CHAIN CARRIERS AND MOLECULAR-WEIGHT DISTRIBUTIONS IN LIVING ISOBUTYLENE POLYMERIZATIONS

The nature of the chain carriers and molecular weight distribution (MWD) in living isobutylene polymerizations are discussed. The evidence presented suggests that the chain carriers in these systems are ion pairs that are in dynamic equilibrium with dormant covalent species (polymeric chlorides). The polymerizations are characterized by "growth periods" and MWDs are described by lBAR, the average run length, which is the average number of monomer units incorporated during one growth period. lBAR can be calculated for constant and varying monomer concentration (lBAR and l0BAR for [M] and [M]0, respectively) from experimental MWD data. The effect of the ionization equilibrium (dependence on solvent polarity, the nature and stability of the counteranion, etc.) on lBAR is shown by evaluating data presently available on living isobutylene polymerizations; the average run lengths are l0BAR = 4.6 for CH3Cl/BCl3 at [M]0 = 1.57 mol/L and l0BAR = 9.9 for CH3Cl/n-C6H14/TiCl4 at [M]0 = 0.6 mol/L. In pure CH3Cl the run length for TiCl4-coinitiated polymerizations is l0BAR = 54 compared to the l0BAR = 4.6 for BCl3 coinitiation. This suggests a higher degree of dissociation or a lower collapse rate in TiCl4-coinitiated systems. lBAR was found to be independent of initiator concentration and to increase dramatically with increasing solvent polarity. Very reactive unpaired ions do not appear to participate in the polymerizations.