadjichristidis1, M. Pitsikalis1, H. Iatrou1, P. Driva1, M. Chatzichristidi1, G. Sakellariou1, D. Lohse2 1University of Athens, Athens, Greece 2ExxonMobil Research and Engineering Company, Annandale, New Jersey Copyright © 2010 by John Wiley & Sons, Inc. All rights reserved. DOI: 10.1002/0471440264.pst150 Article Online Posting Date: September 15, 2010 Abstract

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Abstract This article deals with the synthesis by different strategies, molecular characterization, solution, and bulk properties as well as potential and industrial applications of graft copolymers.

1. Introduction Graft copolymers are composed of a main polymer chain (the backbone) to which one or more side chains (the branches) are chemically connected through covalent bonds. A graft copolymer with one branch can be considered as a miktoarm star copolymer. The backbone and the branches may be homopolymers or copolymers, but they differ in chemical nature or composition (1). The branches are usually equal in length and randomly distributed along the backbone because of the specific synthetic techniques used for their preparation. However, more elaborate recent methods have allowed the synthesis of regular graft copolymers with equally spaced and identical branches and of exact graft copolymers, where all the molecular and structural parameters can be accurately controlled (Fig. 1).

Figure 1. Graft Copolymers: (1) random graft copolymer (identical branches randomly distributed along the backbone); (2) regular graft copolymer (identical branches equally spaced along the backbone); (3) simple graft copolymer (3-miktoarm star copolymer); and (4) graft copolymer with two trifunctional branch points. Exact graft copolymers.

A simple graft copolymer can be represented as Ak-graft-Bm or polyA-graft-polyB or poly(A-g-B), where Ak or polyA is the backbone to which the Bm or polyB branches are grafted. The nomenclature of graft copolymers follows the rules recommended by the IUPAC Commission on Macromolecular Nomenclature (2). Graft copolymers have been mainly used to modify polymer properties because of their unique mechanical, thermal, dilute solution, and melt properties (3-7). The availability of such materials has permitted the researchers to explore the correlation between the structure of the graft copolymers and their properties.

2. Synthesis of Graft Copolymers Three general methods have been developed for the synthesis of randomly branched graft copolymers: (1) the “grafting onto”, (2) the “grafting from”, and (3) the macromonomer method (or

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Page 2 of 38 “grafting through” method) (8) (Fig. 2).

Figure 2. Three general methods of synthesis of randomly branched graft copolymers.

The “grafting onto” method involves the use of a backbone chain containing functional groups X randomly distributed along the chain and branches having reactive chain ends Y. The coupling reaction between the functional backbone and the end-reactive branches lead to the formation of graft copolymers. In the “grafting from” method, active sites are generated randomly along the backbone. These sites are capable of initiating the polymerization of a second monomer, leading to graft copolymers. The most commonly used method for the synthesis of graft copolymers is the macromonomer method. Macromonomers are oligomeric or polymeric chains bearing a polymerizable end group. Macromonomers having two polymerizable end groups have also been reported (8). Copolymerization of preformed macromonomers with another monomer yields graft copolymers. The advantages and disadvantages of each method are presented in Table 1. Table 1. Techniques for the Synthesis of Graft Copolymers Technique

Advantages

Disadvantages

Grafting “onto”

Control of the backbone molecular weight with narrow molecular weight distribution

Low grafting density.

Grafting “from”

Control of the branch molecular weight with narrow molecular weight distribution Control of the backbone molecular weight with narrow molecular weight distribution High grafting density Can afford polymer brushes

Grafting “through”

Control of the branch molecular weight with narrow molecular weight distribution

Rather low branch molecular weight Cannot afford polymer brushes Difficult control of branch molecular weight Broad branch molecular weight distribution Branches cannot be isolated for characterization Low branch molecular weight. The backbone cannot be isolated for characterization

High grafting density Can afford polymer brushes

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3. Grafting “Onto” Methods In the grafting “onto” method, reaction of preformed polymeric chains having functional groups, with other polymeric chains having active chain ends, takes place. In most cases, the incorporation of functional groups is performed by chemical modification of the backbone (9-14). A common procedure is the chloro(bromo) methylation of polystyrene (eq. 1, and the subsequent reaction with living polymeric chains.

(1)

Using this method polystyrene-g-poly(ethylene oxide) (PS-g-PEO) graft copolymers were prepared (9, 12). The chloromethylation of PS was performed using a CCl4 solution of PS with chloromethyl methyl ether, with SnCl4 as the catalyst. The reaction conditions were controlled in such a way so as to give low chloromethyl content (< 10 wt%). A similar synthetic approach was adopted for the synthesis of PS-g-polyisoprene (PS-g-PI) graft copolymers (10, 11). To avoid the side reactions involving lithium-chlorine exchange, the –CH2Cl groups were transformed to the –SiMe2Cl group before reaction with the living polymers. PS-g-poly(2vinylpyridine), PS-g-P2VP, and PS-g-P4VP graft copolymers were prepared by partial chloromethylation of PS polymeric chains (13, 14). Grafting efficiency, that is, the ratio of the number of –CH2Cl groups used for the grafting reaction, and the number of the incorporated groups on the backbone, was as low as 40% in this case. Chloromethylation of poly(N-vinyl carbazole) (PVCz) was also used to synthesize poly(N-vinyl carbazole)-g-PI graft copolymers (15). The chloromethyl methyl ether method was used in the presence of ZnCl2. The grafting efficiency of PI chains was also found to be limited. Bromomethylation of anionically prepared PS backbone was used for the synthesis of PS-g-poly (methyl methacrylate) (PS-g-PMMA) copolymers (16). The polydispersity index of the resulted grafts was higher than the polymeric precursors, indicating the presence of undesirable reactions. By taking advantage of living PS and PI anions with the pyridine rings of P2VP, P2VP-g-PS, and P2VP-g-PI, graft copolymers were synthesized (17) (eq. 2).

(2)

An excess of living chains compared to the 2VP groups was added. The polydispersity index of the final products was higher than the polydispersity of the backbone and the grafted chains, especially in the case of P2VP-g-PI samples. Graft copolymers having as backbone random terpolymers of styrene, maleic anhydride, and ethylhexyl methacrylate, or diethylfumarate, were synthesized by radical polymerization (18). In all these cases, PEO were the grafted chains. Polyethylene glycol monomethyl ether was grafted to the backbones after reaction with the succinic anhydride groups. When the reaction was allowed to proceed to high conversions, gelation occurred. This was attributed to the presence of difunctional polyethylene glycol, which connects many backbones forming high molecular weight and cross-linked products. The grafting “onto” methodology was also followed for the preparation of graft copolymers having poly(glycidyl methacrylate) as the backbone and a mixture of PI- and PS-grafted chains (19). The epoxy groups were used as grafting sites for living PS and PI anionic chains. Graft terpolymers

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Page 4 of 38 having poly[styrene-co-4-(vinylphenyl)-1-butene] as the backbone and either PS or PI as grafted chains were also prepared (20, 21). The synthesis of the backbone involved the anionic copolymerization of styrene with 4-(vinylphenyl)-1-butene. The backbone was partly hydrosilylated, and the incorporated chlorodimethylbutylsilyl active groups were reacted with living PS or PI chains. The graft terpolymers exhibited low polydispersity index comparable to the one of the precursors. Poly(carboxymethylcellulose)-g-poly(N-isopropylacrylamide) (CMC-g-PNIPAM) graft copolymers were synthesized (22) through formation of amide bonds. Amino-terminated NIPAM polymeric chains were obtained by radical polymerization in water. The coupling reaction between the carboxyl groups of the backbone and the terminal amine group of the grafts, performed in water at pH = 8 in the presence of 1-[3-(dimethylamino propyl)-3-ethyl-carbodiimide, resulted in the formation of the desired products. The composition of the graft copolymers and dilute solution measurements confirmed that the PNIPAM chains were efficiently grafted to the backbone. Well-defined polybutadiene-b-polystyrene (PBd-g-PS) graft copolymers were synthesized using hydrosilylation reactions (23). Catalytic hydrosilylation of the 1,2 Bd units (~10%) of PBd introduced chlorosilane groups (Fig. 3). Linking reactions between living polystyrene anions and the Si Cl groups of the backbone (1) gave PBd-g-PS graft copolymers with randomly placed single PS branches. When HSiCl2CH3 was used in the hydrosilylation step, difunctional branching sites were introduced in the backbone, resulting in the formation of P(Bd-g-S)2 double grafts.

Figure 3. Butadiene–styrene graft and double graft copolymers.

Block graft copolymers are copolymers having a backbone composed of a diblock copolymer. Grafted chains can be attached to one or both of the backbone blocks (Fig. 4). Block grafts with a triblock as the backbone are also possible.

Figure 4. Block-graft copolymer. In this case only one of the backbone blocks is grafted.

Using anionic polymerization techniques, poly{styrene-b-[(4-vinyl phenyl-dimethylsiloxane)-gisoprene]}, P[S-b-(VS-g-I)], block graft copolymer was synthesized (24, 25). The block copolymer P (S-b-VS) was prepared by the sequential addition of S and 4-vinyl phenyl dimethyl vinylsiloxane. Living PI chains were reacted with the vinylsilane groups of the block copolymer to form the block graft copolymers. In a similar fashion, poly{4-methylstyrene-b-[(styrene-co-3-methylstyrene)-g-2-vinyl pyridine]} block graft terpolymers were synthesized (26). The backbone was prepared by coordination polymerization and sequential addition of 4-methyl styrene and a mixture of styrene and 3-methyl styrene. Chlorination of the methyl groups of the substituted styrenic monomeric units generated the grafting sites. The final coupling was accomplished by adding a THF solution of the chlorinated terpolymer to living P2VP chains at low temperature. PS-g-PI, (PS-g-PI)-b-PS, and (PS-g-PS)-b-(PS-g-PI) copolymers have been synthesized by using a combination of TEMPO (2,2,6,6-tetramethylpiperidinyl-1-oxy) living free-radical and anionic polymerization (27) (Fig. 5).

