Living Carbocationic Polymerization of p-Methoxystyrene Using
p-Methoxystyrene Hydrochloride/SnBr4 Initiating System:
Determination of the Absolute Rate Constant of Propagation
for Ion Pairs
posted on 2004-10-19, 00:00authored byPriyadarsi De, Rudolf Faust
The carbocationic polymerization of p-methoxystyrene (p-MeOSt) was studied in CH2Cl2
at −30 °C using 1-chloro-1-(p-methoxyphenyl)ethane (p-MeOStCl) as initiator in conjunction with different
Lewis acids, BF3·O(C2H5)2, BCl3, ZnCl2, TiCl4, SnCl4, and SnBr4. The best results, moderate rates,
theoretical Mns, and low polydispersities, were obtained in conjunction of SnBr4. The living nature of the
polymerization was verified by linear first-order ln([M]0/[M]) vs time and linear Mn vs conversion plots
in the temperature range of −60 to −20 °C. The number-average molecular weight of the polymers
increased in direct proportion to monomer conversion up to Mn = 120 000 and agreed with the calculated
molecular weight, assuming that one polymer chain forms per molecule of p-MeOStCl. Kinetic studies
suggest that the polymerization is first order in [SnBr4], and the rate, Mn, and polydispersity index are
not affected by excess proton trap, 2,6-di-tert-butylpyridine. The living nature of the polymerization was
further demonstrated by chain extension experiment. The stability of the propagating chain end of poly(p-MeOSt) and monomeric chain end of p-MeOSt+ was studied, and a slow decomposition of the active
chain end was observed under monomer starved conditions. Employing the model compound p-MeOStCl
in conjunction with SnBr4 in CH2Cl2 UV−vis spectroscopy was used to determine the equilibrium constant
of ionization (Ki) at −30 and −60 °C. From Ki values and the apparent rate constant of propagation
(kapp), the absolute rate constant of propagation for ion pairs, kp± = 1.07 × 105 L mol-1 s-1 for −30 °C and
3.83 × 104 L mol-1 s-1 for −60 °C, was calculated. To determine kp± separately, competition experiments
were carried out in the presence of a nucleophile, phenylsilane. NMR spectroscopy, gel permeation
chromatography, and MALDI−TOF MS analysis suggested complete capping of the polymeric cation and
the absence of side reactions. From the limiting conversion and limiting number-average degree of
polymerization kp± was calculated using the known rate constant of capping. The kp± values obtained
from the competition experiments agreed well with those determined from the UV−vis spectroscopy.