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.在合成聚合物中，交替共聚物（Alternating copolymers）拥有最清晰明确的微观结构，是制备仿生合成聚合物的极具潜力的构筑基元。当前最为成熟的合成策略采用供体-受体单体对、带有不同取代基的对映异构体，以及通过离子聚合与活性自由基聚合制备的各类专门设计的环状单体。本文报道了稀土金属基催化体系在乙烯与一系列未保护极性苯乙烯及非极性苯乙烯直接共聚中的催化行为。针对乙烯与对甲氧基苯乙烯的共聚反应，吡啶侧臂芴基负载钇催化剂（pyridyl side-armed fluorenyl-supported yttrium catalyst）无催化活性；其钪同系物则表现出中等活性，可得到无规共聚物；而半夹心芴基钪催化剂则生成了凝胶产物。与之相反，甲基取代氮杂环卡宾（N-heterocyclic carbene, NHC）侧臂芴基钪催化剂展现出最高活性，达3.19 × 10^5 g·mol_Sc^–1·h^–1，其活性是带有空间位阻较大的三甲基苯基取代NHC芴基配体的同系催化剂的10倍，尽管该催化剂无法引发任何极性苯乙烯的均聚反应。该催化体系的性能还拓展至其他极性苯乙烯底物，包括间甲氧基苯乙烯、6-甲氧基-2-乙烯基萘、对甲硫基苯乙烯、二苯基(4-乙烯基苯基)膦以及对(N,N-二乙氨基)苯乙烯。尽管聚合条件存在差异，所有所得共聚物均具有伪交替微观结构。特别地，当以对(N,N-二甲基氨基)苯乙烯作为共聚单体时，可生成完美交替共聚物，且共聚单体转化率高达83%。本研究明确了前驱体的结构因素、电子效应与其催化性能、所得共聚物组成及序列分布之间的构效关系。