Penultimate unit effect in ethene/propene copolymerization promoted at high temperature by single center catalysts

This work reports on the microstructure of ethene/propene copolymers prepared in solution at high polymerization temperature with single center catalytic systems. Three different organometallic complexes were used as catalysts: a sterically hindered, highly regio- and stereo(iso)-specific metallocene, rac-[methylenebis(3-tert-butyl-1-indenyl)]zirconium dichloride [rac-H2C-(3-(t)BuInd)(2)ZrCl2], the prototypical poorly isospecific hydrogenated metallocene without any substituent on the indenyl ligand, rac-ethylenebis(tetrahydroindenyl)zirconium dichloride [rac-(EBTHI)ZrCl2], and the so-called "constrained geometry" half-sandwich complex, {eta(1):eta(5)-([tert-butylamido)dimethylsilyl](2,3,4,5-tetramethyl-1-cyclopentadienyl)}titanium dichloride [Me2Si(Me4Cp)((NBu)-Bu-t)TiCl2]. Copolymers microstructure was assessed through C-13 NMR analysis, and triad distribution data were elaborated through a statistical method that allows one to determine the reactivity ratios of the comonomers, r(1) and r(2). The product of reactivity ratios r(1)r(2) of ethene/propene copolymers prepared with any of the catalysts increased to values higher than 1 when the copolymerization temperature was raised from 50 to 90 degrees C, thus indicating a blocky nature of the copolymers. Such an increase is due to different causes. The sterically hindered highly isospecific metallocene rac-CH2-(3-Bu-t-Ind)(2)ZrCl2, which at 50 degrees C already tends to produce blocky copolymers, increases its blockiness further on, mainly and surprisingly because of a remarkable r(2) increase. With [rac-(EBTHI)ZrCl2], which at 50 degrees C tends to produce alternate copolymers, one observes a strong increase of r(1). With [Me2Si(Me4Cp)((NBu)-Bu-t)TiCl2] as well a remarkable increase of r(1) is obtained. Copolymerizations promoted by rac-CH2-(3-Bu-t-Ind)(2)ZrCl2 could be described by a second-order Markovian copolymerization model: the relative reactivity of ethene with respect to propene was found to decrease moving from EE to PE, to EP, and finally to PP as the last inserted monomeric units, and an inserted propene unit was thus found to bring about a higher propene reactivity also when it was in the penultimate position.