Development of Group 3 Catalysts for Alternating Copolymerization of Ethylene and Styrene Derivatives

Alternating copolymers have the clearest and most defined microstructures among manmade polymers, having been promising building blocks to access synthetic polymers able to mimic biomaterials. The most successful approaches employ donor-acceptor monomer couples, enantiomers with different substituents, as well as specially designed cyclic monomers containing various units through ionic and living radical polymerizations. Herein we report the catalytic behaviors of rare-earth metal-based catalyst systems toward the direct copolymerization of ethylene with a series of unmasked polar styrenes and nonpolar styrenes. For the copolymerization of ethylene with para-methoxystyrene, the pyridyl side-armed fluorenyl-supported yttrium catalyst was inert, while its scandium analogue displayed moderate activity to give a random copolymer; the half-sandwich fluorenyl scandium catalyst provided a gel product. In contrast, the methyl-substituted N-heterocyclic carbene (NHC) side-armed fluorenyl scandium catalyst showed the highest activity, 3.19 x 10(5) g mol(Sc)(-1) h(-1), which was 10 times higher than its analogue bearing the steric bulky trimethylphenyl-substituted NHC fluorenyl ligand, although it could not initiate any polar styrene homopolymerization. The catalytic performance was extended to the other polar styrenes, such as meta-methoxystyrenes, 6-methoxy-2-vinylnaphthalene, para-methylthiostyrene, diphenyl(4-vinylphenyl)phosphine, and para-(N,N-diethylamino)styrene. All of the resultant copolymers are composed of pseudo-alternating microstructures despite polymerization conditions. In particular, when para-(N,N-dimethylamino)styrene was used as the comonomer, a perfect alternating product was generated with an as high as 83% comonomer conversion. The relationships among the structural factors and electronics of the precursors and their catalytic performances and the resultant copolymer compositions and the sequence distributions were established.