TEMPERATURE EFFECTS ON THE LIVING CATIONIC POLYMERIZATION OF ISOBUTYLENE - DETERMINATION OF SPONTANEOUS CHAIN-TRANSFER CONSTANTS IN THE PRESENCE OF TERMINATIVE CHAIN TRANSFER

The living cationic polymerization of isobutylene (IB) was studied usi ng the 2-chloro-2,4,4-trimethylpentane (TMPCl)/TiCl4/2,6-di-tert-butyl pyridine (DTBP) system in hexane (Hex)/methyl chloride (MeCl) (60/40 a nd 40/60, v/v) solvent mixtures at various temperatures ranging from - 25 to -80 degrees C. From the Arrhenius plots of the apparent rate con stants for propagation, negative apparent activation energies were obs erved and calculated to be -8.5 and -6.9 kcal/mol using Hex/MeCl, 60/4 0, v/v and 40/60, v/v solvent mixtures, respectively. At temperatures less than or equal to -69 degrees C, linear first-order plots and line ar M-n versus conversion plots were obtained, suggesting the absence o f termination and chain transfer. Irreversible termination was conspic uously observed from the curved ln([M](0)/[M]) versus time plots when the polymerizations were carried out at temperatures greater than or e qual to -40 degrees C in both solvent mixtures. M-n versus conversion plots, however, exhibited linear growth of M-n, on the theoretical lin e with increased conversion indicating the absence of chain transfer t o monomer. Structural analysis of products obtained at -40 degrees C u sing H-1 NMR spectroscopy revealed the presence of olefinic end groups , increasing in content with increased conversion. On the basis of the se results, it is concluded that termination at higher temperature inv olves terminative chain transfer, that is, beta-proton elimination fro m the living chain ends with the eliminated proton being instantaneous ly entrapped by a proton trap, DTBP. By a kinetic treatment of the ter minative chain transfer, the spontaneous chain-transfer constants (k(t r)/k(p)'s), zero order in monomer, were determined for the first time. In Hex/MeCl (40/60, v/v), k(tr)/k(p) was calculated to be 1.1 x 10(-2 ) and 7.2 x 10(-4) mol/L at -25 and -40 degrees C, respectively. In He x/MeCl (60/40, v/v), k(tr)/k(p), was calculated to be 3.3 x 10(-3) mol /L at -40 degrees C.