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Figure 5. Polystyrene–polyisoprene graft and block copolymers.

The PS backbone of the PS-g-PI was synthesized by living free-radical batch copolymerization of styrene and p-chloromethyl styrene. PI living chains prepared by anionic polymerization, end capped with 1,1-diphenylethylene, reacted with the chloromethyl groups to give the grafts. In a similar way the synthesis of the (PS-g-PI)-b-PS block graft copolymers was performed. The synthetic approach of the (PS-g-PS)-b-(PS-g-PI) graft-block-graft copolymer involved first the preparation of the PS-g-PS grafts, followed by the copolymerization of styrene and p-chloromethylstyrene by living radical polymerization using the nitroxide end group of the PS backbone as the initiating site. Finally, living diphenylethylene-capped PI chains, prepared by anionic polymerization, were reacted with the chloromethyl groups of the extended PS backbone. Model block-double graft copolymers and terpolymers of styrene, butadiene, and isoprene of the type poly[S-b-(PBd 1,2-g-X)] were recently synthesized (28), where X is either S, Bd, I or S-b-I by a combination of anionic polymerization, hydrosilylation, and chlorosilane linking chemistry. The backbone PS-b-1,2PBd was synthesized by sequential addition of the monomers. The vinyl groups of the PBd-1,2 block were hydrosilylated with HSiCH3Cl2 and used for the attachment of living chains X, resulting in the formation of the corresponding block double graft copolymers. The grafting onto approach and a combination of ATRP and “click” chemistry was employed for the synthesis of graft copolymers consisting from a PMMA backbone and poly(1-ethoxyethyl acrylate), PEEA branches (29). The backbone was prepared by the direct copolymerization of an azide containing monomer, 3-azidopropyl methacrylate (AzMA), and methyl methacrylate (MMA), leading to a polymethacrylate chain having randomly distributed azide groups. The alkyne-containing initiator propargyl 2-bromopropionate was used for the ATRP of 1-ethoxyethyl acrylate leading to polymers bearing end alkyne groups. The backbone's azide groups were subjected to copper(I) catalyzed “click” 1,3-dipolar cycloaddition reaction with the alkyne terminated PEEA branched affording the desired graft copolymer. The pure product was isolated by preparative GPC. The 1-ethoxyethyl protective group can be removed leading to the formation of amphiphilic graft copolymers having poly (acrylic acid) branches (Fig. 6).

Figure 6. Schematic depiction of the synthesis of block and graft copolymers using “click” chemistry.

This approach was efficient for the synthesis of graft copolymers. However, it required the preparation of 3-azidopropyl methacrylate (a two-step procedure) whose purification is challenging, because of its thermal sensitivity. Therefore, the use of, the easy to handle and polymerize, glycidyl methacrylate, GMA, was reported for the introduction of the azide groups through the ring opening of the oxirane rings (30). According to this procedure, MMA and GMA were copolymerized by ATRP followed by the reaction with NaN3 for the introduction of azide groups along the polymer chain (Fig. 7). In a separate reactor, poly(ethylene oxide) monomethyl ether pentynoate, MePEO-P was prepared by the reaction of poly(ethylene oxide) monometheyl ether with 4-pentynoic acid. The azide groups of the backbone and the alkyne end-groups of MePEO-P were then subjected to the “click” reaction leading to the synthesis of the desired amphiphilic graft copolymers. Because of steric hindrance effects, part of the azide groups remained unreacted.

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Figure 7. Copolymerization of GMA and MMA, ring-opening of the epoxide ring in the presence of azide, and synthesis of brush copolymer by a “grafting onto” click technique.

In another contribution (31), an alkyne-containing polymeric backbone was reacted under typical “click” reaction conditions with azido-terminated side chains to produce molecular brushes (densely grafted polymers). The polymeric backbone with alkynyl side groups on essentially every monomer unit (PHEMA-alkyne) was obtained in two steps starting with the synthesis of poly(2-hydroxyethyl methacrylate) (PHEMA), followed by the esterification of the pendant hydroxyl groups with pentynoic acid. The azido-functionalized polystyrene-N3, poly(n-butyl acrylate)-N3, or poly(n-butyl acrylate)-bpolystyrene-N3 were prepared by ATRP and the modification of the chain end-bromine groups via nucleophilic substitution reactions with NaN3. On the other hand, poly(ethylene oxide)-N3 was prepared from the corresponding hydroxyl-terminated PEO chains by conversion to mesylateterminated PEO (PEO-OSO2CH3) by reaction with methanesulfonyl chloride, followed by transformation of the mesylate chain-end groups into azido groups via reaction with NaN3 (Fig. 8). The grafting density was found to depend on the molecular weight, the chemical nature, and the concentration of the linear side chains.

Figure 8. Synthesis of brush polymers via combination of ATRP and click reactions.

The grafting “onto” strategy and a combination of RAFT polymerization and “click” chemistry were employed for the synthesis of graft copolymers having polymethacrylate backbone and poly(vinyl acetate) branches (32). Propargyl methacrylate with its acetylene function protected with a silyl group was first polymerized via the RAFT process, using cyanoisopropyl dithiobenzoate (CPDB) as the RAFT agent. The polymer was then deprotected under mild acidic conditions to afford a polymer backbone bearing acetylene functionalities at each monomer unit. In parallel, an azide-functionalized xanthate (ethoxythiocarbonylsulfanyl-acetic acid 3-azido-propyl ester) was employed to prepare low polydispersity poly(vinyl acetate) end-functionalized with azido groups. The two polymers were then conjugated by “click” reaction leading to the synthesis of graft copolymers. This procedure was monitored by UV–vis spectroscopy (Fig. 9).

Figure 9. Synthesis of comb polymers.

4. Grafting “from” Methods In the grafting “from” method, the backbone is chemically modified to introduce active sites capable of initiating the polymerization of a second monomer. The number of grafted chains can be controlled by the number of active sites generated along the backbone assuming that each one of them participate in the formation of one branch. However, mainly because of kinetic and steric hindrance effects, there may be a difference in the lengths of the produced grafts.

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Page 7 of 38 Anionic grafting techniques were used for the synthesis of PI-g-PS and PBd-g-PS copolymers (3337). The active sites were generated by metallation of the allylic double bonds of the polydiene backbone, by organometallic compounds such as n-BuLi, in the presence of strong chelating agents that facilitate the reaction (eq. 3). The most common metallation procedure involves the use of n-BuLi in the presence of N,N,N′,N′-tetramethylethylenediamine (TMEDA). The activated polydiene then reacts with the styrene. The graft copolymers exhibited well-defined molecular characteristics. (3) By using the grafting “from” technique, PMMA-g-( -butyrolactone) (38) copolymers were synthesized. Anionically polymerized PMMA was treated with 18-crown-6 complex of potassium hydroxide resulting in a macromolecular initiator (2) (eq. 4).

(4)

The carboxylate active groups were used as initiating sites of -butyrolactone. It was found that the grafting efficiency was high, and the density of grafting chains could be easily controlled. Poly (ethylene-g-styrene) copolymers were also prepared (39). The approach for the synthesis of the backbone involved the copolymerization of ethylene and p-methyl styrene by metallocene catalysts, such as [C5(CH3)4(Si(CH3)2N-t-C4H9)]TiCl2/C2H5(Ind)2ZrCl2. The methyl groups of the pmethylstyrene units were then metallated with s-BuLi/TMEDA complex. The active sites produced were used to initiate the anionic polymerization of styrene. The efficiency of the metallation was found to be 65%. Anionic and ring opening polymerization reactions have been employed for the synthesis of graft terpolymers bearing poly(p-hydroxystyrene), P(OHS) backbone, and PEO-b-PPO branches, that is, P (OHS)-g-(PEO-b-PPO) and P(OHS)-g-(PPO-b-PEO) (40) (Fig. 10). p-tert-Butoxystyrene was polymerized anionically followed by postpolymerization hydrolysis of the p-tert-butoxy group to yield the P(OHS) backbone. The pendant hydroxyl functions were transformed to the corresponding phenoxides in the presence of t-BuP4. These active sites were subsequently used for the sequential polymerization of EO and PPO, or vice versa, for the synthesis of the desired graft terpolymers. Because of the presence of chain transfer reactions, the samples were contaminated with the linear block copolymers. Characterization data revealed a high degree of grafting. However, quantitative calculations were difficult to be made.

Figure 10. General synthetic scheme for the preparation of the core-shell copolymers.

