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 × 105 g molSc–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.