Methylaluminoxane

528

Organometallics 2011, 30, 528–533 DOI: 10.1021/om100900f

Ethylenebis(indenyl)zirconium Dichloride/Methylaluminoxane-Catalyzed Copolymerization of Ethylene and 1-Alkene-n-trimethylsilanes Sami Heikki Lipponen and Jukka Veli Sepp€al€a* Faculty of Chemistry and Materials Sciences, Polymer Technology, Aalto University, P.O. Box 16100, FI-00076 Aalto, Finland Received September 17, 2010

In this study a series of new copolymers, namely, poly(ethylene-co-3-butenyltrimethylsilane), poly(ethylene-co-4-pentenyltrimethylsilane), and poly(ethylene-co-5-hexenyltrimethylsilane), were synthesized via ethylenebisindenylzirconium dichloride/methylaluminoxane (Et(Ind)2ZrCl2/MAO)catalyzed polymerizations. In parallel with these polymerizations, also poly(ethylene-coallyltrimethylsilane), poly(ethylene-co-vinyltrimethylsilane), poly(ethylene-co-1-hexene), and poly(ethylene-co-1-decene) were synthesized in similar conditions. All of the synthesized polymers were characterized thoroughly (molar masses with GPC and the microstructures with 1H NMR and 13C NMR), and the influence of the Lewis acidic trimethylsilane (-Si(CH3)3) comonomer in the polymerization performance is discussed. With the shortest silane monomers (vinyl-Si(CH3)3 and allylSi(CH3)3) the electronic influence of the silicon was seen in regiospesific insertions and increased rate of chain termination via β-hydrogen abstraction. In parallel with that, the influence of the -Si(CH3)3 group in the polymerization performance of 3-butenyl-Si(CH3)3 and 4-pentenylSi(CH3)3 was clearly minor, and finally, the 5-hexenyl-Si(CH3)3 was found to copolymerize with ethylene-like 1-alkenes. In addition to these results, a new, clear indication of the presence of an “allylic activation” mechanism was found in ethylene/allyl-Si(CH3)3 and ethylene/4-pentenylSi(CH3)3 copolymerizations. Introduction Polyolefins are relatively inert materials interacting weakly with other materials. They suffer from poor adhesion, printability, paintability, and compatibility, and improving these properties has long been an important research subject. To adjust the properties of the polyolefins, the pendant side chains have been added in the polymer chain by copolymerization. The heteroatom-containing comonomers are usually used to adjust the hydrophobic nature of the polyolefin to more hydrophilic. The most convenient way of introducing functional groups in a polyolefin chain is peroxide grafting during melt processing. Polyethylene and polypropylene grafted with acrylic acid, maleic acid, or maleic acid anhydride are widely used as surface modifiers and compatibilizers.1,2 One of the major challenges with peroxide grafting is that extensive chain scission and loss in molar mass can occur with some polymers, notably with PP. Also significant homopolymerization of acrylic acid monomer can take place. In parallel with grafted polyolefins, ethylene can directly be copolymerized with some vinyl monomers (acetates *To whom correspondence should be addressed. E-mail: jukka. [email protected]. (1) Datta, S.; Lohse, D. J. In Polymeric Compatibilizer: Uses and Benefits in Polymer Blends; Datta, S., Lohse, D. J., Eds.; Carl Hanser Verlag: Germany, 1996. (2) Rothon, R. N.; Hancock, M. In Particulate-Filled Polymer Composites; Rothon, R. N., Ed.; Rapra Technology Limited: UK, 2003; pp 5-52. (3) Knuuttila, H.; Lehtinen, A.; Nummila-Pakarinen, A. In Advances in Polymer Science, Long Term Properties of Polyolefins; Albertson, A. C., Ed.; Springer-Verlag: Germany, 2004; Vol. 169, pp 13-28. pubs.acs.org/Organometallics

