STATIC AND AB-INITIO MOLECULAR-DYNAMICS STUDY OF THE TITANIUM(IV)-CONSTRAINED GEOMETRY CATALYST (CPSIH2NH)TI-R-CHAIN BRANCHING( .2. CHAIN TERMINATION AND LONG)

We present a comprehensive survey of chain termination and long chain branching processes for the ''constrained-geometry'' olefin polymeriza tion catalyst (CpSiH2NH)Ti-R+ (R = ethyl, propyl) based on static and dynamic density functional theory. Car-Parrinello molecular dynamics c alculations are used to locate reaction pathways and estimate free ene rgies of activation, and conventional static calculations are used to ascertain stationary points and relative energies. We have examined th ree distinct chain termination processes: (a) beta-hydrogen transfer t o the monomer, (b) beta-hydrogen transfer to the metal, and (c) olefin C-H sigma-bond metathesis. We find alternative (a) to be the most via ble one [Delta H-el(double dagger)(R = ethyl) = 32 kJ/mol; Delta F-dou ble dagger(R = ethyl) = 40.1; Delta F-double dagger(R = propyl) = 43 /- 8 kJ/mol at 300 K], whereas pathways (b) [Delta H-el(double dagger) (R = propyl) = 67; Delta F-double dagger(R = propyl) = 57 +/- 3 at 300 K] and (c) [Delta H-el(double dagger)(R = ethyl) = 93; Delta F-double dagger(R = ethyl) = 91.7 kJ/mol; Delta F-double dagger(R = propyl)= 8 7 +/- 5 at 300 K] have much higher activation barriers. In addition, w e investigated an unconventional long chain branching mechanism (d), w here a polymer chain binds to the metal center in apt eta(2)-agostic f ashion via aliphatic hydrogens, followed by activation of one aliphati c C-H bond and transfer of the hydrogen to the a-carbon to-bond metath esis). For this process we have found a large electronic barrier of De lta H-el(double dagger)(R = ethyl) = 77 kJ/mol and a free energy barri er of Delta F-double dagger(R = ethyl) = 72.3 kJ/mol; Delta F-double d agger (R = propyl) = 70 +/- 3 kJ/mol at 300 K. On the basis of our dat a, we favor the conventional long chain branching mechanism consisting of chain termination via mechanism (a) to produce a vinyl-terminated chain and reincorporation of the terminated chain into the polymer. In this study we have calculated free energy barriers for each of the af orementioned processes by a conventional static calculations and by a Car-Parrinello ''first principles'' molecular dynamics simulations. Th e agreement of the two methods is exceptional, demonstrating utility o f first principles molecular dynamics to determine free energy barrier s.