Cationic grafting techniques have been used for the synthesis on poly(ethyl vinyl ether-gethyloxazoline) graft copolymers (41). A random copolymer of ethyl vinyl ether with a small quantity of 2-chloroethyl vinyl ether was synthesized by cationic polymerization. The pendant alkyl groups were the initiating sites of the cationic polymerization of ethyloxazoline grafts. In a similar way, acid chloride groups were incorporated along the polymer chain by partial hydrolysis of poly(vinyl acetate) (3) and reacting the produced hydroxyl groups with phosgene or diphosgene (42). These groups were the initiating sites for the polymerization of 2-phenyl or 2-methyl oxazoline in the presence of potassium iodide (eq. 5)

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(5)

Poly(isobutylene-g-indene) and poly(isobutylene-g-styrene) copolymers were synthesized by cationic polymerization (43). The backbone was prepared by cationic copolymerization of isobutylene and pchloromethylstyrene, using TiCl4/t-BuCl as the initiator. The chloroalkyl groups were used, in the presence of Al(C2H5)2Cl, as the initiation sites of the cationic polymerization of indene or styrene. The grafting efficiency was as low as 40–50%. [MMA-co-(p-chloromethylstyrene)]-g-[2-methyl-2oxazoline] amphiphilic copolymers were prepared by combining radical and cationic polymerization techniques (44). The synthesis of the backbone was performed by radical copolymerization of MMA and p-chloromethylstyrene in the bulk. The cationic macroinitiator was dissolved in benzonitrile along with 2-methyl-2-oxazoline (MeOXz) and the production of PmeOXz grafts took place at 110°C. A convenient route for the preparation of poly(methylphenylsilane)-g-PS was presented (45). The synthetic approach involved the alkali metal mediated reductive-coupling reaction of dichloromethylphenylsilane with sodium in toluene at 100°C (Fig. 17). The phenyl groups were then bromomethylated (Fig. 17), and the incorporated bromomethyl groups were used as the initiators of the polymerization of styrene by atom transfer radical polymerization (ATRP) (eq. 6).

(6)

Polypropylene-g-PS copolymers were synthesized by combination of metallocene and TEMPO living free-radical polymerization techniques (46). The backbone was synthesized by copolymerization of propylene and a TEMPO-functionalized derivative containing a -double bond. The TEMPO groups were then used for the polymerization of styene by living free-radical polymerization (Fig. 11).

Figure 11. Synthesis of polypropylene-g-polystyrene.

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By using ATRP, poly(2-hydroxyethyl methacrylate)-g-PS (PHEMA-g-PS) and PHEMA-g-poly(n-butyl acrylate) (47) were synthesized. The backbone, composed of trimethylsilyl-protected 2-hydroxyethyl methacrylate, was synthesized by using the ATRP methodology, followed by deprotection of the hydroxyl group. Subsequent esterification (eq. 7) with 2-bromoisobutyryl bromide resulted in poly[2-(2-bromoisobutyryloxy)ethyl methacrylate] (7). PS or poly(n-butyl methacrylate) was then grafted from the macromolecular initiator (eq. 7). Subsequent analysis revealed that all the initiation sites participated in the polymerization of the second monomer.

(7)

In a later work, copolymers of PVC with various grafted chains like poly(butyl acrylate), PMMA, PS, and poly(methyl acrylate) were synthesized (48). The backbone was a random copolymer of vinyl chloride and vinyl acetate. The carbonyl substituted alkyl halide groups were used as the initiator for the polymerization of these monomers by ATRP. Metallocene and ATRP techniques were combined for the synthesis of graft ter- and quaterpolymers poly(ethylene-co-styrene) as backbone and one of PS, PMMA, PMMA-b-PS, PMMA-bpolymethylacrylate, and PMMA-b-PHEMA as grafted chain (49). The aromatic rings of styrene were partially brominated and the C–Br groups that were formed were subsequently used as initiating sites for ATRP polymerization. ATRP techniques were employed for the synthesis of poly[poly(ethylene glycol) methyl ether acrylate]-g-poly(methoxymethyl methacrylate), PPEGME-g-PMOMMA, amphiphilic graft copolymers (50) (Fig. 12). PPEGME was initially prepared via ATRP using methyl 2-bromopropionate as the initiator, CuBr as the catalyst, and tris[2-(dimethylamino)ethyl]amine, Me6TREN, as the ligand. Treatment of the polymer with lithium diisopropylamine, LDA, and 2-bromopropionyl chloride led to the introduction of initiation sites for ATRP along the backbone at the -carbon of the ester groups. Subsequent polymerization of MOMMA through these initiation sites provided the desired graft copolymer. Double hydrophilic graft copolymers were obtained by the hydrolysis of the PMOMMA side chains to the corresponding poly(methacrylic acid), PMAA, chains.

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Figure 12. Synthesis of double hydrophilic graft copolymer PPEGMEA-g-PMAA.

Polymer brushes were prepared by ATRP techniques. 2-(Trimethylsilyloxy)ethyl methacrylate was polymerized in anisole at 90°C using tosyl chloride as the initiator, 4,4′-di(5-nonyl)-2,2′-bipyridine, dNbpy, as the ligand, and CuBr as the catalyst (51) (Fig. 13). The silyloxy protective group was removed by treatment with KF, and tetrabutylammonium fluoride and the produced hydroxyl groups were then esterified after reaction with 2-bromoisobutyryl bromide. These initiation sites were subsequently used for the polymerization of n-butyl acrylate, nBuA, leading to the synthesis of very densely grafted copolymers poly[2-(2-bromopropionyloxy) ethyl methacrylate]-g-PnBuA. It was found that at the initial stages the grafting efficiency was quite low, but it was increased up to 87% at 12% monomer conversion.

Figure 13. Outline for macro-initiator and molecular brush synthesis.

Poly(vinylidene fluoride-co-chlorotrifluoroethylene), P(VDF-co-CTFE), was prepared by ATRP methods (52) (Fig. 14). The pendant chlorine moieties were further employed as initiation sites for the ATRP of either styrene or tBuA leading to the synthesis of P(VDF-co-CTFE)-g-PS and P(VDF-coCTFE)-g-PtBuA graft copolymers. The experimental conditions were optimized to avoid the formation of crosslinking by products. In the case of the P(VDF-co-CTFE)-g-PS copolymers, SEC analysis revealed the presence of PS homopolymer, produced by thermal radical polymerization. The hydrolysis of the tBu-groups from the P(VDF-co-CTFE)-g-PtBuA copolymers led to the synthesis of amphiphilic graft copolymers with PAA side chains.

Figure 14. Graft copolymerization from P(VDF-co-CTFE) via ATRP.

Commercially available syndiotactic PS, sPS, was treated with chloro-1,5-cyclooctadiene iridium(I) dimer and bis(pinacolato)diboron to give the pinacolboronate ester-functionalized sPS (53) (Fig. 15). Subsequent oxidation using a mixture of H2O2 and NaOH in an aqueous THF solution generated hydroxylated sPS. The hydroxyl groups were then reacted with 2-bromo-2-methylpropionyl bromide to afford initiation sites for the ATRP of either MMA or tBuA leading to the synthesis of the corresponding sPS-g-PMMA and sPS-g-PtBuA graft copolymers. The pure copolymers were obtained after extraction with hot acetone to remove the PMMA and PtBuA homopolymers that were formed during the copolymerization reactions.

Figure 15. Preparation of polar graft copolymers of syndiotactic polystyrene.

A combination of ROMP and ATRP was employed for the synthesis of graft copolymers. Norbornene derivatives containing triethylene glycol monomethyl ether or n-octyl ester side groups were copolymerized with another norbornene derivative bearing triethylene glycol ester containing 2bromopropionate end groups by ROMP using the Grubb's catalyst Cl2(PCy3)2Ru( =CHPh) (54) (Fig. 16). The pendant 2-bromopropionate side groups served as initiators for the ATRP of tBuA leading to the formation of graft copolymers. Subsequent hydrolysis under mild acidic conditions provided amphiphilic graft copolymers, consisting of a polynorbornene-based backbone and PAA side chains

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Page 11 of 38 (eq. (8).

(8)

Figure 16. Synthesis of norbornene monomers and random graft copolymers.

The same Grubb's catalyst was also employed for the ROMP of cis-3,4-bis(2-bromo-2isobutyrylmethyl)-cyclobutene leading to the synthesis of a 1,4-PBd chain bearing two initiation sites for ATRP per monomer unit (55) (Fig. 17). These sites were then used for the polymerization of styrene, MMa, and tBuA for the synthesis of the respective graft copolymers. The SEC analysis revealed the presence of a bimodal distribution for the synthesis of the PBd-g-PS copolymers because of the presence of cross-linking coupling reactions involving the PBd backbone.

Figure 17. Graft copolymer synthesis.

Statistical copolymers of MMA and 2-hydroxyethyl methacrylate, HEMA, were prepared by ATRP using ethyl 2-bromoisobutyrate as initiator, CuBr as the catalyst, 2,2′-bipyridine as the ligand and caprolactone, -CL, as the solvent in scCO2 (56) (Fig. 18). The hydroxyl groups along the backbone were further employed to promote the enzymatic ROP of -CL leading to the synthesis of P(MMA-coHEMA)-g-P( -CL) graft copolymers. The SEC analysis showed that the macroinitiator was quantitatively consumed. Alternatively, a one-step approach was employed meaning that the radical copolymerization of MMA and HEMA, and the enzymatic ROP of -CL took place simultaneously. In this procedure, P( -CL) homopolymer was obtained as well.

Figure 18. Synthesis of graft copolymers by the “grafting from” technique.

Block-graft copolymers were also prepared by the grafting “from” method. A Reaction of the block copolymers P(S-b-V) (25) with n-BuLi in THF at –30°C for 1 h leads to metallation of the vinyl groups of the vinyl segments. Then D3 was added for the formation of P[(S-b(VS-g-DMS)] block graft copolymers. The molecular characterization revealed that the number of the introduced PDMS branches was between 170 and 325. The synthesis of the poly[styrene-b-(hydroxystyrene-g-ethylene oxide)-b-styrene] (25), P[S-b-(HS-gEO)-b-PS], has been reported. The backbone was a triblock copolymer, poly(styrene-b-tbutoxystyrene-b-styrene) (8), prepared by anionic polymerization by sequential addition of

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Page 12 of 38 monomers. The protected t-butyl group was removed by treatment with HBr leading to the formation of P(S-b-HS-b-S) triblocks (9) (eq. 9). The metallation of the hydroxyl groups was performed in THF using either cumyl potassium or diphenylethylene potassium. The addition of EO generated the block-graft copolymers.