Published on Web 01/14/2011

and acrylates) via radical polymerization in pipe line.3 However, the reaction conditions are rather drastic (>2500 bar, >300 °C), and milder synthesis methods, e.g., coordination copolymerizations, are favored. In these cationic group IV metal (Ziegler-Natta or metallocene)-catalyzed polymerizations the reaction conditions would be tolerable and the synthesized polymers more homogeneous. As a drawback, most of the polar groups are strong inhibitors of the ZieglerNatta and metallocene catalysts.4-7 Still, these catalysts are less sensitive toward weakly interacting monomers (Lewis acids) and also some direct applications for the Lewis acidfunctionalized olefins have recently been found.8 The metallocene-catalyzed polymerizations of Lewis acidic monomers have normally proceeded well if the electronic influence of the heteroatom (B, Si, Al) is diminished by a long spacer. In the reported studies, 5-hexenyl-9-BBN,9 7-octenyldiisobutylaluminum,10 and 7-octenyldimethylphenylsilane8,11,12 have been found to copolymerize with olefin (4) B€ ohm, L. L.; Berthold, J.; Enderle, H.-F.; Fleissner, M. In Metalorganic Catalysts for Synthesis and Polymerization: Recent Results by Ziegler-Natta and Metallocene Investigations; Kaminsky, W., Ed.; Springer-Verlag: Germany, 1999; pp 3-13. (5) Brintzinger, H. H.; Fischer, D.; M€ ulhaupt, R.; Rieger, B.; Waymouth, R. M. Angew. Chem. 1995, 34, 1143–1170. (6) Boffa, L. S.; Novak, B. M. Chem. Rev. 2000, 100, 1479–1493. (7) Yanjarappa, M. J.; Sivaram, S. Prog. Polym. Sci. 2002, 27, 1347– 1398. (8) Lipponen, S.; Lahelin, M.; Sepp€al€a, J. Eur. Polym. J. 2009, 45, 1179–1189. (9) Chung, T. C.; Lu, H. L.; Li, C. L. Polym. Int. 1995, 37, 197–205. (10) Nam, Y.; Shiono, T.; Ikeda, T. Macromolecules 2002, 35, 6760–6762. r 2011 American Chemical Society

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Table 1. Metallocene/MAO-catalyzed Copolymerization Resultsa run

comonomer

in feed mol/mol

in polym mol %

activity kg/mol 3 P 3 h

Mw kg/mol

PD

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

1-hexene 1-decene 5-hexenyl-Si(CH3)3 5-hexenyl-Si(CH3)3 4-pentenyl-Si(CH3)3 4-pentenyl-Si(CH3)3 3-butenyl-Si(CH3)3 allyl-Si(CH3)3 allyl-Si(CH3)3 allyl-Si(CH3)3 allyl-Si(CH3)3 vinyl-Si(CH3)3 vinyl-Si(CH3)3 vinyl-Si(CH3)3

0.35 0.25 0.25 1.0 0.25 0.8 0.35 0.25 0.35 0.50 1.0 0.25 0.50 1.0

2.0 1.4 1.3 4.7 0.5 1.8 1.9 1.4 1.8 2.3 3.7 0.3 0.5 0.7

10 000 13 000 12 000 11 000 17 000 5500 7300 5200 6100 5800 5300 1900 4400 2600 1300

450 240 260 260 140 380 180 240 48 45 38 25 92 63 39

3.7 2.9 3.1 3.2 2.9 3.9 3.1 3.4 2.0 2.0 2.0 2.0 2.1 2.1 2.0

a

Conditions: Et(Ind)2ZrCl2 = 1 μmol; Al/Zr = 2000; Pethylene = 0.7 bar; T =40 °C; time = 20 min.; toluene 300 mL, stirrer speed 600 rpm.

(ethylene or propylene) smoothly, and their polymerization performance was similar compared to long 1-alkenes (e.g., 1-decene, 1-dodecene). However, when the Lewis acidic monomer contains a heteroatom in the vicinity of the double bond, its behavior at the metallocene cation may be altered largely. Several authors have homopolymerized13,14 and copolymerized7,15,16 allylsilanes with ethylene by metallocene catalysts. Even though the propagation of allylsilane monomers was rather good, they were also sensitive to chain termination via β-hydrogen abstraction, resulting in low molar mass polymers.15 In the present study, we have copolymerized different trimethylsilane monomers (CH2dCH-(CH 2)n-Si(CH3)3, n = 0, 1, 2, 3) and ethylene by using a metallocene/MAO catalyst. The copolymerization performances of silane monomers were determined by observing the formed microstructure of the copolymer, which was then compared with the microstructure of polyethylene-co-1-hexene and polyethylene-co-1-decene synthesized in similar conditions.

