ABSTRACT
The stereospecific radical polymerization of vinyl esters, methacrylates, and alpha-substituted acrylates was studied. Fluoroalcohols, as a solvent, have remarkable effects on the stereoregularity of the radical polymerizations of vinyl acetate, vinyl pivalate, and vinyl benzoate, affording polymers rich in syndiotacticity, heterotacticity, and isotacticity, respectively. This method was successfully applied to the polymerization of methacrylates to give syndiotactic polymers. The steric repulsion between the entering monomer and the chain-end monomeric unit bound by the solvent through hydrogen bonding is important for the stereochemical control in these systems. Lewis acid catalysts, such as lanthanide trifluoromethanesulfonates and zinc salts, were also effective for the stereocontrol during the radical polymerization of methyl methacrylate, to reduce the syndiotacticity and alpha-(alkoxymethyl)acrylates to synthesize isotactic and syndiotactic polymers. Radical polymerization of the methacrylates bearing a bulky ester group, such as the triphenylmethyl methacrylate derivatives, gave highly isotactic polymers, as in the case of anionic polymerization. In addition, the control of one-handed helical conformation was attained in the radical polymerization of 1-phenyldibenzosuberyl methacrylate using chiral neomenthanethiol or cobalt(II) complexes as an additive.
ABSTRACT
Novel alpha-(alkoxymethyl)acrylates bearing a chiral substituent, racemic and optically active benzyl alpha-(menthoxymethyl)acrylates ((+)- and (-)-BMnMA), were synthesized and polymerized by radical and anionic methods. The effects of the chiral substituent on the stereoregularity and chiroptical property of the obtained polymers were investigated. 13CNMR spectra of the polymers obtained by anionic polymerization of the racemic monomer showed sharper peaks than that of the radically obtained polymers, indicating that the former polymers possessed higher stereoregularity than the radically obtained polymers. The polymers prepared from optically pure (-)-monomer with anionic initiators had low solubility in common organic solvents. However, the THF-soluble parts were highly isotactic, and exhibited lower specific rotation ([alpha]25(365) = -92 degrees) than those of the monomer ([alpha]25(365) = -226 degrees) and the radically obtained polymer ([alpha]25(365) = -201 degrees). The circular dichroism (CD) spectra of the copolymers prepared from a (+)- and (-)-monomer mixture with various optical purity suggested that the highly isotactic (-)-polymer may possess a chiral helical conformation.