(9)

P(S-g-t-BMA) and PS-b-P(S-g-t-BMA) block graft copolymers were prepared using TEMPO living and atom transfer radical polymerization techniques (57). The backbone of the P(S-g-t-BMA) copolymer was synthesized by TEMPO living radical copolymerization of styrene and pchloromethylstyrene. Subsequently, the chloromethyl groups in the presence of CuBr and bipyridine were used as initiation sites of ATRP of t-BMA. In the case of PS-b-P(S-g-t-BMA) copolymer, the synthesis was performed in a similar way.

5. Macromonomer Method The synthesis of graft copolymers by the macromonomer method is characterized by its own specific features (58-61). The number of branches is determined by the ratio of the macromonomer and comonomer molar concentrations and their copolymerization behavior, described by the reactivity ratios r1 and r2. These parameters determine how random the placement of the branches along the backbone will be. It is evident that during the copolymerization, the relative concentrations of the macromonomer and the comonomer change with time, leading to the formation of graft copolymers with subsequently different number of branches. In addition, the copolymerization is not homogeneous throughout the course of the reaction since phase separation may occur. For the above-mentioned reasons, it can be concluded that the graft copolymers prepared by this method are generally characterized by increased compositional and chemical heterogeneity. The synthesis of macromonomers can be accomplished by almost all the available polymerization techniques. Among them, living polymerization methods offer unique control over the molecular weight, the molecular weight distribution, and chain-end functionalization. Anionic polymerization is one of the best methods for the synthesis of well-defined macromonomers. Functional initiation or termination by a suitable electrophilic reagent are the best ways for the incorporation of the reactive end-groups (62). According to this methodology, living polystyryllithium initially reacts with ethylene oxide to form the less reactive alkoxide, followed by the reaction with methacryloyl chloride for the synthesis of the macromonomer (63) (eq. 10).

(10)

These macromonomers were then copolymerized with vinyl monomers mainly by free-radical techniques (64) and also other methods as by metallocene catalysts (65) to provide graft copolymers. The reaction of polystyryllithium with p-chlorovinylbenzene at low temperatures affords

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Page 13 of 38 macromonomers with styryl end groups (66) (eq. 10). (10) Allyllithium was used to polymerize hexamethylcyclotrisiloxane, D3 (Fig. 30). Subsequent termination with chlorotrimethylsilane yielded polydimethylsiloxane, PDMS macromonomers carrying allyl end groups (67) (eq. 11).

(11)

Macromonomers have also been used as intermediate structures for the synthesis of highly branched or dendritic macromolecules through the convergent approach. When a bifunctional compound carrying a polymerizable vinyl group and a group capable to link a living polymer chain is slowly added into a living polymer solution, consecutive macromonomer formation and macromonomer addition reactions can take place. Bifunctional compounds that have been used are the 4-(chlorodimethylsilyl)styrene, CDMSS, and vinylbenzyl chloride, VBC (68). Depending on the degree of branching and the molecular weight of the living polymers the final product may be a hyperbranched structure or a star polymer. Possible reactions involving the reaction of polystyryllithium with VBC are given in Figure 19.

Figure 19. Possible reactions of polystyryl lithium with VBC.

The characteristic of this procedure is that the complex architectures are formed in a one-pot reaction. However, there is no absolute control during the progress of the synthesis and the products are rather polydisperse with high molecular weight heterogeneity. Especially, in the case of VBC, several side reactions may take place imposing extra difficulties in obtaining well-defined polymers. Monomeric or dimeric termination of the living PS chains, -proton abstraction from VBC, and lithium-halogen exchange are possible side reactions. By choosing the suitable reaction conditions (solvent, temperature, reaction time, etc.), these side reactions can be minimized but cannot be eliminated. The star or dendritic structures prepared were characterized from the existence of a functional styryl group at the focal point or the core, respectively. Therefore, it was possible to copolymerize these complex macromonomers with styrene or MMA for the synthesis of graft copolymers imposing star polymers or dendrimers as side chains (68). The products, shown in Figure 20, had very broad molecular weight distributions and were characterized by increased compositional and molecular heterogeneity.

Figure 20. Macromonomers with styrene or MMA for the synthesis of graft copolymers imposing star polymers or dendrimers as side chains.

Atom transfer radical copolymerization of MMA with methacryloyl terminated poly(dimethylsiloxane), PDMS, macromonomers was performed, and the results concerning the reactivity of the system were

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Page 14 of 38 compared with those obtained by conventional radical polymerization (69). It was concluded that, as in the previous case, the macromonomer in ATRP is more reactive than in the classical free-radical polymerization. Furthermore, the graft copolymers from ATR copolymerization had a much lower chemical and compositional heterogeneity and narrower molecular weight distributions. Examples of graft copolymers synthesized using anionically prepared macromonomers are given in Table 2. Table 2. Graft Copolymers Prepared by Macromonomers Synthesized Anionically Macromonomer End group PS PS PS PS PS PDMS PMMA PI PS-b-PI PDMS PDMS PS-b-PDMS

PtC4H9 Maa PI PBd PEO PS PEO PDMS PS, PI, PDMS aPoly(t-butyl

Comonomer

Methacrylate MMA pMMA Vinylbenzene Norbornene Norbornene pButadiene Vinylbenzene Methacrylate Ethyl acrylate Methacrylate MMA pStyrene Vinylbenzene Methacrylate MMA Methacrylate Styrene Allyl Ethylene Methacrylate Styrene Diol Diphenylmethyl diisocyanate + butanediol p4-Vinylpyridine Vinylbenzene pStyrene Vinylbenzene Norbornene Norbornene Norbornene Norbornene Allyl Propylene Methacrylate Styrene Methacrylate MMA Methacrylate MMA

Copolymerization Reference Radical Radical

(70, 71) (68)

ROMP Anionic

(72) (73)

Radical GTP Radical

(74) (75) (76)

GTP Radical Ziegler–Natta Radical Polycondensation

(75) (77) (69) (78) (79)

Radical

(80)

Metallocenes

(81)

ROMP ROMP Metallocene ATRP ATRP, radical Metallocene

(82) (83) (84) (85) (86) (67)

methacrylate).

Cationic polymerization has also been used for the synthesis of macromonomers, especially after the development of living cationic polymerization techniques (87). Macromonomers were prepared by the cationic ring opening polymerization of tetrahydrofuran (THF) using methyltrifluoromethane sulfonate, followed by termination with 3-sodio-propyloxydimethylvinylsilane to give a macromonomer with vinyl silane end groups (88) (eq. 12). (12) These macromonomers were copolymerized with vinyl acetate (Vac) using azo-bisisobutyronitrile

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Page 15 of 38 (AIBN) as radical initiator to produce PVAc-g-PTHF graft copolymers. Subsequent saponification using NaOH provided poly(vinyl alcohol)-g-PTHF graft copolymers. Termination of living PTHF (13) with 3-(dimethylamino propyl) isocyanide leads to the formation of macromonomers having end isocyanide groups (89) (eq. 13).

(13)

p-Iodomethylstyrene was used as a functional initiator to promote the polymerization of 2-phenyl-2oxazoline leading to the formation of macromonomers having styryl end groups (90) (eq. 14).

(14)

Radical copolymerization with styrene produced PS-g-poly(2-phenyl-2-oxazoline) graft copolymers. Other examples of graft copolymers prepared using macromonomers, which were synthesized by cationic polymerization, are given in Table 3. Table 3. Graft Copolymers Prepared by Macromonomers Synthesized by Cationic Polymerization Macromonomer End group Poly(2-alkyl-2oxazoline)

Comonomer

Copolymerization Reference

Diethanolamine -Caprolactone + Polycondensation (91) 4,4′-methylene di (phenylisocyanate) (Meth)acrylate (92)

Poly(2-alkyl-2oxazoline) PPO, poly (Meth)acrylate Styrene, MMA (epichlorohydrin) Polyisobutylene Methacrylate MMA

Radical

(93)

GTP

(94)

Free-radical polymerization has been the most common technique for the synthesis of macromonomers because of its less demanding experimental conditions, the absence of special purification procedures of reagents used, and its applicability to a wide variety of monomers. However, the method suffers many disadvantages as the poor control over the molecular weight, the molecular weight distribution, and the degree of functionalization. The basic method for the incorporation of the functional end groups involves the use of chain-transfer agents (95). Poly(methyl methacrylate) (PMMA) macromonomers have been prepared using thioglycolic acid as a chain-transfer agent, followed by a reaction with glycidyl methacrylate (96) (15).

