Silyl Functionalization of Polyolefins

Bhanu P. S. Chauhan* and Bharathi Balagam

Nanomaterials Laboratory of “Center for EngineeredPolymeric Materials” (CePM), Department of Chemistryand Graduate Center, City UniVersity of New York atCollege of Staten Island, 2800 Victory BouleVard,Staten Island, New York 10314

ReceiVed NoVember 23, 2005ReVised Manuscript ReceiVed February 10, 2006

Synthetic modification of polymers by introducing thefunctional groups is an efficient strategy to generate newmaterials with enhanced or specific physical and chemicalproperties.1 In particular, the functionalization of diene-basedpolymers has generated intense interest, due to the availabilityof double bonds for various transformations such as oxidation,2

epoxidation,3 hydroboration,4 and hydroformylation.5 Similarly,the hydrosilylation reaction of unsaturated polymers offers auseful and convenient method for preparing silane-modifiedpolymers which may find potential applications as rubbermaterials, adhesives and drug delivery agents.6,7 Significantadvances in this area have been made by groups of Rempel8

and Cole-Hamilton,9 who have investigated the hydrosilylationreactions of polybutadienes in the presence of RhCl(PPh3)3 andH2PtCl6 catalysts to generate silasubstituted polymers.

Recent disclosures from our laboratory have introducednew strategies for the generation of catalytically active metalnanoclusters and their utility as potent recyclable catalysts forvarious transformations.10-12 To the best of our knowledge,metal nanocluster catalyzed functionalization of carbon-basedpolymers has not been reported in the literature. In this com-munication, we describe a first example of a highly selec-tive, mild, and clean synthetic route to silyl-functionalizedorganic polymers by hydrosilylation of polybutadiene (PBD)using readily synthesized and recyclable Pt nanoclusters ascatalysts.

Initial investigations of Pt nanoclusters catalyzed hydrosily-lation reactions ofPBD-1 were examined in the presence offour model hydrosilanes2a-2d (Scheme 1). In an optimizedprocedure, a Schlenk tube was charged with presynthesized Ptnanoclusters (0.01 g, 0.001 mmol Pt),12 and then filled withnitrogen. ThePBD-1 (0.054 g, 1 mmol) dissolved in drybenzene (2 mL) was added to the Schlenk followed by injectionof 2a (0.13 mL, 1.2 mmol) under a constant flow of nitrogen.After few minutes of stirring, the reaction mixture turned intolight brown homogeneous solution, indicating the formation ofsoluble nanoclusters.12 The progress of the reaction wasmonitored by1H NMR spectroscopy. After the completion ofthe reaction, the solid catalyst was separated by centrifugation.The filtrate was evacuated to obtain the crude product, whichwas purified by passing through the silica gel column (hexane).The pure product thus obtained was analyzed by GPC and1H,13C, DEPT,29Si NMR spectroscopies.

Gel permeation chromatography (GPC) was used to determinethe molecular weight and chain length properties of function-alized polymers with reference to polystyrene standards (Figure1). The GPC chromatograms of the products have clearly beenshifted toward the high molecular weight region, while retaining

a narrow molecular weight distribution (Mw/Mn ∼ 1.4-1.5).This analysis confirmed that no other side reactions such aschain scission, cross-linking, etc. occurred during the course ofthe hydrosilylation reaction, leaving the large-scale molecularstructure intact.

The structure and regioselectivity of functionalized polymers3a-3d were determined by1H, 13C, DEPT and29Si NMRstudies (Figure 1).1H and 13C NMR studies of the productssuggested that the hydrosilylation ofPBD-1 occurs selectivelyvia an anti-Markovnikov addition, i.e., the Si-atom beingattached at the terminal position of the 1,2-PBD unit (â-product).Particularly noteworthy is the hydrosilylation ofPBD-1 withsilane2d, since silane2d is known to be more susceptible toprovide the MarkoVnikoV product as demonstrated in thepresence of RhCl(PPh3)3.8 However, the Pt nanocluster catalyzedreaction of PBD-1 with silane 2d still led to quantitativeformation of the desired hydrosilylated product inanti-Mark-oVnikoV fashion.

The 13C NMR was used to differentiate betweenR and âisomers of the hydrosilylated polymer by using the distortionlessenhancement by polarization transfer (DEPT) technique. The

* Corresponding author. E- mail: Chauhan@mail.csi.cuny.edu. Tele-phone: (718) 982-3902. Fax: (718) 982-3910.

Figure 1. (A) GPC traces of thePBD-1 and Polymers3a, 3b, 3c, and3d and (B)13C (DEPT) NMR spectra of3d andPBD-1.

Scheme 1. Hydrosilylation of 1,2-Polybutadiene

Scheme 2. Hydrosilylation of 1-Heptene Using Pt Nanoclusters

2010 Macromolecules2006,39, 2010-2012

10.1021/ma052508y CCC: $33.50 © 2006 American Chemical SocietyPublished on Web 02/25/2006

selectivity towardâ-product was verified for all the productsby the DEPT technique. For instance, the DEPT spectrum of3d (Figure 1) shows one upward resonance at 33.70, which canbe assigned to the methine carbon (CH), and three downwardsignals at 10.76, 26.72, and 37.85 are attributed to the threemethylene (CH2) carbons of polymer backbone of theâ-product.This experiment clearly indicates the highly regioselectiveformation of theâ-product, since theR-isomer would havegenerated two methine, one methylene carbon and one methylgroup.

