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 k p 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 for pMeSt, 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.

Reactivities of carbocations and monomers in carbocationic polymerization and copolymerization

“…However, this agreement is not surprising since the E values were obtained assuming that capping rates of the styrenes carbocations are diffusion limited (with k diff = 3 × 10 9 L mol −1 s −1 ) 12, 15. And according to the k 12capp values calculated,47 the difference between the reactivities of p MeOSt + and p MeSt + is by a factor 2 × 10 6 while it should be less than 200 according to the E values.…”

“…New data on the reactivity scales have been just published by Faust and coworkers,47 for which the propagating carbocations are those of poly( p MeSt) + (∼∼∼M). In the presence of monomer M 2 , termination by capping occurred after the addition of one M 2 unit, giving poly( p MeSt) n M 2 Cl.…”

“…This is in contradiction with the k 12capp values calculated in this article for the series of carbocations (1) capped by butadiene (2) (last line in Table 4). 47…”

Structure−Reactivity Scales in Carbocationic Polymerizations: The Case of α-Methylstyrene

“…In these experiments, polymerizations, which proceed in the absence of chain transfer and conventional termination, were carried out in the presence of π-nucleophiles or hydride donors, which terminate the chains by forming a stable cation or a cation that can rapidly eliminate a cationic fragment (e.g., Me 3 Si + ), or by hydride transfer. Therefore, the cationic polymerization of monomer M 1 with a terminating nucleophile M 2 results in limiting conversions of M 1 . − The terminating nucleophile M 2 can also be addressed as a capping agent because all of the polymeric chains have M 2 or a fragment from M 2 as an end group. From the limiting conversion ( x ∞ ) or from the corresponding limiting number-average degree of polymerization (DP ∞ ), the reactivity ratio, k p / k c , can be calculated from the following relationships k normalp k normalc 1.5pt = 1.5pt ln false( 1 − 1.5pt x ∞ false) ln false( 1 − 1.5pt false[ normalI false] 0 / false[ normalM 2 false] 0 false) k normalp k normalc 1.5pt = 1.5pt ln false( 1 − 1.5pt DP ∞ false[ normalI false] 0 / false[ normalM 1 false] 0 false) ln false( 1 − 1.5pt false[ normalI false] 0 / …”

“…Therefore, the cationic polymerization of monomer M 1 with a terminating nucleophile M 2 results in limiting conversions of M 1 . [6][7][8][9][10][11][12] The terminating nucleophile M 2 can also be addressed as a capping agent because all of the polymeric chains have M 2 or a fragment from M 2 as an end group. From the limiting conversion (x ¥ ) or from the corresponding limiting number-average degree of polymerization (DP ¥ ), the reactivity ratio, k p /k c , can be calculated from the following relationships…”