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Page 16 of 38

(15)

-(Bromoethyl) acrylate has also been used as a chain-transfer agent for the synthesis of PMMA macromonomers carrying acrylic end groups (97) (eq. 16). (16) Table 4 summarizes examples of graft copolymers prepared by macromonomers that were synthesized by free-radical polymerization methods. Table 4. Graft Copolymers Prepared by Macromonomers Synthesized by Free-Radical Polymerization Macromonomer PMMA PMMA

End group

Comonomer Copolymerization Reference

Methacrylate Acrylic acid Acrylate Styrene, vinyl chloride, vinyl acetate Polyvilylpyrrolidone pStyrene Vinylbenzene Poly(4pStyrene vinylpyridine) Vinylbenzene PMMA Aromatic Terephthalic dicarboxy acid + bisphenol PS Carboxyl Ethyl cellulose PMMA Dihydroxyl eCaprolactone Poly(ethyl Methacrylate Methacrylic methacrylate), poly acid (n-butyl methacrylate) PMMA Acrylate Styrene

Radical Radical

(98) (97)

Radical

(99)

Radical

(99)

Polycondensation (100) UV irradiation

(101)

Polycondensation (102) Radical

(103)

ATRP

(104)

The evolution of the “living” free-radical polymerization techniques [mainly TEMPO (2,2,6,6tetramethylpiperidinyl-1-oxy) and ATRP (atom transfer radical polymerization) methods] very soon led to the synthesis of macromonomers. These methods combine the advantages of the free-radical polymerization with those of the living polymerization techniques, despite the fact that control over the functionalization reaction is not always comparable to the anionic polymerization methods (105, 106). A multifunctional reagent (14) containing reactive sites [TEMPO, oxazoline, and t-butyl dimethyl silyl protected hydroxyl (TBDMS) group] able to initiate anionic, cationic, and “living” free-radical polymerization was used to prepare a variety of macromonomers (107). Styrene was polymerized at high temperatures through the alkoxyamine functional group to produce macromonomers with

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Page 17 of 38 oxazoline end groups (15) (eq. 17). Cationic copolymerization of these macromonomers with oxazoline using methyl triflate as initiator leads to the formation of polyoxazoline-g-PS graft copolymers.

(17)

Poly(dimethylaminoethyl methacrylate) macromonomers were prepared by ATRP using allyl 2bromoisobutyrate (ABIB (16))/CuBr/tris[2-di(butyl acrylate) aminoethyl] amine, (BA6-TREN) or allyltrichloroacetamide/CuBr/BA6-TREN as the initiation systems (108) (eq. 18).

(18)

A difunctional initiator suitable to promote the ATRP of vinyl monomers was prepared by the reaction of 2-bromoisobutyryl bromide with cis-3,4-bis(hydroxymethyl)-cyclobutene (109) (Fig. 21). This initiator was used for the sequential polymerization of styrene and tert-butyl acrylate, tBuA, in the presence of CuBr as the catalyst and N,N,N′,N′,N″-pentamethyldiethylenetriamine, PMDETA, as the ligand leading to the synthesis of PtBuA-b-PS-b-PtBuA macromonomers bearing cyclobutene functions at the middle of the PS block. Subsequent polymerization of these macromonomers using the second generation Grubb's catalyst, (IMesH2)-(Cy3P)RuCl2(CHPh), afforded the corresponding molecular brushes bearing polybutadiene, PBd, backbone and PS-b-PtBuA branches. Hydrolysis of the tert-butyl groups under mild acidic conditions led to the synthesis of PBd-g-(PS-b-PAA) graft terpolymers.

Figure 21. Graft copolymer synthesis.

Group transfer polymerization has also been used for the synthesis of macromonomers. The functional end groups are incorporated mainly using the suitable initiation system (110). Trimethylsilyloxy ethyltrimethylsilyl dimethyl ketene was used as initiator for the synthesis of PMMA macromonomers (17) in the presence of tetrabutylammonium benzoate that was the catalyst. Treatment with dilute HCl provided the –OH functional macromonomers (18), followed by the reaction with acryloyl chloride to give the final macromonomer structure (111) (eq. 19).

(19)

Polyacrylate macromonomers were prepared using triphenyl phosphine and trimethylchlorosilane in

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Page 18 of 38 the presence of zinc halide that was the catalyst (112). The resulting polymers (19), having trimethylphosphinium end groups, were transformed to the corresponding macromolecular ylide (20) by reaction with sodium ethanolate. The subsequent Witting reaction of the ylide provides the macromonomer (21), where R = methyl, ethyl, or n-butyl (eq. 20).

(20)

More examples concerning the synthesis of macromonomers by group transfer polymerization and the subsequent formation of graft copolymers are given in Table 5. Table 5. Graft Copolymers Prepared by Macromonomers Synthesized by Group Transfer Polymerization Macromonomer

End group

Poly(vinyl alcohol) pVinylbenzene Poly(2-phenylp1,3,2-dioxaborole) Vinylbenzene Methyl, hexyl, nVinyl butyl acrylate PMMA pVinylbenzene PMMA Methacrylate

Comonomer Copolymerization Reference Styrene

Radical

(113, 114)

Styrene

Radical

(115)

Styrene

Radical

(112)

Styrene

Radical

(116)

n-Butyl acrylate

ATRP, radical

(117)

Polycondensation techniques have been employed for the synthesis of macromonomers. Depending on the nature of the functional end group, the copolymerization with comonomers can be performed with addition polymerization techniques, giving rise to very interesting graft copolymer structures. Poly( -benzyl-L-glutamate) macromonomers were prepared by polymerization of benzyl(S)-3-(2,5dioxo-1,3-oxazolidin-4-yl)propionate (21) (or -benzyl-L-glutamate-N-carboxyanhydride) using the primary amino group of the N-methyl-N-4-vinylphenethyl) ethylene diamine (22)(118, 119) (eq. 21).

(21)

Subsequent reaction with aminoalcohols provides another type of macromonomer with side –OH groups (eq. 22).

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Page 19 of 38

(22)

Characteristic examples concerning the synthesis of macromonomers prepared by polycondensation reactions and the subsequent formation of graft copolymers are given in Table 6. Table 6. Graft Copolymers Prepared by Macromonomers Synthesized by Polycondensation Methods Macromonomer End group

Comonomer Copolymerization Reference

Polyguanamine Isopropenyl substituted triazine Polyurethane Methacrylate Polyamine pVinylbenzene

Styrene, MMA Radical

(120)

MMA 2Hydroxyethyl methacrylate

(121, 122) (123)

Radical Radical

Macromonomers have been also prepared by post-polymerization reactions. Poly(ethylene oxide) (PEO) or poly(propylene oxide) (PPO) has one or two –OH end groups depending on the method of their preparation. These –OH groups may react with (meth)acryloyl chloride, p-vinyl benzyl chloride, norbornenyl chloride, 4-vinyl benzoyl chloride etc for the synthesis of the corresponding macromonomers (124, 125) (eq. 23). (23) Hydrosilyl-terminated PDMS chains may undergo hydrosilylation reactions with comonomers having double bonds to produce macromonomers (126, 127) (eq. 24).

(24)

Poly(ethylene glycol), PEG, macromonomers with end cyclooctene groups were copolymerized with cyclooctene using the suitable Grubbs catalyst, that is, bis(tricyclohexylphosphine)benzylidine ruthenium(IV) dichloride to provide the polycyclooctene-g-PEG graft copolymer (128) (eq. 25). Subsequent hydrogenation using p-toluenesulfonhydrazide produced the corresponding polyethylene-g-PEG graft copolymer.

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Page 20 of 38

(25)

Poly(ethylene-co-propylene), (PE-co-PP), chains having end-vinylidene groups were prepared by metallocene-mediated copolymerization of ethylene and propylene using Cp2ZrCl2/MAO as the catalytic system (129). Subsequent hydroalumination with diisobutylaluminum hydride, DIBAL-H, and oxidation with dry air at 110°C transformed the alkylaluminum end groups to hydroxyl end groups. The obtained hydroxyl groups were then reacted with methacryloyl chloride to afford the methacryloyl terminated (PE-co-PP) macromonomers. Conventional radical and ATRP of these macromonomers with MMA were performed for the synthesis of the graft terpolymers PMMA-g-(PE-co-PP) (Fig. 22).

Figure 22. Synthesis of PMMA-g-EPR graft copolymer.

A similar, but more elaborate approach was followed for the synthesis of PS-g-PE graft copolymers (130). Bis-[N-(3-tert-butylsalicylidene)methylaminato] zirconium dichloride activated by MAO was found to selectively produce a vinyl-terminated linear polyethylene with high functionality (>90%). Hydroalumination/oxidation reaction was employed to transform the end-vinyl groups to hydroxylgroups. The synthesis of the corresponding methacryloyl-terminated PE macromonomers was conducted through the reaction of the end-hydroxyl groups with 2-bromo-2-methyl-propionylbromide, followed by the dehybromination of the -bromoisobutyrate group to the corresponding methacryloyl ester functionality after the reaction with excess 1,8-diazabicyclo[5.4.0]undec-7-ene, DBU. The grafting through ATRP of these macromonomers with styrene was then employed to produce the PSg-PE graft copolymers. PnBuA macromonomers with methacrylate or acrylate end-groups were prepared by nucleophilic substitution of the end-bromine group of the PnBuA chains, produced by ATRP, with methacrylic or acrylic acid, respectively, in the presence of DBU (131) (Fig. 23). These macromonomers were then copolymerized with DMAEMA, acrylic acid, AA, or dimethyl acrylamide, DMAA, to give the corresponding graft copolymers PDMAEMA-g-PnBuA, PAA-g-PnBuA, and PDMAA-g-PnBuA.

Figure 23. Copolymerization of poly(BA)-MM with DMAEMA.

Allyl-terminated PS macromonomers were obtained after the reaction of the end-bromine groups of

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Page 21 of 38 the PS chains, which were produced by ATRP, with allyltrimethylsilane, ATMS, in the presence of TiCl4 (132). This reaction proceeded via the formation of the PS cation and Ti2Cl9– mimicking the carbocationic polymerization. The carbocation addition to ATMS affords the allyl-terminated macromonomer. Copolymerization of these macromonomers with propene using zirconocenes activated with MAO led to the synthesis of PP-g-PS graft copolymers (Fig. 24).

