Cationic polymerization temperature effects

Results of these orienting experiments compiled in Table 3 in regard to the effect of temperature, medium polarity, initiator concentration, monomer concentration, and coinitiator concentration are similar to those reported by others36"39 for cationic polymerization of a-methylstyrene. For example, decreasing temperature, the molecular weight increases and increasing medium polarity, the yield increases. 

Hoogenboom R, Fijten MWM, Schubert US (2004) The effect of temperature on the living cationic polymerization of 2-phenyl-2-oxazoline explored utilizing an automated synthesizer. Macromol Rapid Commun 25 339-343... 

The cationic polymerization was further evidenced by studying the effect of added proton scavengers, such as ammonia and trimethylamine, and the copolymerization with a-methylstyrene and isobutylvinylether. It is now believed that both radical and cationic polymerization of styrene are able to proceed by ionizing radiations even in bulk at room temperature, and the latter polymerization is much more predominant in the absence of cation scavengers, though it is effectively suppressed in their presence. 

With these catalysts, the cation complexes with the monomer so weakly that a solid surface and low polymerization temperatures are required to achieve sufficient orientation for stereospecificity. Braun, Herner and Kern (217) have shown that lower polymerization temperatures are required (in n-hexane diluent) to obtain isotactic polystyrene as the alkyl metal becomes more electropositive (RNa, —20° C. RK, —60° to —70° C. and RRb, —80° C.). They correlate isotacticity with the polymerization rate as a function of catalyst, temperature or solvent. However, with Alfin catalysts, stereospecific polymerization of styrene is unrelated to rate (226). A helical polymerization mechanism as proposed by Ham (229) and Szwarc (230) is also inadequate for explaining the temperature effects since the probability for adventitious formation of several successive isotactic placements should have been the same at constant temperature in the same solvent for all catalysts. 

The active site in chain-growth polymerizations can be an ion instead of a free-radical. Ionic reactions are much more sensitive than free-radical processes to the effects of solvent, temperature, and adventitious impurities. Successful ionic polymerizations must be carried out much more carefully than normal free-radical syntheses. Consequently, a given polymeric structure will ordinarily not be produced by ionic initiation if a satisfactory product can be made by less expensive free-radical processes. Styrene polymerization can be initiated with free radicals or appropriate anions or cations. Commercial atactic styrene polymers are, however, all almost free-radical products. Particular anionic processes are used to make research-grade polystyrenes with exceptionally narrow molecular weight distributions and the syndiotactic polymer is produced by metallocene catalysis. Cationic polymerization of styrene is not a commercial process. 

The effects of temperature on cationic polymerizations can be described with caution by an Arrhenius expression like... 

Initiation of Poly merization of Vinyl Monomers Propagation Reactions Termination and Transfer Processes Kinetics of Cationic Polymerization of Olefins Temperature Effects... 

If the cationic polymerization is carried out at room temperature or higher, both the rate and the molecular weight of the product obtained are lower. This is probably the result of an increased rate of transfer to monomer competing more effectively with the propagation process at the higher temperatures (Eq. 20.21). 

Cationic polymerization is terminated by the presence of contaminants, including water. In experiments to note the combined effects of temperature and humidity on tack-free time, temperature of the substrate was controlled by means of the oscillating stage, as described above, and humidity was controlled by conducting the experiments in an environmental chamber. Results are shown in Table XI. 

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