Selective co-oligomerization of ethylene and styrenes by half-titanocene catalysts and synthesis of polyethylenes with 4-aryl-1-butyl branches

Copolymerization of ethylene and styrene with the catalytic system Cp*TiMe3 -B(C6F5)(3) under suitable conditions affords a new polymer having a polyet hylenic backbone with 4-phenyl-1-butyl branches as the main solid product. Analysis of the polymerization mixtures evidences the formation of signific ant amounts of unsaturated ethylene-styrene co-oligomers; A detailed charac terization of the oligomeric fraction by CC-MS and NMR analysis suggests so me selectivity of the co-oligomerization, i.e., the prevailing formation of phenylhexenes and the absence of phenylbutenes. These unexpected results h ave been ascribed to the multisite nature of the catalytic system, containi ng a species able to co-oligomerize ethylene and styrene to phenylhexenes a nd another species able to copolymerize the formed g-phenyl-l-hexene with e thylene. A mechanism involving Ti(II) active species and metallacycle inter mediates has been proposed to explain the observed selectivity in the co-ol igomerization, by analogy with the mechanism proposed for the selective tri merization of ethylene to l-hexene by the same catalyst. Extension of this study to catalysts based on other half-titanocenes activated by methylalumo xane results in similar findings; optimization of the reaction conditions f or the CpTiCl4-MAO catalyst results in styrene conversions to unsaturated c o-oligomers >90%. Dual-catalyst systems,with one component co-oligomerizing ethylene and styrene and the other copolymerizing the formed g-phenyl-l-he xene with ethylene, have been used to selectively produce polyethylenes wit h 4-phenyl-1-butyl branches. Use of substituted styrenes, i.e., p-methylsty rene, p-tert-butylstyrene, p-chlorostyrene, and m-methylstyrene, as the com onomers results in the formation of polyethylenes having the corresponding 4-aryl-1-butyl branches as well as oligomeric fractions analogous to those obtained from styrene.