Figure 24. Synthesis of allyl-terminated polystyrene macromonomers and their copolymerization with propylene.

More examples of macromonomers prepared by postpolymerization reactions are provided in Table 7. Table 7. Graft Copolymers Prepared by Macromonomers Synthesized by Post-Polymerization Reactions Macromonomer End group PEO PDMS PPO PEO PEO Oligo(ahydroxyalkanoic acid) PEO

Comonomer Copolymerization Reference

Acrylate Styrene Methacrylate Styrene Norbornene Norbornene derivative pn-Butyl Vinylbenzene methacrylate Methacrylate Acrylamide Methacrylate t-Butyl acrylate Allenyloxy

Radical Radical ROMP

(124) (127) (133)

Radical

(125)

Radical Radical

(134) (135)

Coordination polymerization

(136)

Other methods have also been employed for the synthesis of macromonomers. They do not have always general applicability but can be successfully used in specific cases. For example, polypropylene macromonomers with vinylidene end groups were prepared using metallocenecatalyzed polymerization. Bis(cyclopentadienyl)zirconium dichloride/methylaluminoxane (Cp2ZrCl2 /MAO) and other catalytic systems were used. The olefinic end groups were obtained through the hydrogen elimination mechanism. These macromonomers were used either directly to produce graft copolymers after copolymerization with olefins using metallocene catalysts or were transformed to other structures by postpolymerization reactions. Addition of thioacetic acid introduced thiol functional groups, whereas hydroboration and subsequent reaction of the resulting hydroxyl group with methacrylic acid produced methacryloyl-terminated polypropylenes. Radical copolymerization with MMA provided PMMA-g-PP graft copolymers (137). Styryl amylose amide (VAA) was prepared from maltopentose-substituted styrene (VM5A) by phosphorylase-catalyzed polymerization of glucose-1-phosphate, Glu-1P (138) (Fig. 25).

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Page 22 of 38

Figure 25. Preparation of the macromonomer VAA.

Subsequent radical copolymerization with acrylamide gave the corresponding graft copolymers. Poly(lactic acid) macromonomers bearing methacryloyl or acryloyl end functions were prepared by the ROP of lactide using 2-hydroxymethyl methacrylate or 2-hydroxymethyl acrylate as initiator and was then copolymerized with MMA by ATRP (139). The reactivities of the macromonomers were similar to those obtained from the corresponding low molecular weight monomers (2-hydroxyethyl methacrylate and 2-hydroxyethyl acrylate). Structural analysis revealed that homogeneous distribution of the branches was achieved when a mixture of methacryloyl- and acryloyl-terminated macromonomers were copolymerized with MMA. Ring opening metathesis polymerization was employed for the synthesis of polynorbornene macromonomers (24). A well-defined molybdenum initiator of the type Mo(CHCMe2C6H5)(N-2,6-iPr 2C6H3)(OR)2, [R = OC(CH3)3, OCCH3(CF3)2] was used. The polymers produced were cleaved from the initiator fragment in a Witting-like reaction using p-(CH3)3SiC6H4CHO, according to a wellestablished method (eq. 26). Deprotection under basic conditions gave macromonomers having phenyl end groups (25). Subsequent reaction with norbornene carboxylic acid chloride yields macromonomers with norbornene end groups (140).

(26)

[Top of Page]

6. Regular Graft Copolymers The term regular graft copolymers corresponds to graft copolymers with identical equally spaced branches along the backbone. Regular graft copolymers of PI-g-PS have been prepared. The synthetic approach involves the selective replacement of one chlorine atom of methyltrichlorosilane by polystyrene followed by step-growth polymerization of the produced (PS)Si(CH3)Cl2 with , – dilithium PI. The polydispersity of the reaction products is high (Mw/Mn = 2.0–3.0), but it is reduced by fractionation (Mw/Mn = 1.2–1.5). By using SiCl4 instead of Si(CH3)Cl3 and by replacement of two chlorines by PS, graft copolymers with tetrafunctional branching points are synthesized (141) (Fig. 26).

Figure 26. Synthetic approach to regular PI–PS graft copolymers.

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Page 23 of 38

This work can be further extended to the synthesis of graft copolymers bearing hexafunctional branching points (142). For this purpose the hexachlorosilane Cl3Si(CH2)3SiCl3 is reacted with PSLi for the replacement of four chlorines atoms (twice upon each Si atom). Subsequent step-growth polymerization of the produced Cl(PS)2Si(CH2)3Si(PS)2Cl with , –dilithium PI yields the desired multigtaft structures. [Top of Page]

7. Exact Graft Copolymers The term exact graft copolymers refers to molecules where all the molecular and structural characteristics, such as the backbones’ and branches’ molecular weights and molecular weights distribution, the number of branches, and the specific grafting points on the backbone, can be controlled and varied at will. The parameters that are most difficult to control are the number and the spacing distribution of branches along the backbone chain. Exact graft copolymers having rather simple structures, that is, one or two branches, have been prepared so far. Among these are the A2B and AA′B single graft copolymers (143-150), the H- and -shaped copolymers (151-153) (Fig. 27).

Figure 27. Exact graft copolymers: AA′B, H-shaped, and

-shaped.

Several techniques have been used for the synthesis of these structures, and mainly these are anionic polymerization and controlled chlorosilane chemistry. A new promising methodology was reported for the synthesis of exact graft copolymers with polyisoprene backbone and polystyrene branches (154) (Fig. 28).

Figure 28. Synthesis of exact graft copolymers of styrene and isoprene.

The method is based on the use of 1,4-bis(1-phenylethenyl)benzene (27), which upon reaction with living polyisoprenyllithium leads to the formation of a polyisoprene macromonomer. This macromonomer is subsequently linked with polystyryllithium, followed by the anionic polymerization of isoprene. The living single graft copolymer (27) then reacts again with 26, and the same reaction sequence is repeated to produce the exact graft copolymer with two branches. In principle, it is possible to continue the synthesis of graft copolymers with more branches, using the same reaction series. A novel strategy, termed as the iterative process, was developed recently by Hirao and co-workers (155). A specific reaction sequence is performed repeatedly in a step-wise fashion to build up the desired branched structure. Highly selective and quantitative reactions are needed to perform this

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Page 24 of 38 approach several times. According to this methodology, 3-bromopropyl-terminated PS is prepared by anionic polymerization using 3-(tert-butyldimethylsilyloxy)-1-propyl-lithium (SiOPLi) as initiator followed by treatment initially with Bu4NF and then with PPh3 and CBr4. SiOP-end-functionalized PSLi, -SiOP-PSLi, is then reacted with the DPE derivative (1) to afford an -SiOP- -DPEfunctionalized living PS. In situ reaction of living PS with the 3-bromopropyl-terminated PS yields SiOP-in-chain-DPE-functionalized PS. Finally, another PSLi is further reacted with the DPE moiety to introduce a PS branch. These steps can be repeated several times to give an exact PS comb polymer (156) (Fig. 29). The same approach is applied for the synthesis of graft copolymers composed of a PS backbone and PI branches.

Figure 29. Synthesis by the iterative methodology of exact PS combs with up to five branches.

Alternatively, exact graft copolymers having PMMA backbone and PS branches can be prepared employing the following reaction sequence (Fig. 30): (a) synthesis of in-chain-functionalized AB diblock copolymer anions bearing SiOP groups at the junction point, (b) transformation of the SiOP groups to benzyl bromide, BnBr, groups, and (c) coupling of the living SiOP in-chain-functionalized AB diblock copolymer with the BnBr in-chain -functionalized AB diblock copolymer. This procedure can be repeated several times since the BnBr function is regenerated in each reaction cycle (157).

Figure 30. Synthesis of a series of exact graft copolymers composed of PMMA backbone and PS branches by iterative methodology using in-chain-functionalized AB diblock copolymer anion.

[Top of Page]

8. Purification and Molecular Characterization of Graft Copolymers Molecular characterization is an essential step in the study of graft copolymers since the knowledge that is gained is essential in deducing the structure–property relationships. Graft copolymers, like all copolymers, may present molecular weight, compositional, and architectural (distribution in the number of branches and the position of the branching points) heterogeneity (158-160). Following the synthesis of graft copolymers, the next step is to thoroughly characterize the obtained samples to determine their average molecular weight, average composition, molecular and compositional homogeneity, and to obtain information on the architectural characteristics of the material. In addition, information about the molecular size of the individual macromolecule may be desired. Because of the imperfections present in the synthetic methodology used for the preparation of nearly every synthetic macromolecule, the resulting material is a very complex system whose identity can only be revealed by the employment of a combination of analytical techniques (158-160). Sometimes a number of purification procedures must be followed before the desired materials can be isolated and characterized. Graft copolymers may contain varying amount of impurities, such as homopolymers (ie, backbone or branches resulting from incomplete coupling) or copolymers of different architecture, as a result of the synthetic strategy followed for their preparation. These heterogeneities in all cases affect the final properties of the material. Fractionation is a powerful tool usually employed to separate heterogeneous mixtures of polymers before the final characterization

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Page 25 of 38 of each fraction (159). Copolymers can be fractionated according to their molecular weight or chemical composition (160). The mostly used fractionation methods that result in the isolation of actual samples (fractions) are batch fractionation and column-elution fractionation. In some cases, extraction with solvents selective for a specific impurity can be used, that is, in separating excess homopolymer branches.

