The Stereocomplex Formation in Poly(methyl methacrylate) and the Stereospecific Polymerization of Its Monomer

Abstract

The stereocomplex formation between isotactic and syndiotactic poly(methyl methacrylate) (PMMA) molecules is studied in detail, using the techniques of thin layer chromatography (TLC), ultracentrifugation, X-ray diffraction, viscometry, and high-resolution NMR. It is shown that the 1 :1 (iso/synd) complex is formed as the primary structure; further associations between the 1 :1 complex and free syndiotactic molecules, when they exist in solution, occur to form the 1 :2 (iso/synd) complex. The so-called "stereoblock" PMMA samples prepared with n-butylmagnesium chloride and diphenylmagnesium are analyzed by using tic technique. Evidence is thus shown for the stereochemical identity between the stereoblock PMMA and the stereocomplex which is formed by isotactic and syndiotactic PMMA in the ratio 1 :2 (iso/synd). On the basis of this result, we conclude that the 1 :2 stereocomplex may be formed during stereospecific polymerization of methyl methacrylate (MMA) with different Grignard's types of catalysts. In relation to the above conclusion stereospecific polymerizations of MMA at-50°C with n-butylmagnesium chloride in toluene were carried out in the presence of either isotactic or syndiotactic PMMA. The results suggest that the stereospecific polymerization of MMA is closely linked to the stereocomplex formation. The presence of the isotactic polymer chains promotes preferentially the polymerization of MMA to the syndiotactic polymer chains, while the presence of the syndiotactic polymer chains appears to be a prerequisite for initiating the polymerization to the isotactic polymer chains so far as this catalytic system is employed at-50°C. KEY WORDS Stereocomplex Formation/ Poly(Methyl Methacrylate) / Thin Layer Chromatography/ Ultracentrifugation / X-ray Diffraction / Stereoblock / Grignard / Stereospecific Polymerization / The first documented study of specific interactions between tactic methyl methacrylate polymer chains is probably that reported by Watanabe, et al. 1 who recorded the gel formation phenomena observed upon admixture of solutions of the isotactic and the syndiotactic polymer. A fairly large number of publications on the phenomena have been presented to date and at present it is recognized that stereocomplexes are formed between tactic chains of poly(methyl methacrylate) (PMMA). 2-6 However, these works have mainly concerned the effects of solvent, polymer concentration and the stereoisomeric composition upon the stereocomplex formation, and the structure and the formation mechanism of stereo-complexes are still open for discussion to some extent. with 9-fluorenyllithium and other organometallic catalysts under specified conditions and presumed as a "stereoblock" polymer characterized by high contents of both isotactic and syndiotactic sequences in the same chain. 7-9 A possible polymerization mechanism for such a stereoblock formation has been proposed. 10 • 11 Recently Liquori and his associates 12 have, however, confirmed the stereochemical identity between the stereoblock PMMA and the stereocomplex which is formed by isotactic and syndiotactic PMMA in the ratio 1 :2 (iso/synd). This result allows us to presume that under some specific conditions, stereospecific polymerizations of methyl methacry-Iate (MMA) may proceed through a reaction mechanism in which the stereocomplex formation plays an essential role. A crystallizable type of PMMA was obtained One of the aims of this study is to present more 46