Experimental Section Materials and Methods. The commercial comonomers (vinyltrimethylsilane >99.5%, allyltrimethylsilane >99%, 1-hexene >97%, and 1-decene >97%) were from Aldrich. The reagents for comonomer synthesis, 1,4-pentadiene (>99% Fluka), 1,5heksadiene (>97% Aldrich), dimethylchlorosilane (>98% Aldrich), methylmagnesium bromide (Aldrich, 3.0 M solution in diethyl ether), allyl bromide (>97% Aldrich), and (trimethylsilylmethyl)magnesium bromide (Aldrich, 1.0 M solution in diethyl ether) were used without further purification. The synthesized comonomers were purified by distilling under argon. Prior to usage, all the comonomers were dried with molecular sieves. The polymerization catalysts Et(Ind)2ZrCl2 and MAO (4.82 wt % in toluene) were purchased from Witco GmbH and used as delivered. Toluene (Merck, >99.5%) was dried by refluxing it over sodium/benzophenone, and it was (11) Lipponen, S.; Sepp€al€a, J. J. Polym. Sci., Polym. Chem. 2002, 40, 1303–1308. (12) Lipponen, S.; Sepp€al€a, J. J. Polym. Sci., Polym. Chem. 2004, 42, 1461–1467. (13) Zeigler, R.; Resconi, L.; Balbontin, G.; Guerra, G.; Venditto, V.; De Rosa, C. Polymer 1994, 35, 4648–4655. (14) Habaue, S.; Baraki, H.; Okamoto, Y. Macromol. Chem. Phys. 1998, 199, 2211–2215. (15) Byun, D.-J.; Shin, S.-M.; Han, C. J.; Kim, S. Y. Polym. Bull. 1999, 43, 333–340. (16) Liu, J.; Nomura, K. Macromolecules 2008, 41, 1070–1072.

subsequently distilled under nitrogen. Ethylene (grade 3.5 from Air Liquide, Liege, Belgium) was further purified by passing it through columns with 3 A˚ molecular sieves, a copper catalyst (BASF R3-11), and activated Al2O3. All the monomer syntheses were performed in a two-necked round-bottom flask (50-100 mL) with a magnetic stirrer and a condenser. The reaction vessel was filled with an inert gas (argon), and the reagents were injected with a syringe through a silicone rubber septum. Polymerizations were carried out at 40 °C in a 0.5 dm3 B€ uchi reactor equipped with a mechanical stirrer. The reactor was vacuumed (vacuum 5 min and purged with nitrogen) three times before adding MAO, toluene, and the comonomer. The ethylene was fed into the reactor, and the pressure was kept constant at 0.7 bar (84 mmol/L). The catalyst (Et(Ind)2ZrCl2, Al/Zr = 2000) was added, and the reaction was stopped after 20 min by pouring the reaction mixture into 300 mL of acidic ethanol. After filtration, the product was washed once with ethanol and dried under vacuum. Analysis. The 1H NMR, 13C NMR, and DEPT studies were performed on a Varian Gemini 2000 300 MHz NMR spectrometer. Two additional 1H NMR analyses were carried out also with a Bruker Avance DPX400 spectrometer. The synthesized comonomers were dissolved in deuterated chloroform (CDCl3), and the 1H NMR spectra were recorded at room temperature. The internal reference peak (CHCl3) was placed at δ 7.26 ppm. To run 1H NMR spectra from the copolymers, the samples were dissolved in d8-toluene, and the spectra were recorded at 90100 °C. As an internal standard, the C6D5CD2H residue resonance peak was placed at δ 2.1 ppm. The 13C NMR and DEPT analyses of the copolymers were run in a trichlorobenzene/ d6-benzene (90/10) solution at 100 °C. As internal standard, the resonance of the isolated methylene units in the polyethylene chain was placed at 29.83 ppm. The molar masses and molar mass distributions were analyzed with a Waters Alliance GPCV 2000 gel permeation chromatography apparatus, equipped with four Waters Styragel columns (HMW 7, 2*HMW 6, HMW 2), a refractive index detector, and a viscometer. 1,2,4-Trichlorobenzene was used as a solvent at a temperature of 140 °C and was applied at a flow rate of 0.9 mL/min. Narrow molar mass distribution poly styrene standards were used to calibrate the columns. The Mark-Houwink K and R parameters for the samples were determined with a reference sample of linear low-density polyethylene with a known molar mass and molar mass distribution.

