A unified view of ethylene polymerization by d0 and d0fn transition metals.: 3.: Termination of the growing polymer chain

We present a systematic investigation of chain-termination processes for a number of d(0) [L]MR(0,+,2+) fragments (M = Sc(III), Y(III), La(III), Lu(III), Ti(IV), Zr(TV), Hf(IV), Ce(TV), Th(IV), and V(V); I, = NH(CH)-2NH2- [1], N(BH2)(CH)(2)(BH2)N2- [2], O(CH)(3)O- [3], Cp-2(2-) [4], NHSi(H-2)C5H42- [5], [(oxo)(O(CH)(3)O)](3-) [6], (NH2)(2)(2-) [7], (OH)(2)(2-) [8], (CH3)(2)(2-) [9], NH(CH2)(3)NH2- [10], O(CH2)(3)O2- [11], and DPZ [12]; R = C2H5, C3H7) involved in ethylene polymerization. Our calculations show that beta-hydrogen transfer to the monomer is the dominant chain-termination mechanism under the usual experimental conditions. beta-Hydrogen elimination (i.e., hydrogen transfer to the metal) can only compete in the limit of very small monomer concentrations or if monomer complexation is otherwise disfavored. The activation barrier for beta-hydrogen transfer to the monomer is only weakly dependent on the character of the metal center and the auxiliary ligand. The thermodynamic driving force as well as the kinetic barrier of beta-hydrogen elimination is highly dependent on the metal, but only weakly dependent on the auxiliary ligand set. We lay out rules to affect BHE and BHT barriers, and, by comparing the termination activation barriers with data on ethylene insertion barriers, we provide guidelines along which successful ethylene polymerization catalysts may be designed.