The regioselectivity of the products was further establishedby comparing the spectroscopic results with the hydrosilylationreaction of 1-heptene using four model silanes,2a-2d (Scheme2). The 1H, 13C and29Si NMR analysis of hydrosilylation of1-heptene showed formation of correspondingâ-products andwere in good agreement with those of hydrosilylated polymers.

For example, the hydrosilylation ofPBD-1 using silanes2a, 2b, 2c, and2d gave single29Si resonances at 7.06, 18.01,7.02/-20.36, and-1.7 ppm, respectively (Figure 2), which are Figure 2. 29Si NMR spectra of3a, 3b, 3c, and3d.

Table 1. Hydrosilylation of 1,2-Polybutadiene Using Pt Nanoclustersa

a Conditions: [PBD-1]) 1.0 mmol; [silane]) 1.2 mmol; Pt-nanocluster) (0.010 g). Solvent: benzene.b Yields are based on isolated products.

Macromolecules, Vol. 39, No. 6, 2006 Communications to the Editor2011

comparable to the Si shifts observed for theâ-hydrosilylated1-heptene (7.7, 17.46, 6.94/-21.05,-2.66). The29Si NMR, withonly one detectable29Si resonance (Figure 2) in conjunctionwith 13C NMR experiments, confirmed the high regioselectivityof these reactions toward formation of theâ-adduct.

To widen the scope of hydrosilylation of polybutadienes, wehave screened a variety of functional silanes,2e-2l (Table 1).Almost complete functionalization (95-98%) of the 1,2-vinylgroups of the polymer was achieved with variety of chloro- andtrialkylsilanes. Regardless of the nature of the silanes, all thecatalytic reactions led to high yields and selectivity to corre-spondingâ-products as evidenced by1H, 13C, and29Si NMRspectroscopy.

To evaluate the stability and reactivity of Pt nanoclusters,we investigated the recyclability studies for several cycles. Inevery recycle, the catalyst was separated and recovered bycentrifugation, washed thoroughly with fresh benzene, and driedunder vacuum. We found a consistent activity and selectivityup to five recycles. The solvent effect has also been studied forthe hydrosilylation ofPBD-1 using different solvents. Thenonpolar solvents such as benzene, toluene, and hexane andpolar solvents such as dichloromethane, ether, and chloroformwere found to be equally efficient for the hydrosilylation ofPBD-1. Exceptionally, the solvent THF (tetrahydrofuran) hasshown only 50% hydrosilylation even after long reaction periods.

In view of the above-described results, the present methodoffers a highly efficient synthetic route to silyl-functionalizedpolymers in terms of high yields, selectivities, avoidance of sidereactions and ease of product separation and purification.Additional investigations into their morphological studies andsubsequent modifications of these polymers by other transfor-mations are currently underway.

Acknowledgment. B.P.S.C. acknowledges supports fromNYSTAR funding through the CART program, RF collaborative

grants, and NIST research grants. The PI (B.P.S.C.) is alsoaffiliated with the“ Macromolecular Assembly Institute” (MMA)of CUNY.

Supporting Information Available: Text giving the generalexperimental data and data for the preparation of the Pt nanoclustercatalyst, general procedure for the hydrosilation of the polybuta-dienes, and NMR analysis of3a-3k (including structural drawingsof 3a-3k). This material is available free of charge via the Internetat http://pubs.acs.org.

References and Notes

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Hamilton, D. J.J. Chem. Soc., Chem. Commun.1989, 1688.(4) Chung, T. C.; Raate, M.; Berluche, E.; Schulz, D. N.Macromolecules

1988, 21, 1903.(5) Mohammadi, N. A.; Ling, S. S. M.; Rempel, G. L.Polym. Prepr.

1986, 27, 95.(6) Marciniec, B. In ComprehensiVe Handbook on Hydrosilylation;

Pergamon Press: Oxford, U.K., 1992; Chapter 6, p 215.(7) Tremont, S. J.; Collins, P. W.; Perkins, W. E.; Fenton, R. L.; Forster,

D.; McGrath, M. P.; Wagner, G. M.; Gasiecki, A. F.; Bianchi, R.G.; Casler, J. J.; Ponte, C. M.; Stolzenbach, J. C.; Jones, P. H.; Gard,J. K.; Wise, W. B.J. Med. Chem.1993, 36, 3087.

(8) Guo, X.; Farwaha, R.; Rempel, G. L.Macromolecules.1990, 23,5047.

(9) Iraqi, A.; Seth, S.; Vincent, C. A.; Cole-Hamilton, D. J.; Watkinson,M. D.; Graham, I. M.; Jeffrey, D.J. Mater. Chem.1992, 2, 1057.

(10) Chauhan, B. P. S.; Rathore, J. S.; Chauhan, M.; Krawicz, A.J. Am.Chem. Soc.2003, 125, 2876.

(11) Chauhan, B. P. S.; Rathore, J. S.; Chauhan, M.; Tariq, B.J. Am.Chem. Soc.2004, 126, 8493.

(12) Chauhan, B. P. S.; Rathore, J. S.J. Am. Chem. Soc.2005, 127,5790.

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2012 Communications to the Editor Macromolecules, Vol. 39, No. 6, 2006