Results Monomer Synthesis. 4-Pentenyltrimethylsilane (4-pentenylSi(CH3)3) and 5-hexenyltrimethylsilane (5-hexenyl-Si(CH3)3)

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Scheme 1. Identified 13C NMR Shifts of the Synthesized Copolymers

were synthesized via hydrosilylation and Grignard reactions following similar procedures used in our previous study.11 3-Butenyltrimethylsilane (3-butenyl-Si(CH3)3) was synthesized by the coupling reaction of allyl bromide and (trimethylsilylmethyl)magnesium bromide.17 The raw products were purified by distillation, and their 1H NMR spectra were recorded at ambient temperature. 3-Butenyltrimethylsilane. Bp: 116-118 °C. 1H NMR (CDCl3, δ): (CH3)3Si- (s, -0.01); Si-CH2- (t, 0.60, J = 8.5 Hz); CH2dCH-CH2- (m, 2.06); CH2dCH-CH2(dd, 4.89, J=10.5 Hz; 4.99, J=17 Hz); CH2dCH-CH2(m, 5.82-5.95). 4-Pentenyltrimethylsilane. Bp: (200 mbar) 70-71 °C. 1H NMR (CDCl3, δ): (CH3)3Si- (s, -0.02); Si-CH2- (t, 0.50, J=8.4 Hz); -CH2-CH2-CH2- (m, 1.39); CH2dCH-CH2(q, 2.07, J = 6.6 Hz); CH2dCH-CH2- (m, 4.945.04); CH2dCH-CH2- (m, 5.74-5.89). 5-Hexenyltrimethylsilane. Bp: (15 mbar) 44-45 °C. 1H NMR (CDCl3, δ): (CH3)2Si- (s, -0.03); Si-CH2- (t, 0.48, J = 9.5 Hz); -CH2-CH2-CH2-CH2- (m, 1.25-1.45); CH2dCH-CH2- (q, 2.05, J = 6.9 Hz); CH2dCH-CH2(dd, 4.93, J = 7.2 Hz; 4.99, J = 17 Hz); CH2dCH-CH2(m, 5.74-5.88). Polymerizations. The polymerizations were performed at low temperature and pressure to ensure that the catalyst activities remained constant during the polymerizations. Therefore, the values for the activities as well as the formed microstructures were expected to well represent the true nature of the active metal center. The polymerization results are summarized in Table 1. The structures of the synthesized copolymers were determined on the basis of 13C NMR analysis. The observed shifts in the 13C NMR spectrum were related to the spesific carbons in the polymer main and pendant chains (Scheme 1). The determination was partly done by comparing with the previous data.11,12,18 Still, to identify some of the resonances (e.g., at 37.70 and 38.24 ppm in poly(ethylene-co-4-pentenylSi(CH3)3), Scheme 1C), also some additional DEPT analysis was performed. As a result, most of the resonances were related (17) Sakurai, H.; Hosomi, A.; Kumada, M. J. Org. Chem. 1969, 34, 1764–1768. (18) Seger, M. R.; Maciel, G. E. Anal. Chem. 2004, 79, 5734–5747.

Figure 1. 1H NMR spectra of (A) poly(ethylene-co-1-decene), (B) poly(ethylene-co-5-hexenyl-Si(CH3)3), and (C) poly(ethylene-co-4-pentenyl-Si(CH3)3).

