Propene polymerization with the isospecific, highly regiospecific rac-Me2C(3-t-Bu-1-Ind)2ZrCl2/MAO catalyst.: 1.: Influence of hydrogen on initiation and propagation:: Experimental detection and theoretical investigation of 2,1 propene insertion into the Zr-H bond

The influence of molecular hydrogen on propene polymerization promoted by the regio- and isospecific rac-Me2C(3-t-Bu-1-Ind)(2)ZrCl2/MAO catalytic system has been investigated both by experiment (liquid propene, T-p = 50 degrees C) and molecular modeling (quantum mechanics/molecular mechanics). A competitive secondary (2,1) propene insertion into the Zr-H bond, originated after chain transfer to hydrogen, is detected by C-13 NMR analysis of these polymers: increasing amounts of the new CH3-CH(CH3)-CH(CH3)-CH2-chain-end group are observed when the hydrogen partial pressure is increased from 1 to 4 bar. The ratio between primary (1,2) and secondary initiations is found to be constant and nearly equal to 5. On the other hand, the complete regiospecificity of propene insertion during chain propagation is preserved. A combined quantum mechanics/molecular mechanics study on the model cation [rac-H2C(3-t-Bu-1-Ind)(2)Zr-H(propene)](+) shows that secondary propene coordination, followed by secondary insertion into the Zr-H bond which generates the observed structure of the saturated end group, is even favored over primary coordination/ insertion, thanks to a stabilizing beta-agostic bond between the metal center and one methyl hydrogen of the secondary coordinated propene. Two different mechanisms have been identified for primary propene insertion into the Zr(n-propyl) and Zr(isopropyl) species, with energy barriers of nearly 4 and 9 kcal/mol, respectively. The experimentally observed ratio of [1,2]/[2,1] approximate to 5 for initiations is explained in terms of side reactions that can transform the "slow" propagating Zr(isopropyl) site into the "fast" Zr(n-propyl) one. The model [rac-H2C(3-t-Bu-1-Ind)(2)Zr(isopropyl)](+) cation shows indeed comparable energy barriers for monomer insertion and beta-H transfer followed by associative displacement with a primary propene to generate the "fast" propagating species.