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Highly Stereoselective Coordination Polymerization of b-Myrcene from a Lanthanide-Based Catalyst: Access to Bio-Sourced Elastomers Saliha Loughmari,1,2,3,4,5 Abderrafia Hafid,6 Aicha Bouazza,5 Abdelaziz El Bouadili,5 Philippe Zinck,1,2,3,4 Marc Visseaux1,2,3,4 Lille Nord de France, F-5900 Lille, France Universite ENSCL, UCCS, CCM, F-59652 Villeneuve d’Ascq, France 3 USTL, UCCS, CCM, F-59655 Villeneuve d’Ascq, France 4 CNRS, UMR8181, F-59652 Villeneuve d’Ascq, France 5 e et Science de l’Environnement, USMS, FST-BM, B.P 523, 23 000 Be ni-Mellal, Maroc Laboratoire de Chimie Applique 6 ni-Mellal, Maroc Laboratoire de Chimie Organique et Analytique—Equipe COOA, USMS, FST-BM, B.P 523, 23 000 Be Correspondence to: M. Visseaux (E-mail:
[email protected]) 1 2
Received 26 January 2012; accepted 21 March 2012; published online 16 April 2012 DOI: 10.1002/pola.26069
ABSTRACT: Polymerization of b-myrcene with neodymium borohydride-based coordination catalysts is very efficient, affording polyb-myrcene (polymyrcene, PMy) with high selectivity. With stoichiometric amounts of n-butylethyl magnesium (BEM) as co-catalyst, good control over macromolecular data along with cis-stereoselectivity up to 98.5%, are obtained. In the presence of excess BEM, high level of transfer reactions efficiency between neodymium and magnesium is clearly evidenced whereas the selectivity switches to 3,4-rich. Combining the neodymium pre-catalyst with
triisobutyl aluminum in the presence of a boron activator affords PMy in good yield, but the polymer material displays low solubilC 2012 Wiley Periity, likely due to the occurrence of crosslinking. V odicals, Inc. J Polym Sci Part A: Polym Chem 50: 2898–2905, 2012
INTRODUCTION There is nowadays a growing interest to find alternates to petro-sourced monomers to produce polymers and elastomers. Beyond sugar derivatives and ester bio-monomers, isoprene, such as sugar cane-derived ethylene, is now available as a green resource from bio-fermentation,1 and it has just been used by the GoodYear Company in the elaboration of a tire made of poly(bio-isoprene).2 Alternately, immediately available natural conjugated dienes such as myrcene (7-methyl-3-methylene-octa-1,6-diene, C10H16), also called b-myrcene, can be advantageously used for such purpose, as monomer for the preparation of 100% biosourced elastomeric materials with applications usually devoted to polybutadiene and related petro-sourced polymers. Surprisingly, this terpene found in many essential oils has been until now quite ignored by the polymer scientists’ community. The first—and until now unique—complete study reporting the polymerization of myrcene by ZieglerNatta-type catalysts was published in 1960, but unfortunately, the 1,4-stereoregularity was not determined.3 Since then, the preparation of said cis-4 or trans-5 poly-b-myrcene (polymyrcene, PMy) were briefly mentioned in patents, with full experimental details lacking. On the other hand, PMy
was prepared by anionic polymerization with sec-BuLi, affording a material containing around 85% of 1,4-cis enchainments, along with about 15% of 3,4-defects.6 Myrcene was also used as a component of anionic triblock copolymers,7 and of statistical copolymers prepared by free radical polymerization.8 More recently, it was described that myrcene can be fairly efficiently homopolymerized by free radical process in aqueous media in the presence of cyclodextrin, yielding mixtures of cis- and trans-PMy.9 Cis-1,4-poly-b-myrcene (75% regular) has also been found as a natural component in mastic resin from Pistacia Lentiscus L.10
KEYWORDS: biomass; coordination polymerization; elastomers;
metal-organic catalysts/organometallic catalysts; b-myrcene; neodymium; polymyrcene; rare earths; stereoregular; stereospecific polymers; Ziegler-Natta polymerization
Although the bibliography reporting lanthanide-based catalysts for the stereoselective polymerization of conjugated dienes is well documented,11 yet such process has never been applied to myrcene to our knowledge. In the course of our studies aiming at exploring the potential of lanthanide borohydrides in polymerization catalysis,12 we report herein our results with Nd/Mg and Nd/borate/Al catalytic combinations, including also examination in coordinative chain transfer polymerization (CCTP) conditions. A tentative explanation is given to account for the high cis-selectivity observed.
