Control of sequence distribution in ethylene/alpha-olefin copolymers using multi-state metallocene catalysts: Efforts toward the synthesis of high-melting ethylene copolymers

In metallocene-catalyzed olefin polymerization, much attention has been directed toward elucidating the relationship between catalyst structure and polymerization behavior. Most studies have focused on establishing the effects of ligand structure on polymerization selectivities. We have been investigating strategies for controlling the sequence distribution of ethylene copolymers using conformationally dynamic metallocene ligand structures.; A series of unbridged mixed-ligand 1-substituted-2-arylindenyl zirconocenes were synthesized and studied in the MAO-activated copolymerizations of ethylene/propylene (EP) and ethylene/1-hexene (EH). These mixed-ligand metallocene catalysts, like their parent bisindenyl metallocenes, exhibited high comonomer selectivities. The steric size of the 1-alkyl substituent did not appear to have any significant effect on comonomer selectivity or copolymer sequence distribution. A 2-phenylindene/2-phenylindole zirconium complex was also synthesized to investigate the electronic effect of replacing the C1-carbon in the cyclopentadienyl ring of one ligand with a nitrogen. While it was found that this complex was less productive than the parent bis(2-phenylindenyl)zirconium catalyst, it incorporated comonomer just as efficiently. No significant effect on the copolymer sequence distribution was observed.; A 1-benzyl substituent produced a catalyst which copolymerized EP and EH with reactivity ratio products greater than one (r_er_p = 2.4, r_er_h = 2.2) to give rubbery, semi-crystalline, high-melting copolymers. It was proposed that a hemilabile intramolecular interaction of the pendant benzyl with the metal center, coupled with the conformational dynamics of the metallocene ligands, produced a unique catalyst system with unusual selectivity for ethylene. Collectively, the reactivity ratios, thermal analysis and solvent fractionation results gave evidence that these copolymers contained long crystallizable ethylene sequences. A comparison of a series of EH copolymers with similar ethylene compositions revealed a sensitive dependence of the melting transition to the comonomer sequence distribution.; To determine the intermolecular effect of catalyst counteranions on bis(2-arylindenyl)metallocene-catalyzed EP copolymerization selectivities, a variety of cocatalysts were used to activate two bis(2-arylindenyl)zirconocene dimethyl complexes. It was found that in these conformationally dynamic systems, the copolymerization reactivity ratios r_e and r_p were unaffected by the variation in metallocene activators and alkylaluminum additives. However, an effect on the stereospecificity and productivity of the catalysts, similar to that found in propylene homopolymerizations, was observed.