A-methylstyrene polymerization

Polymerization leads to a contraction in the volume of the system so that the equilibrium of a monomer-polymer system shifts in the direction of the reaction as the hydrostatic pressure increases. This was demonstrated by Weale (40) in his studies of a-methylstyrene polymerization under high pressure. The ceiling temperature increased from 61° C at 1 atm to 170° C at 6480 atm. 

It is difficult to follow the polymerization kinetics if the initiator does not absorb in the UV region. The kinetics of such a system has been derived for a-methylstyrene polymerizations initiated by mixtures of BuOTiCb and H20 [218]. As shown in Eq. 59, reaction between the components of the initiating system is assumed to be the rate-determining step. 

The formation of stable carbenium ions can be observed visually and/ or spectroscopically. For example, styrene and a-methylstyrene polymerizations are generally colorless because the growing carbenium ions absorb at approximately 340 nm (cf., Sections II.B and IV.B.l). However, these systems may turn brown or dark red at longer reaction times due to formation of indanyl carbenium ions (A 440 nm) [14,26,325] and other delocalized carbocations similar to those in Eq. (121). The stable cyclic diaryl carbenium ions are generated by hydride transfer from the initially formed indanyl end groups [Eq. (124)] in styrene polymerizations, and by methide transfer in a-methylstyrene polymerizations. The prerequisite for this termination is therefore intramolecular transfer by Friedel-Crafts alkylation protons liberated in the first stage can then reinitiate polymerization. 

Recently, Soares studied a-methylstyrene polymerization with iodine in liquid sulfur dioxide [229,230], which is known as a unique solvent [231]. Polymers are formed in a 1 1 mixture of liquid S02 and methylene chloride, the latter being needed to keep the solution homogeneous. The molecular weight control is, however, not as good as in the other systems, accompanied by broad MWDs. 

Sometimes, an apparently ideal model of the growing species such as f-butyl halide for isobutene polymerization and cumyl halide for a-methylstyrene polymerization may not be sufficiently reactive. In both cases the ionization ability and initiation efficiency for the monomeric species is much lower than that for the dimeric/macromolecular species ... 

Wang, Q. Quyoum, R. Gillis, D. J. Tudoret, M.-J. Jeremic, D. Hunter, B. K. Baird, M. C. Ethylene, styrene, and a-methylstyrene polymerization by mono(pentamethylcyclopentadienyl) (Cp ) complexes of titanium, zirconium, and hafnium Roles of cationic complexes of the type [Cp MR2] (R = alkyl) as both coordination polymerization catalysts and carbocationic polymerization initiators. Organometallics 1996,15, 693-703. 

Substances capable of undergoing cationic polymerization, such as isobutene, styrene and a-methylstyrene, polymerize by a cationic mechanism if subjected to irradiation with y or X-rays, provided that they have been rigorously purified from impurities, in particular from traces of water. Scheme 5.5 presents a kinetic scheme of cationic polymerization which takes into account that an impurity X interferes with the polymerization. The initiation comprises the generation of free ions A" and B , the latter being a free electron or a molecular ion. 

Albeit little studied, a couple of well-defined organoaluminum species have been used as Lewis acids for the direct polymerization of styrene derivatives, yet with a poor polymerization control so far. Chen and coworkers reported that A1(C6F5)3 directly initiates a very rapid styrene polymerization at —78°C to quantitatively convert 1,000 equiv. of styrene to poorly defined PS (PDI = 3.9) within 5 min (TON = 22,800 h ), via a process likely to involve the zwitterionic styrene-alane adduct 6 (Scheme 6) [23]. fii an analogous manner, the N202-supported penta-coordinate A1 cation 7 (Fig. 2) was found to readily mediate a-methylstyrene polymerization, yet to produce a broadly disperse material [24]. Overall, the direct polymerization of styrene-type monomers by discrete aluminum cationic initiators is beneficial to catalytic activity however, the poor control of such polymerization reactions certainly hampers their potential usefulness at this stage. 

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