Additional Supporting Information may be found in the online version of this article. C 2012 Wiley Periodicals, Inc. V
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SCHEME 1 Polymerization of myrcene with Nd(BH4)3(THF)3/BEM dual catalyst.
RESULTS AND DISCUSSION
obtained for isoprene with the same catalyst. Little enhancement is observed rising the temperature (Runs 3 and 7), with up to 87% yield at 80 C in 2 h. A kinetic study showed that after 20 min of reaction at 70 C, the polymer yield is already 66% (Fig. 1), which corresponds to an average activity for the first 20 min of 80 kg pol molNd1 h1. The slow conversion progresses after this initial period of 20 min is likely due to the high viscosity of the reaction mixture.
Polymerization With Nd(BH4)3(THF)3/n-Butylethyl Magnesium Catalyst We have shown that when associated to stoichiometric amounts of n-butylethyl magnesium (BEM) (n-BuEtMg), Nd(BH4)3(THF)3 gives rise to efficient catalysts toward the polymerization of isoprene.13 We observed that such combinations are also highly capable to polymerize myrcene (Scheme 1). Selected results are gathered in Table 1.
In general, we found that molecular weights are quite well in accordance with theoretical values expected for two polymer chains growing per magnesium metal. One can thus consider that initiation efficiency of the catalyst is optimal although size exclusion chromatography (SEC) data must be viewed with caution since they are given with reference to polystyrene standards. From Runs 9 to 14, it can be seen
Although the catalyst is poorly efficient at room temperature (Run 1), significant improvement is observed at increased temperature with more than 80% polymer yield after 2 h at 50 C (Run 2). The average activity over 2 h period reaches 20 kg pol molNd1 h1 under these conditions, which can be compared with the value of 50 kg pol molNd1 h1 TABLE 1 Polymerization of Myrcene with Nd(BH4)3(THF)3/BEM
Runa
BEM
T ( C)
Time (h)
Yield (%)
Mn (SEC)d
PDIe
Mn (th)f (g mol1)
Mn (RMN)g (g mol1)
Selectivity (%)h cis-/trans-/3,4-
1b
1
22
120
80
32,300
2.02
16,300
–
90.9/7.8/1.3
2
1
50
2
82
27,600
1.51
16,700
–
91.0/7.2/1.8
3
1
70
2
84
21,000
1.40
17,200
–
90.8/7.5/1.7
4
1
70
0.5
69
20,900
1.29
14,100
–
90.0/8.4/1.6
5
1
70
24
90
18,800
1.33
18,300
–
86.7/11.6/1.7
6
1
70
2
92
70,900
1.70
63,200
–
88.7/9.5/1.8
7
1
80
2
87
19,800
1.43
17,700
–
88.6/10.3/1.1
8i
1
70
4
90
66,800
1.79
36,700
–
98.5/0/1.5
9
2
70
24
85
7,800
1.75
8,600
8,100
91.8/0k/8.2
10
3
70
24
90
5,800
1.64
6,100
5,500
ndl/17.1
c
11
5
70
24
80
3,200
1.78
3,300
3,200
nd/54.7
12
10
70
26
56
3,200
1.27
1,200
2,500
nd/59.9
13
20
70
26
50
3,100
1.30
500
2,300
nd/64.7
14
j
70
240
56
3,200
1.43
800
2,700
nd/86.2
20 5
NNd ¼ 2.10 mol; [My]/[Nd] ¼ 300; VMy ¼ Vtol ¼ 1 mL; [Mg]/[Nd] ¼ 1. VMy ¼ Vtol ¼ 0.5 mL. [My]/[Nd] ¼ 1,000. d Determined by SEC versus PS standards (see ‘‘Experimental’’ Section). e PDI ¼ M w /M n . f For two chains per Mg: M n (th) ¼ 136 yield % [My]/2[Nd]. g Determined by integration of the methyl terminal signal relative to the lateral CMe2 fragment (considered for low M n samples only). a
h
3,4-Percentage determined by H NMR and cis-/trans- determined by C NMR. In situ prepared Cp*Nd(BH4)2(THF)x. j In the absence of Nd pre-catalyst. k