to specific carbons in the copolymers. Only the exact resonances generated around the branching point in the poly(ethylene-co-vinyl-Si(CH3)3) remained unclear (Scheme 1F). Still, these results verified that the comonomers were incorporated into the polymer chain and that the insertion occurred randomly, with no comonomer blocks forming. Polymerization Performance of Larger Silane Comonomers: 5-Hexenyl-Si(CH3)3, 4-Pentenyl-Si(CH3)3. The polymerization performance of 5-hexenyl-Si(CH3)3 resembled well the one obtained for 1-alkenes (Table 1, runs 4 and 5 vs runs 2 and 3). All of the comonomers resulted in similar results regarding the catalyst activity and the comonomer uptake. Also the molar masses were decreased similarly when compared with the molar mass of the homopolyethylene (Table 1, run 1). Contrary to these results, the polymerization performance of one methylene shorter silane monomer, 4-pentenyl-Si(CH3)3, differed largely, as the catalyst activity was reduced by half and the comonomer uptake was clearly poorer (Table 1, runs 6 and 7 vs runs 2-5). These results were slightly unexpected and cannot be explained directly, e.g., by impure monomer or inaccurate 13C NMR analysis (monomer and copolymer NMR spectra in Supporting

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Scheme 2. Mechanisms for Different Unsaturations in the Polyethylene Chaina

a

A-C, 1-hexene as an example comonomer, P = polyethylene chain.

Information). Due to the poor propagation rate of 4-pentenyl-Si(CH3)3, also the reduction in molar mass was not as pronounced. The reduced molar mass of poly(ethylene-co-1-alkene) versus homopolyethylene is usually due to the high rate of chain termination of the last inserted long 1-alkenes. To gain more information on the chain termination mechanisms, the microstructures of these copolymers were studied in more detail by observing their 1H NMR spectra from the unsaturated area (Figure 1). In general, the β-hydrogen chain transfer reaction was found to be the dominating termination mechanism, which resulted in vinyl chain end after ethylene insertion (Scheme 2A and Figure 1, resonances at 4.91-5.02 and 5.70-5.84 ppm) and vinylidene chain end after 1,2 inserted comonomer (Scheme 2B and Figure 1, resonances at ∼4.8 ppm). The multiple/overlapping resonances at 5.4-5.5 ppm in the 1H NMR spectrum have ordinarily been explained by the internal vinylene unsaturation (cis þ trans) formed after chain termination of secondary inserted 1-alkene (Scheme 2C).19 However, in our case the situation was more complicated due to some undefined resonances at 5.25.3 ppm. As a matter of fact, these resonances at 5.2-5.5 ppm appeared more clearly in the spectrum of poly(ethyleneco-4-pentenyl-Si(CH3)3), and actually a coupling pattern could be hypothesized (Figure 1C, two doublets at 5.215.29 ppm, J(d)=8 Hz, J(dd)=15 Hz, and two triplets at 5.41 and 5.46 ppm, J(t) = 6 Hz, J(dt) = 15 Hz). This coupling (19) Camurati, I.; Cavicci, B.; Dall’Occo, T.; Piemontesi, F. Macromol. Chem. Phys. 2001, 202, 701–709.

pattern can be assigned to a microstructure where the internal trans-vinylene unsaturation locates next to the branching point.20 These kinds of internal unsaturations are often explained by the “allylic activation” mechanism,20-25 and this mechanism is in agreement with our observations as well (2,1-insertion, β-hydrogen abstraction, allylic activation, and propagation via ethylene insertion, Scheme 2D). According to this hypothesis, the allylic activation was strongly present alongside the β-hydrogen abstraction, whenever any of the above-mentioned comonomers were inserted via a secondary 2,1-insertion mechanism. In addition, the tendency for allylic activation was higher in copolymerization of 4-pentenyl-Si(CH3)3 and ethylene. This promotes the previous observation that the polymerization performance of 4-pentenyl-Si(CH3)3 differs from that one of 5-hexenylSi(CH3)3 and 1-alkenes. Polymerization Performance of Short Silane Comonomers: Vinyl-Si(CH3)3, Allyl-Si(CH3)3, and 3-Butenyl-Si(CH3)3. When a Lewis acidic monomer contains the heteroatom in the (20) Busico, V.; Cipullo, R.; Friederichs, N.; Linssen, H.; Segre, A.; Castelli, V. V. A.; van der Velden, G. Macromolecules 2005, 38, 6988– 6996. (21) Resconi, L.; Camurati, I.; Sudmeijer, O. Top. Catal. 1999, 7, 145– 163. (22) Busico, V.; Caporaso, L.; Cipullo, R.; Landriani, L.; Angelini, G.; Margonelli, A.; Segre, A. L. J. Am. Chem. Soc. 1996, 118, 2105–2106. (23) Richardson, D. E.; Alameddin, N. G.; Ryan, M. F.; Hayes, T.; Eyler, J. R.; Siedle, A. R. J. Am. Chem. Soc. 1996, 118, 11244–11253. (24) Margl, P. M.; Woo, T. K.; Bl€ ochl, P. E.; Ziegler, T. J. Am. Chem. Soc. 1998, 120, 2174–2175. (25) Lehmus, P.; Kokko, E.; Leino, R.; Luttikhedde, J. G.; Rieger, B.; Sepp€al€a, J. Macromolecules 2000, 33, 8534–8540.

