Solvent influence on cationic polymerization, 3. Copolymerization of isobutene and cyclopentadiene

Abstract

The influence of solvents on cationic polymerizations is well-known I). Starting from model calculations a rule could be established describing the influence of the solvent on the ratio of copolymerization reactivity ratios rl and rZzt3):"If in a copolymerization via ionized cationic chain ends the comonomer smaller in size, forming a cation with low charge delocalization, is indicated as monomer 1 and monomer 2 exhibits opposite behaviour, i. e. the latter is larger in size and the cation is stabilized by charge delocalization, then r2 /rl should decrease during the transition into a more polar solvent".The usefulness of this rule was shown by an extensive comparison with literature data (see part 1 of this series2)). Part 3 deals with theoretical and experimental investigations on the cationic copolymerization of isobutene (IB) with cyclopentadiene (CPD) according to the following equations: IB-IB+(1-1) k l l , IB-CPD' (1-2) 4 2 , CPD-IB+ (2-1) k21 ., CPD-CPD ' (2-2) k22 , IB+ + IB IB+ + CPD CPD' + IB CPD' + CPD r, = k l l / k 1 2 ; r2 = kZ2/kalThe analysis of these processes by means of thermochemical kinetics4) leads to expectation values for the reactivity ratios of copolymerization.A method for the estimation of the reaction enthalpies of the first propagation steps in cationic polymerizations was given in ref.'). Using this method the following reaction enthalpies for the gas phase are calculated for steps (1-1) to (2-2):AHG/(kJ * mo1-l) = -104 (1-l), -122 (1-2), -90 (2-l), -108 (2-2).The Hammond principle is valid because of the distinctly negative values of these reaction enthalpies. This means that one can assume the direct proportionality AHT(i-j) a AH*(i-j), where AH* represents the activation enthalpyQ. This was a) Part 2 cf.3).