Macromolecules zyxwvu 1991,24, zyxwvu 2703-2708 2703 zyxwvutsrqp Styrene zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHG f"' zyxwvutsrqponmlkjihgfedcbaZYXWVU Model Studies of End Capping of Mono- and Biended Polystyrene Anions. Stereoisomerism at the Chain Ends 5 Mukesh C. Bheda and Harry W. Gibson' Department of Chemistry, NSF Science and Technology Center: High Performance Polymeric Adhesives and Composites, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061 -0212 Received July 27, 1990; Revised Manuscript Received November 29, 1990 ABSTRACT Model studies of end capping of living polystyryl anions, with and without a-methylstyrene at the chain ends, with chlorotrimethylsilane and chloromethyl methyl ether were done. In 1H NMR spectra multiple peaks for Si(CH& and CHzOCHs group protons were observed. However, Si(CH& peak patterns for polymers with and without a-methylstyrene at the chain ends were quite different. Trimethylsilyl end- group protons and methine or methylene protons of the polystyrene chain were not coupled, as determined by decoupling experiments. Various parameters such as initiator, solvent, temperature, and molecular weights did not change the peak pattern. This study indicates that the multiple peaks are due to stereoisomerism at the chain ends. Due to steric factors, selective stereoisomer formation is indicated when a-methylstyrene is the terminal unit before end capping. Further, the chain-end tacticities correspond to bulk tacticities and statistical analyses of multiple peaks gave P, = 0.55, which is consistent with the reported values for an- ionically prepared polystyrene. Thus, end-group spectroscopy provides a means of determining bulk tac- ticity. Introduction Polyrotaxanes are two component systems in which a linear backbone polymer is threaded through macrocy- cles. This association of linear and cyclic species may result in polymers with improved solution, thermal, and mechanical properties.'+ Our goals were to synthesize cyclic polystyrene (PS) of low molecular weights (ca. 1500- 2500) so as to have 30-50 ring atoms in the macrocycles and to use these macrocycles for making polyrotaxanes. The synthesis and characterization of cyclicpolystyrene has been reported in the literature. A broad range of mo- lecular weights for cyclic polystyrene has been synthe- sized.4*+14 Our early efforts to synthesize cyclic polystyrene using sodium naphthalide as initiator and dichlorodimethylsi- lane (DCDMS) as cyclization agent (difunctional termi- nator) presented us with two problems: (i) multiple peaks for the dimethylsilyl group protons in the lH NMR spectrum and (ii) incorporation of very low amounts of the terminator. The latter problem was corrected by using better experimental and analytical techniques. However, the first problem of multiple peaks persisted. DCDMS has been used for the synthesis of cyclic polymers;10J2 however, there is no report on the charac- terization of the dimethylsilyl group incorporated into cyclic polymers. Further, multifunctional chlorosilanes have been extensively used for the synthesis of star-shaped polymers;16 however, no detailed characterization of incorporated silanes has been done. Thus, the origin of the multiple peaks in our polymer was puzzling. Hence we decided to do model reactions by end capping living polystyryl anions (prepared by using n-butyllithium and sodium naphthalide as initiators) with chlorotrimethyl- silane (CTMS) and chloromethyl methyl ether (CMME). The lower molecular weights of the model polymers enabled us to study the nature of the end groups by spectroscopic methods. Experimental Section THF was distilled once from sodium-benzophenone and then styrene and n-butyllithium were added to it; the solution was freeze-thawed on the vacuum line (ca. IOa Torr) five to six times, distilled on the vacuum line, and stored in the glovebox ([HzO] = 0.6 ppm, [Oz] = 1.0 ppm). 0024-9297 zyxwvutsr f 91 f 2224-2703$02.50 f 0 Poly(styry1)lithium solutions in cyclohexane and benzene were freeze-thawed five to six times and distilled on the vacuum line, and the pure solvents were stored in the glovebox. CMME and CTMS were distilled under nitrogen and the middle fractions were freeze-thawed five to six times, distilled on the vacuum line, and stored in the glovebox. Styrene was passed through alumina and titrated against dibu- tylmagnesium until light yellow and distilled under a vacuum of 38 Torr (Nz) at 57 "C. The middle fraction was freeze-thawed five to six times on the vacuum line and distilled freshly before use. Sodium naphthalide was prepared in 30 mL of T H F by mixing 1.0 g of sodium and 10.0 g of naphthalene. It was allowed to stir for 1 day at 25 "C in the glovebox and titrated against 0.5 N HC1 before use. Styrene was polymerized and then end capped with CTMS or CMME in the glovebox. Styrene (4.0 mL) polymerization in THF-benzene (5545 v/v, 110.0 mL) was initiated by using n-bu- tyllithium or sodium naphthalide. Polymerizations were done at 10-15 "C for 30 min. Living polymers were end capped with2-3 equiv (relative to the initiator) of the end-capping agents. In some cases, 1.1-2.0 equiv of a-methylstyrene were added to the living polystyrene and after 15 min the resulting living polymer 0 1991 American Chemical Society