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Scheme 3. Dominating Insertion and Termination Mechanism of (A) Vinyl-Si(CH3)3 and (B) Allyl-Si(CH3)3 on the Metallocene Catalyst

vicinity of the double bond, its behavior at the metallocene cation is altered largely. This was clearly seen in the studies of Guram et al.26,27 where the vinyl-Si(CH3)3 was found to coordinate with the cationic metallocene strongly via a secondary 2,1-insertion mechanism. This was explained to be due to the electropositive nature of silicon, which drives the electron density at the internal olefinic carbon and therefore favors the 2,1-insertion (Scheme 3A).26,27 Contrary to vinylSi(CH3)3, allyl-Si(CH3)3 was found to coordinate with the metallocene catalyst via the primary 1,2-insertion mechanism.26 The behavior of allylsilanes was rationalized on the basis of the formed polar transition state (Scheme 3B), which was stabilized by the β-effect of silicon.28 This was also the reason for the promoted β-hydrogen abstraction after allylsilane insertions in the studies of Guram,27 Byun,15 Brandow,29 and Casey.30 In our results these above-mentioned behaviors of short silane monomers were clearly seen. When ethylene was copolymerized with vinyl-Si(CH3)3 using Et(Ind)2ZrCl2/ MAO catalyst, the influence of the 2,1-insertion in the polymerization performance was seen in weak comonomer uptake, low molar mass, and decreased catalyst activity (Table 1, run 1 vs 13-15). The strong presence of secondary insertion was promoted by 1H NMR analysis, which revealed resonances that can be attributed to internal double bonds formed after β-hydrogen abstraction of secondary inserted vinyl-Si(CH3)3 (Figure 2A, Scheme 3A). The high coupling constant of these double-bond protons indicated that they were in trans position. Contrary to the vinyl-Si(CH3)3, the copolymerization of allyl-Si(CH3)3 and ethylene proceeded slightly better, as the comonomer uptakes were clearly higher (Table 1, runs 9-12). However, these polymerizations also suffered from low molar mass copolymers and reduced catalyst activities. Both these disadvantages were in agreement with the formation (26) Guram, A. S.; Jordan, R. F. Organometallics 1990, 9, 2190–2192. (27) Guram, A. S.; Jordan, R. F. J. Org. Chem. 1993, 58, 5595–5597. (28) Brook, M. A., Ed. Silicon in Organic, Organometallic and Polymer Chemistry; John Wiley & Sons: New York, 2000; pp 552-607. (29) Brandow, G. C.; Mendiratta, A.; Bercaw, J. E. Organometallics 2001, 20, 4253–4261. (30) Casey, C. P.; Carpenetti, D. W.; Sakurai, H. Organometallics 2001, 20, 4262–4265.

Figure 2. Unsaturated area in 1H NMR spectra of (A) poly(ethylene-co-vinyl-Si(CH3)3), (B) poly(ethylene-co-allyl-Si(CH3)3) after normal workup (2 h in acidic ethanol), and (C) poly(ethylene-co-allyl-Si(CH3)3) after extended workup (24 h in acidic ethanol). The values in parentheses are the relative intensities of the resonances versus the intensity of the resonances at 4.9-5.05 ppm.

