Control of stereochemistry in vinyl polymers via rational monomer design: Cyclopolymerization of chiral bis(methacrylates).

摘要

Free radical (FR) and group transfer polymerization (GTP) methods typically give atactic polymers. In order to prepare stereoregular optically active polymers by FR and GTP methods, novel divinyl monomers incorporating asymmetric templates have been designed. This also enabled the criteria for induction of stereochemistry in the vinyl polymer backbone and the factors influencing formation of stable secondary structures to be probed. Cyclopolymerization of divinyl monomers can be utilized to enhance the stereoregularity of the polymers.*; Monomers 1-6 were synthesized and polymerized by GTP and FR methods. The monomers cyclopolymerized well under both sets of conditions to give totally soluble polymers. The steric bulkiness of the monomer controlled the extent of isotacticity of the polymer. The bulkier the template the higher the isotacticity. Poly-6 by GTP at {dollar}{lcub}-{rcub}78spcirc{dollar}C had isotacticity {dollar}fsb{lcub}mm{rcub} = 97%.{dollar} The highly isotactic polymers showed very high optical rotations with mean residue molar rotations (molar rotation per repeat unit) between {dollar}-{dollar}1107 and {dollar}-{dollar}1489.; In order to determine the secondary structure of the highly isotactic poly -3, -4, and -6, both computer modeling and free radical copolymerization with achiral monomers were carried out. Results suggest that the polymers assumed helical conformations which was confirmed by circular dichroism spectroscopic studies. The helix was stable in solution, and the polymers were thermally robust with onset thermal decomposition temperatures {dollar}rm Tsb{lcub}d{rcub}{dollar} higher than {dollar}300spcirc{dollar}C.; Other monomers incorporating the binaphthyl unit and cycloadducts derived from asymmetric Diels-Alder reactions were designed to probe the effects of the steric barrier and ring size on the stereochemistry of polymerization. For the binaphthyl-templated monomers, an 11-membered ring was too small for complete cyclopolymerization to occur. Steric barriers exerted no effect when the ring size was 17. Optically active polymers with 13-membered ring repeat units showed improved stereochemistry by GTP. However, no secondary structure was observed for these polymers based on the results of copolymerization with an achiral monomer, benzyl methacrylate. For monomers based on Diels-Alder cycloadducts, the bulkiness of the template imposed no influence on stereochemical control since templates were far removed from the reactive centers. ftn*Please refer to the dissertation for diagrams.