Hyperbranched polyisobutylenes (PIBs) have been synthesized by carbocationic copolymerization of an inimer (initiator-monomer, IM) and isobutylene (IB). The IM used was 4-(2-methoxyisopropyl)styrene (p-methoxycumylstyrene, pMeOCumSt), in conjunction with TiCl4 as a co-initiator in
methylcyclohexane (MeCHx)/methyl chloride (MeCl) 60/40 v/v solvent mixture at −80 °C, in the presence
of 2,6-di-tert-butylpyridine (DtBP) as a proton trap. The effect of reaction conditions on the molecular
weights (MW), molecular weight distributions (MWD), and branching frequencies (BR) was investigated.
Hyperbranched PIBs with MWs up to M
n ≈ 8 × 105 g mol-1 and MWD as low as 1.2 were obtained within
15−60 min reaction time, where under the same conditions the 2-chloro-2,4,4-trimethylpentane (TMPCl)
monofunctional initiator would have yielded MWs in the range of M
n ≈ 104 g/mol. BRs were determined
by selective destruction of the branching points (links) and taking the ratio of the MWs before and after
link destruction. The experimental BR values, ranging from 3 to 57, agreed reasonably well with values
calculated using the formula BR = (M
n/M
n(theo)) − 1, where M
n is the total MW of the hyperbranched PIB,
and M
n(theo) would be the MW if the IM would act as initiator only. The process was scaled up to 400 g of
PIB/batch, sufficient for meaningful physical characterization.
The kinetics of isobutylene (IB) polymerization initiated by the 2-chloro-2,4,4-trimethylpentane (TMPCl)/TiCl4 in methylcyclohexane (MeCHx)/methyl chloride (MeCl) or hexane (Hx)/MeCl 60/ 40 v/v at -80 °C were investigated in the range of [TMPCl]0/[TiCl4]0 > 1 using a real-time fiber-optic mid-IR monitoring technique. The rate of IB consumption was monitored by following the disappearance of the 1655 cm -1 IR band, characteristic of the C dC stretching vibration. Both the area and the height of the 1655 cm -1 band were found to be directly proportional to IB concentration under the conditions investigated. From polymerization rate data the overall rate constant kp′ ) kpKeq was calculated, where kp is the rate constant of propagation and Keq ) k1/k-1 is the equilibrium constant for the dormant/active equilibrium characteristic of living IB polymerization. The investigation revealed that kp′ was very sensitive to solvent quality. kp′ ≈ 0.5-1.7 L 2 mol -2 s -1 was found using MeCHx, while kp′≈ 1.7-3.4 L 2 mol -2 s -1 was obtained using Hx from different providers and/or with and without further purification. GC analysis and real-time IR monitoring revealed the presence of carbonyl contaminants in the solvents, which were found to form a complex with TiCl4. Regardless of solvent quality, the reaction order in TiCl4 was found to be invariably 1 under the conditions investigated. Monomer concentration and temperature were found to have a profound effect on the polymerization; kp′ increased exponentially with an increase of [IB]0 or a decrease in temperature. The apparent activation enthalpy of propagation was found to be ∆Ha ) -26.4 kJ/mol.
This article features macromolecular engineering via carbocationic polymerization, the focus of research of the recently established Macromolecular Engineering Research Centre (MERC) at the University of Western Ontario. The fundamental philosophy of MERC is interdisciplinary research with a strong industrial orientation, while emphasizing the quest for fundamental understanding of polymerization processes and polymer structure‐property relationships. First, a brief overview of living polymerizations in general, and living carbocationic polymerizations in particular will be given. This latter technique is of interest because some monomers (e. g., isobutylene) can be polymerized by cationic techniques only, to yield polymers with unique properties (e. g., polyisobutylene with superior chemical and oxidative stability, low permeability and high damping). This will be followed by an overview of our research strategy and a summary of our latest results. These include the development of a fiber‐optic mid‐FTIR method for the real‐time monitoring of low temperature polymerization processes, the discovery that selected epoxides initiate effectively the living carbocationic polymerization of isobutylene, fundamental studies into the mechanism and kinetics of living carbocationic polymerization, and the design and synthesis of various polymer architectures (e. g., branched homo‐ and block copolymers) with improved properties and nanostructured phase morphologies.
SynopsisCreep and dynamic mechanical measurements were carried out on a series of hyperbranched polyisobutylenes ͑PIBs͒, having a range of molecular weights ( у 10 6 daltons͒ and branching frequencies ͑3-57 branches/molecule͒. For all samples, the molecular weight of the branches was higher than the entanglement molecular weight of linear PIB, by as much as a factor of 10; nevertheless, only for molecular weights of approximately half-million daltons does the zero-shear viscosity exceed that of linear PIB. Both the viscosity and the length of the entanglement plateau are governed primarily by the branching frequency, rather than by the length of the branches. Such behavior is quite distinct from star-branched polymers. However, the magnitude of the plateau modulus and the temperature dependence of the terminal zone shift factors are the same for hyperbranched, star-branched, and linear PIB.
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