of a stable polar transition state, which, indeed, reduces the propagation rate and allows the chain termination reaction to take place (Scheme 3B). This hypothesis was further supported by the 1H NMR analysis, which showed an overwhelming concentration of chain end allylic silane groups (Figure 2B), which were obviously formed after primary 1,2-insertion of allyl-Si(CH3)3. Most interestingly, this kind of unsaturation (allylic to silicon) is sensitive to electrophilic substitution,28 and therefore the -Si(CH3)3 groups were cleaved off when the normal acidic workup procedure (after the polymerization step) was extended overnight (Figure 2B vs C). In addition to the allylic silane groups in the chain end, also a clear indication of internal vinylene unsaturation20 was found (Figure 2B, triplet at 5.19 ppm, J=7.5 Hz). The intensity of this resonance was reduced ∼40% during the extended acidic treatment, while a new resonance appeared at 4.83 ppm (Figure 2B vs C). That led us to an assumption that this internal vinylene unsaturation was in an allylic

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Scheme 4. Suggested Reaction Pathway to Internal Allylsilane Group

the double bond is negligible in 3-butenyl-Si(CH3)3, and the secondary insertion was hindered due to the steric effects of the bulky -Si(CH3)3 group.

Conclusions

Figure 3. 1H NMR spectra of (A) polyethylene-co-1-hexene and (B) polyethylene-co-3-butenyl-Si(CH3)3 polymerized with Et(Ind)2ZrCl2/MAO catalyst. The spectra were recorded with a Bruker 400 MHz NMR spectrophotometer.

position to silane. Mechanistically this microstructure can be explained by the allylic activation taking place after the chain termination of 1,2-inserted allyl-Si(CH3)3 (Scheme 4). Finally, the copolymerization performance of 3-butenylSi(CH3)3 was studied and compared with those of the other comonomers. With Et(Ind)2ZrCl2/MAO as catalyst, the 3-butenyl-Si(CH3)3 behaved mainly like 1-hexene (Table 1, run 8 vs run 2). The comonomer uptake was high, and, contrary to allyl-Si(CH3)3, the detrimental influence in molar mass was absent. Still, the catalyst activity was low, and, like allyl-Si(CH3)3, also 3-butenyl-Si(CH3)3 favored the primary 1,2-insertion when copolymerized with ethylene (Figure 3B). Actually, no signs of secondary insertion were observed in either of these cases. These strong tendencies for primary insertion were slightly odd because there were clear signs of secondary insertions when other monomers were copolymerized with ethylene (Figure 1, Figure 3A vs B). The electropositive nature of silicon would explain this behavior for allyl-Si(CH3)3, but not for 3-butenyl-Si(CH3)3, as then it should favor the secondary insertion (like vinylsilanes but not as strongly). Most probably the influence of silicon on

Despite the fact that the metallocenes tolerate Lewis acidic monomers, the electronic influence of the heteroatom can be seen as well. The Et(Ind)2ZrCl2/MAO-catalyzed copolymerization of ethylene and short trialkylsilane monomers (vinyl-Si(CH3)3 and allyl-Si(CH3)3) suffered from electronic influence of silicon, which resulted in poor polymerization performance, especially in the sense of molar mass. In these cases, however, the poly(ethylene-co-allyl-Si(CH3)3) contained reactive allylic silane groups, internally and in the chain end, and therefore the functionality/reactivity of these copolymers was higher than the weakly interacting trimethylsilane moiety can solely provide. This reactivity may open up new possibilities to use these copolymers as functional polyolefins. The polymerization performance of the larger trimethylsilane monomers, 3-butenyl-Si(CH3)3 and 4-pentenyl-Si(CH3)3, resembled the polymerization performance of the 1-alkenes. Still, there were some anomalies, which were most likely due to the steric influence of the bulky -Si(CH3)3 group. These anomalies were absent when 5-hexenyl-Si(CH3)3 was used as a comonomer. It was obvious that the spacer of four methylene groups between the double bond and the bulky -Si(CH3)3 group was required to obtain similar polymerization performance when compared with that of 1-alkenes. Finally, these polymerizations resulted in a few new kinds of microstructures, which were explained by the “allylic activation” mechanism. Allylic activation took place especially after primary 1,2-insertion of allyl-Si(CH3)3, and therefore it will be a usable monomer in the future if this mechanism is studied in more detail. Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs.org.