ENANTIOSELECTIVE CYCLOPOLYMERIZATION OF 1,5-HEXADIENE CATALYZED BY CHIRAL ZIRCONOCENES - A NOVEL STRATEGY FOR THE SYNTHESIS OF OPTICALLY-ACTIVE POLYMERS WITH CHIRALITY IN THE MAIN CHAIN

Enantioselective cyclopolymerization represents a novel strategy for the synthesis of optically active main-chain chiral polymers. Cyclopolymerization of 1,5-hexadiene using the optically active catalyst precursor, (R,R)-(EBTHI)ZrBINOL ((R,R)-1) [EBTHI = ethylene- 1,2-bis(eta5-4,5,6,7-tetrahydro-1-indenyl); BINOL = 1,1'-bi-2-naphtholatel, yields optically active poly(methylene-1,3-cyclopentane) (PMCP) with a molar optical rotation of [PHI]28(450) +51.0-degrees (c = 0.80, CHCl3). Cyclopolymerization with (S,S)-1 affords the enantiomeric polymer with a molar optical rotation of [PHI]28(405) -51.2-degrees (c = 0.80, CHCl3). The molar optical rotation for the polymer derived from (R,R)-1 is higher than that of the model compound trans-(1R,3R)-1,3-dimethylcyclopentane and is also temperature dependent, suggesting that the polymer adopts chiral conformations which contribute to the observed optical rotation. The microstructure of the polymer was interpreted by C-13 NMR at tetrad resolution. A statistical model for the microstructure based on an enantiomorphic site control mechanism provided good agreement with the experimental data. On the basis of this model, the enantioface selectivity for the cyclopolymerization of 1,5-hexadiene in the presence of catalysts derived from (R,R)-1 is 91% at 23-degrees-C, indicative of a highly isotactic microstructure. The molar optical rotation of poly(methylene-1,3-cyclopentane) is independent of molecular weight, which provides experimental support for an enantiomorphic site control mechanism. The absolute configuration of PMCP was tentatively assigned on the basis of the sign of the optical rotation of the model compound trans-(1R,3R)-1,3-dimethylcyclopentane and the known enantioface selectivity of 1 with similar alpha-olefins.