Quasi-Living Cationic Polymerization of Styrene and Isobutylene: Measurement of Run Number and Calculation of Apparent Rate Constant of Ionization by TiCl4

The run number (RN), the average number of monomer units added per ionization−termination cycle, was measured at −70 °C from first-order kinetic data for cationic quasi-living polymerization of styrene and isobutylene initiated with 5- tert -butyl-1,3-bis(2-chloro-2-propyl)benzene (bDCC)/TiCl 4 in 60/40 MCHex/MeCl, via analysis of an initiation event termed rapid monomer consumption (RMC). RMC is characterized by an initial period of high polymerization rate followed by slower first-order decay in monomer concentration and is due to a larger ionization rate for the initiator compared to that for the polymer chain end. Styrene RN was found to decrease with increasing initiator concentration while that for isobutylene remained relatively unchanged; the dependence of styrene RN on initiator concentration was attributed to chain transfer to initiator during RMC. RN in the limit of zero initiator concentration was determined to be 39 and 4.7 for 0.5 M styrene and isobutylene, respectively. Apparent rate constants of chain-end ionization, k i, of 1.3 and 15 M -2 s -1 and rate constants of ion-pair collapse of 1.9 × 10 7 and 7.5 × 10 7 s -1 for styrene and isobutylene, respectively, were calculated. The latter were calculated using a value for k p of 1.5 × 10 9 M -1 s -1 for styrene and 7 × 10 8 M -1 s -1 for IB; apparent rate constants of chain-end ionization were determined independently of k p, from k - i / k p and k app .