Structure−Reactivity Scales in Carbocationic Polymerizations

Abstract

The polymerization of p-methylstyrene (pMeSt) in the presence of isobutylene (IB), styrene (St), p-chlorostyrene (pClSt), and 1,3-butadiene (BD) was carried out at -40 °C. By selecting CH 2 Cl 2 /methylcyclohexane (MeCHx) 50/50 (v/v) solvent mixture and the appropriate weak Lewis acid, SnBr 2 Cl 2 monoaddition of IB, St, pClSt, and BD followed by instantaneous termination was achieved, and the polymerizations stopped short of completion. The reactivity ratios, k p /k 12 , where kp is propagation rate constant and k 12 is the cross-propagation rate constant, were calculated on the basis of limiting conversions and limiting molecular weights. According to these values, pMeSt is 3.8 ± 0.5, 4.8 ± 1.1, 7.2 ± 1.6, and 100 ± 6 times more reactive than IB, St, pClSt, and BD respectively. Similar competition reactions between IB/BD, St/BD. and pClSt/BD in conjunction with TiCl 4 as Lewis acid yielded k p /k 12 = 77 ± 5, 7.1 ± 1.0, and 3.2 ± 0.7, respectively. Using k p /k 12 and k p forpMeSt, St, and pClSt, the cross-propagation k 12 values were calculated to establish structure-reactivity scales. Similarly to our earlier observations, comparison of these values indicated that structural differences have a much larger effect on cation reactivity than on monomer reactivity.