9. Determination of Molecular Weight, Constitution, and Composition Many analytical techniques are available for the determination of the primary molecular structure of a block copolymer and its average chemical composition. Among them spectroscopic techniques used for low molecular weight compounds are the most powerful and most widely employed. NMR can provide both qualitative and quantitative information with respect to comonomer composition and stereochemical configuration of polymeric molecules (161, 162). The IR technique provides information on chemical, structural, and conformational aspects of polymeric chains (163). Because of the inherently high sensitivity of UV spectroscopy, the technique is often utilized for the identification and quantitative determination of comonomers in graft copolymers. The average molecular weight of block copolymer is a very important parameter that characterizes a certain copolymer sample, and it is strongly related with the properties of the polymeric material. Therefore, its determination and knowledge is imperative in the study of these materials. A variety of techniques are available for the determination of different average molecular weights of graft copolymers and polymers in general. These include membrane osmometry and light scattering for the determination of the absolute values of Mn and Mw respectively, (158). In the case of light scattering, it should be always kept in mind that the Mw obtained is apparent, unless the dn/dc is high for all parts of the graft copolymer (backbone and branches) and the material is characterized by high molecular weight and compositional homogeneity. Knowledge of the molecular weights of the individual components of the graft copolymer can be combined with the determination of the total molecular weight to determine the average number of branches per macromolecule. This determination can be aided by selectively sampling of individual parts of the graft copolymer during synthesis, when this is possible, or selective degradation of a part of the macromolecule after isolation of the product. Size exclusion chromatography (SEC) has emerged as a powerful analytical technique for the determination of molecular weight and molecular weight and composition distribution of graft copolymers (164). A number of different types of mass/concentration and molecular mass detectors have been developed so far for simultaneous use in SEC instruments, providing a large amount of information in a single run. In cases where the molecular size of the graft copolymer must be known in solution, several methods can be applied. These include primarily static and dynamic light scattering, small angle neutron and X-ray scattering, and viscometry. Static light, small-angle neutron (SANS), and X-ray (SAXS) scattering provide information for the radius of gyration of isolated chains (or parts of them using SANS and contrast variation techniques), whereas dynamic light scattering and viscometry measure the hydrodynamic radius and properties of the molecules (158, 165, 166).

10. Properties and Uses 10.1. Solution Properties Graft copolymers’ properties in solutions have been studied extensively for many decades. Because of the arrangement of branches along the backbone, certain changes in its conformation and effective stiffness are observed. Studies on model graft copolymers have shown that the conformation of the whole molecule becomes more extended compared to linear chains (36, 143, 167). The phenomenon is more pronounced in the case of polymacromonomers (graft copolymers that are derived from the homopolymerization of macromonomers) where each repeating unit of the backbone carries a polymer chain (168-172). Crowding effects are maximal in this case, resulting in

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Page 26 of 38 an increased induced stiffness on the backbone and a bottlebrush conformation. This specific conformation can impart interesting rheological properties in the corresponding solutions of polymacromonomers. Detailed light scattering investigations were performed in the case of poly(isoprene-g-styrene) block copolymers in thermodynamically good solvents, isorefractive for different parts of the molecule (36). This technique enabled the determination of the size of the backbone and the branches independently, leading to the conclusion that segregation exists between the backbone and the grafts, even in nonselective solvents and the molecule adopts a more or less core-shell structure. In solvents selective for one of the parts of the molecule, graft copolymers show the ability for micelle formation (173-182). Micelles, with cores comprised from the insoluble part and coronas comprised from the soluble one, are formed. However, aggregation numbers and size of the micelles are lower than the corresponding ones for linear block copolymers. Critical micelle concentrations are also higher in the case of graft copolymers. This was attributed to the additional constrains imposed to micelle formation because of the presence of a large number of branching points along the backbone. Ideally, these branching points should be situated at the micelle core–corona interface, a requirement that increases the free energy of the system as excluded volume effects between the branches and conformational entropy because of looping of backbone are increased. As a result of the molecular architecture, unimolecular micelles of graft copolymers are more easily formed, especially in the case where the solvent is selective for the branches (176-178). In this case, the backbone can adopt a collapsed conformation protected from unfavorable interactions with the solvent molecules by the well-solvated branches forming the corona. The increased number of branches favors the stabilization of the structure in solution. 10.2. Morphology The peculiar macromolecular architecture of graft copolymers also influences their bulk properties. The arrangement of a relatively large number of branching points along one of the constituent chains (the backbone) imparts some additional constrains in the formation of microphases and the geometry of the final microstructure. Macrophase separation cannot be excluded in graft copolymer systems where there is a large molecular, compositional, and architectural heterogeneity. Nevertheless, the bulk morphology of a graft copolymer substantially influences other properties of the material (mechanical, rheological, optical, etc). Therefore, elucidation of equilibrium morphology and factors affecting ordering are essential in the design of new materials for specific applications. The effect of chain architecture on the phase separation, order–disorder transition, and final morphology of graft copolymer systems has been investigated both experimentally (145, 149, 151153, 183-186) and theoretically (187-189). Studies on model graft copolymers elucidated, using transmission electron microscopy (TEM) and SAXS or SANS, many aspects of microphase separation of these complex macromolecules. In general, microphase separation and long-range order decrease as the number of branches increases. The morphology of graft copolymers can be predicted from the four morphologies of the simple constituting units which is a miktoarm star copolymer. The same is valid in the case of and H exact graft copolymers (Fig. 31). Figure 31. (a) Transmission electron microscopy image of poly(butadiene-g-styrene) randomly branched graft copolymers with two PS branches on every grafting point. (b) Corresponding image for a poly(isoprene-g-styrene) regular graft copolymer with two PS branches on every grafting point. The copolymers contain approximately 61% and 67% by volume PS, respectively. Notice the poor long-range order and the grain structure characteristics.

This is attributed to the presence of the branching points. Each one of them can be considered as a point where interfacial curvature between the two phases changes, and this causes severe frustration during the organization of macromolecules within the microphases. Consequently, microdomain sizes and grain structure are altered. 10.3. Mechanical Properties The viscoelastic and mechanical properties of bulk graft copolymers are directly connected to their

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Page 27 of 38 morphology, which in turn depends on the molecular characteristics of the copolymer. Chemical nature of the backbone and grafts and their mutual solubility as well as the molecular weight and the degree of branching are the primary parameters affecting the mechanical properties of such systems (190-192). For example, Young's modulus, stress at break, toughness, and yielding stress for poly (butyl acrylate-g-styrene) graft copolymers were found to increase, whereas yield elongation decreased as the number of branches increased (at the same branch molecular weight) (190) (Fig. 32).

Figure 32. Variation of the Young's modulus (E), tensile strength at break ( B), and toughness as a function of the number of PS grafts on the backbone for a poly(n-butyl methacrylate-g-styrene) graft copolymer prepared by the macromonomer technique. To convert MPa to psi, multiply by 145, to convert kJ/m2 to ft·lbf/in.2, divide by 2.10.

This behavior was identified as a result of the increase in the hard phase (PS) content in the graft copolymer. Grafting of styrene and vinylacetate onto EPDM (ethylene-propylene-diene terpolymer) improved the tensile strength of the resulting material in comparison to ungrafted EPDM, whereas grafting of N-substituted acrylamides on EPDM gave copolymers with decreased tensile strength (191). Poly(isoprene-g-styrene) copolymers with tetrafunctional branch points were found to have increased strain at break (192) than the commercial thermoplastic elastomers (Fig. 33).

Figure 33. Comparison of the mechanical properties of tetrafunctional multigraft poly(isoprene-gstyrene) copolymers with commercial Styroflex and Kraton copolymers.

Detailed studies on the poly(isoprene-g-styrene) multigraft copolymers (193-197) have shown that both the functionality of the graft copolymers (tri-, tetra-, or hexafunctional) and the number of junction points per molecule greatly influence their mechanical properties. An increase in the functionality causes a change in morphology, leading to a high tensile strength for tetrafunctional (cylindrical) and hexafunctional (lamellae) multigraft copolymers, resulting in about twice the strength of the spherical trifunctional multigrafts of the similar overall composition. Strain at break and tensile strength increase linearly with the number of junction points per molecule. Furthermore, the multigraft copolymers have higher elasticity compared to commercial thermoplastic elastomers. The mechanical properties of polymeric blends compatibilized with graft copolymers have received a relatively larger attention (198-201). 10.4. Compatibilization of Polymer Blends Polymer blending is one of the most general, flexible, and efficient methods for generating new polymeric high performance materials (202). The resulting polymer mixture combines the properties of its components. These properties may in principle be varied by changing the blend composition according to the desired final performance of the material. However, because of entropic reasons, the majority of polymer pairs are incompatible. Graft copolymers are widely used as compatibilizing agents of immiscible blends (202-212). The different parts of a graft copolymer can be chosen and synthesized in such a way that interactions with the constituents of the blends are maximized. The constituents of the graft copolymer may have the same chemical structure with either the components of the blend or a chemical constitution (ie, complementary functional groups) that favors the interactions with the functional groups of the components. In this way, graft copolymers are aiding in the adhesion of the different phases by forming a bicompatible interface. The interface reduces the interfacial energy between the phases in the blend and permits a more stable and finer dispersion of the components. The resulting mechanical properties of the formulation are improved

