Determination of Propagation Rate Constants in Carbocationic Polymerization of Olefins. 1. Isobutylene

Abstract

The propagation rate constant for ion pairs (k p ± ) in the polymerization of isobutylene in conjunction with TiCl4 in hexanes/methyl chloride 60/40 (v/v) at −80 °C has been determined using two different diffusion clock methods. The rate constant k p ± was in the range of (0.3−1.0) × 109 L mol-1 s-1, 4 orders of magnitude higher than presently accepted values. The first method involved on-line UV−vis monitoring of the addition of the π-nucleophiles 1,1-bis(4-methylphenyl)ethylene, 1,1-bis(4-tert-butylphenyl)ethylene, and 2-phenylfuran to hydrochlorinated isobutylene n-mers (n = 2, 3, 36). The apparent rate constants of capping, k c K i, and the rate constant of ionization, k i, have been determined. For a given n the k c K i values were identical independent of the nature and nucleophilicity of the π-nucleophile, which was attributed to diffusion-limited addition. Using the diffusion-limited second-order rate constant of k c ∼ 3 × 109 L mol-1 s-1, K i and k - i have been calculated. From the concentration of active chain ends (determined from K i) and the apparent rate constant of propagation for isobutylene (k p ± [active chain ends], determined separately), the absolute propagation rate constant of k p ± = 1 × 109 L mol-1 s-1 was calculated. The second simple diffusion clock method involved competition experiments, i.e., polymerization carried out in the presence of a π-nucleophile, which stops short of completion when all chain ends are capped. From the limiting conversions and number-average degrees of polymerization, k p ± = (3−6) × 108 L mol-1 s-1 have been obtained.

Primary Hydroxy-Terminated Polyisobutylene via End-Quenching with a Protected N-(ω-Hydroxyalkyl)pyrrole

“…This zero-order dependence on quencher concentration indicated that the rate of quenching was limited by chain end ionization. 40 From run number measurements made under similar conditions at -60°C, Thomas and Storey 41 calculated a value of k i = 9.1 L 2 mol -2 s -1 for the rate constant for ionization of PIB chain ends. The initial firstorder rate constant for alkylation in Figure 8A, divided by the square of the effective TiCl 4 concentration, 42 yielded a similar value of 8.4 L 2 mol -2 s -1 , thus supporting the conclusion that ionization was the rate-limiting step for the quenching reaction.…”

Reactivities of carbocations and monomers in carbocationic polymerization and copolymerization

“…But the conclusions are not convincing since the limiting conversions in the absence of salt effect (i.e., without D t BP) are not larger by more than 20%, showing similar rates of capping and concentrations of P n + and P n ± (if k = k ) for these two experiments. But with a calculated [PIB ± , Ti 2 Cl] of 1.3 × 10 −12 mol L −1 30 and a dissociation constant K D of 10 −6 to 10 −7 mol L −1 15 the unpaired ions concentration should have been 10 2 to 10 3 larger than [PIB ± , Ti 2 Cl], with a much larger initial rate.…”

Cationic polymerization in rotating packed bed reactor: Experimental and modeling

“…The rate constants of propagation ( k p ) in the cationic polymerization vary considerably, e.g., k p 's from 10 3 to 10 9 L mol −1 s −1 have been reported for isobutylene polymerization 13, 14. It is proposed that the discrepancy in k p reported for various cationic polymerizations is due to different mechanistic interpretations and assumptions 15.…”