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Page 28 of 38 since strong adhesion of phases helps in assimilating stresses and stains without disruption of the blend's morphology. The possibility of morphology changes during processing conditions is also minimized. A more recent compatibilization methodology, namely reactive blending, involves the formation of block or graft copolymers in situ (213-223). The method relies on the use of reactive functionalities previously incorporated in one or more polymers of the blend for the production of block or graft copolymers after mixing the individual components and during melt-processing. This concept has been applied to a number of blend systems such as polyamide/polyolefins, polyamide/ABS, PS/olefins, polyester/polyamide using different types of reactions that can occur rapidly at elevated processing temperatures. 10.5. Lithography Graft copolymers have gained the attention of researchers for lithographic applications since the late 1980s because of their ability to combine the properties of the blocks involved in the copolymer. Some work has been done on the use of graft copolymers as lithographic materials (224-226), but in recent years there is an increased interest in the surface modification of patterned substrates via grafting copolymerization. Surface modification finds many applications in device fabrication and is very popular for bioapplications and microfluidic applications. The majority of these studies have been focused on the organization of cells and proteins in surfaces with patterned bioadhesive and nonadhesive areas. Two popular approaches are used to produce such patterns. The first approach involves the selective attachment of the graft copolymer, thus inhibiting protein adsorption, in one of the two areas of the patterned surface, whereas the other approach involves the in situ grafting copolymerization of one area of the patterned surface with antifouling polymers (eg, oligo or poly (ethylene glycol)). The patterned surfaces are created by various lithographic techniques such as photolithography (227, 228), soft lithography (229), and EUV lithography (230). Poly(styrene-g-methyl methacrylate) (PS-g-PMMA) and poly(styrene-g-t-butyl methacrylate) (PS-gtBMA) were investigated as e-beam resist in 1988 (224, 225). These graft copolymers were chosen because of the high dry etch resistance of PS and the high resolution of PMMA. The copolymers were synthesized via radical copolymerization of methacrylate-terminated polystyrene macromonomers. Choi and co-workers investigated the use of poly(dimethyl siloxane)-g-PMMA (PDMS-g-PMMA), PDMS-g-(PMMA-co-PIA) as resist materials for nanoimprint lithography. They found that these siloxane graft copolymers provide superior mold release performance without delamination; they have excellent etch resistance and are capable of producing 50-nm features (226) (see Fig. 34).

Figure 34. (a) 60-nm feature mold used to imprint into (b–d) PDMS-g-PMA-co-PIA, PDMS-gPMMA, and PS-b-PDMS copolymers, respectively. (e) PDMS-g-PMA-co-PIA imprinted at a lower pressure and shorter time.

Falconet and co-workers used conventional photolithography using a commercial photoresist to pattern a surface and then dipped the sample into a solution of functionalized poly(L-lysine)-g-poly (ethylene glycol), PLL-g-PEG/PEG-X (X = biotin or cell adhesive peptide) to cover the photoresist unprotected areas of the wafer with the copolymer. The photoresist was then lifted-off, and the background (the areas that were previous covered with the resist) was filled with nonfunctionalized PLL-g-PEG inhibiting nonspecific interactions outside the interactive patches (227, 228). These authors used the same method with patterned surfaces via nanoimprint lithography (NIL) on PMMA (229) (Fig. 35).

Figure 35. Molecular-assembly patterning by lift-off (MAPL) process. (a) The photoresist-patterned sample (stage I) is dipped into a solution of functionalized PLL-g-PEG/PEG-X (stage II, X = biotin or cell adhesive peptide). The photoresist is removed with an organic solvent in a sonicated bath (stage III) and then, in a last dip and rinse step the sample is incubated in a solution of nonfunctionalized PLL-g-PEG to render the background, between the adhesive regions, and resistant to the adsorption of proteins/cells (stage IV). (b) AFM image of 2-μm wide lines consist of assembled PLL-g-PEG/PEG-biotin with the uncoated substrate surface in the background (stage III). The topographical line scan perpendicular to the patterned lines displays a layer thickness of

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1.5 nm, typical of the copolymer monolayer thickness in air. (c) Confocal laser scanning microscope (CLSM) image of 20- and 50-μm stripes of biotinylated-PLL-g-PEG in a PLL-g-PEG background. The surface was patterned via nano-imprint lithography instead of optical lithography (step I is made with a different process). The fluorescent-labeled streptavidin binds specifically to the biotin areas.

Recently, researchers are investigating patterned polymer brushes from the polymerization of patterned initiator substrates, mostly SAMs initiators. The patterned substrates are prepared by conventional photolithography (231–233), UV or e-beam exposure of the SAMs (234-236), photoinduced graft polymerization (237-239) or soft lithography (microcontact and nanocontact printing (238, 240) and commonly amplified with ATRP. The method generally used involves the creation of binary patterns (that is, patterns of SAMs terminating in one chemical functionality in a background of a polymer or a SAM with different functionality). Dong and co-workers deposited PEG SAMs on a silicon wafer and patterned the surface using photolithography followed by oxygen plasma and lift-off of the resist. The etched regions were then back filled with an initiator for ATRP of sodium acrylate. ATRP was achieved at room temperature in an aqueous medium and resulted in patterned poly(acrylic acid) (PAA) brushes. Avidin was covalently attached only to PAA brushes (231). Schmelmer and co-workers converted patterns created by irradiation of 4′-nitro-1,1′-bi[henyl-4-thiol (NBT) SAMs through e-beam into patterned polymer brush systems. The NBT after e-beam exposure was converted into crosslinked 4′-amino-1,1′-biphenyl-4-thiol (cABT) and the amino groups were diazotized and treated with methylmalonodinitrile giving second-generation initiators for radical polymerization of vinyl compounds to prepare graft copolymers in solution. Patterned PS brushes were produced (234). Sugiura and co-workers used photoinduced polymerization to create functional micropatterned surfaces. Poly(dimethylosiloxane) (PDMS) flat substrates were covered with a reaction mixture solution of polyethylene glycol diacrylate (PEGDA), NaIO4, and benzyl alcohol in water and were exposed through a photomask with a deep UV lamp at 365 nm. In the exposed areas, PEGDA chains were grafted, leaving those areas hydrophilic with low protein adsorption and low cell attachment, whereas the nonexposed PDMS areas showed good protein adsorption and cell attachment properties (237). Hu and co-workers modified the PDMS surface by grafting polymer chains via ultraviolet radiation. These authors grafted acrylic acid (AA), acrylamide (AM), dimethylacrylamide (DMA), 2-hydroxyethyl acrylate (HEA), and PEG onto PDMS to turn the surface hydrophilic and repel peptides (238, 239). 10.6. Other Present and Potential Applications Graft copolymers are steadily assuming an increased importance because of their tremendous industrial application potential (241-252). Graft copolymers of commercial utility include ABS, obtained by grafting acrylonitrile and styrene monomers onto polybutadiene, high impact polystyrene, a family of poly(butadiene-g-styrene) materials, alkali-treated cellulose-g-acrylonitrile, and starch-gacrylonitrile, which are used as “superabsorbent” components in diapers, sanitary napkins, and the like. Graft copolymers containing acrylic monomers are used as pressure-sensitive adhesives (244). Other graft copolymers are essential materials in oil recovery operations. A large number of commercial paint, printing, and coating formulations involve graft copolymers as dispersion stabilizers, rheology modifiers, and final coating properties improvers (eg, corrosion resistivity, environmental compatibility, and so on) (245-252). The potential of using graft copolymer micelles and nanoparticles derived from graft copolymers as drug carriers for controlled drug delivery systems and for environmental purification methodologies is high (253-258). These applications rely on the ability of the micelles to solubilize low molecular weight compounds in their cores. By controlling the molecular characteristics of the copolymers and the chemical nature of the corona, drug-containing vehicles of the desired size, loading capacity, release kinetics, targeting capabilities, biocompatibility, and biodegradability can be produced.

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Page 30 of 38 Formulations in the gel and bulk state are also candidates for this kind of application. Size and loading capabilities of the micellar microcontainers are also important in water treatment systems. The application of graft copolymer membranes in separation processes, like removal of deleterious organic compounds from contaminated drinking water, is also of interest (259-261). With judicious choice of chemical components in conjunction with graft copolymer morphology, design, perselectivity, and permeability of the resulting membranes can be adjusted.

11. Graft Copolymer Production/Consumption It is difficult to accurately measure the amount of graft copolymers produced commercially. Here, we refer to those polymers made either by the reaction of two polymers with each other or by the growth of a chain of one monomer from a polymer, made of a different one. In each case, one has a nonlinear structure where the various segments of the resultant polymer (ie, backbone, branches, and so on) are chemically different. The difficulty in determining how much of this kind of polymer is produced is that it is done in many ways, and it is not clear whether even the producers are always cognizant of what they have made. The grafting often occurs during a mixing process, and the graft polymer itself is not separated from the other components of the blend. However, it is clear that graft copolymer technology is crucial to the modern plastics industry and that graft copolymers are present in a large fraction of the plastic materials made today. The largest portion of this can be found in the styrenic polymers. Impact polystyrene [often called high impact polystyrene (HIPS)] is produced by polymerizing styrene monomer in the presence of polybutadiene. This results in grafting of some of the growing PS chains to the PB ones, giving good impact strength to the final product. Out of the 12.7 million tons (106 t) of PS made worldwide each year (262), perhaps a quarter to a third is HIPS (263-265). ABS is made in a similar fashion as HIPS, except that acrylonitrile is also added to the styrene as a polymerizing monomer (266-269). About 4.9 × 106 t of this polymer is made each year. The next most common use of grafting is probably in the “super-tough” polyamides, made by a grafting reaction between the polyamide and a functionalized ethylene-propylene rubber (270-274). Perhaps 10–20% of the 1.8 × 106 t of polyamide is made this way. There are many other uses of graft copolymers, mostly for compatibilizing various polymer blends (275). As stated above, it is difficult to capture the amount of these made. However, it is safe to say that between 5 and 10% of the 141 × 106 t of plastics made each year contain a significant fraction of graft copolymer.

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