Cationic chain polymerization applications

A variety of classes of monomers undergo cationic chain polymerization, including epoxides, vinyl ethers, propenyl ethers, siloxanes, oxetanes, cyclic acetals and for-mals, cyclic sulfides, lactones, and lactams. Any monomer that undergoes cationic polymerization is a candidate for cationic photopoiymerization, and a suitable monomer may be selected based upon the specific requirements of the application. 

An advantage of this type of photopolymerizations is that as they are non-radical chain polymerizations, they are insensitive to oxygen. In addition, as the cation is relatively stable, the reaction is able to continue in the dark. Applications of this chemistry may be found in the fields of coatings, adhesives, printing inks, and also for photocurable composites and microelectronic photoresists. 

Butyl rubber, a copolymer of isobutylene with 0.5—2.5% isoprene to make vulcanization possible, is the most important commercial polymer made by cationic polymerization (see Elastomers, synthetic-butyl rubber). The polymerization is initiated by water in conjunction with AlC and carried outatlow temperature (—90 to —100° C) to prevent chain transfer that limits the molecular weight (1). Another important commercial application of cationic polymerization is the manufacture of polybutenes, low molecular weight copolymers of isobutylene and a smaller amount of other butenes (1) used in adhesives, sealants, lubricants, viscosity improvers, etc. 

This article surveys the research work on the synthesis and modification reactions of poly(ethyleneimine) as well as its applications to metal complexation processes. Poly-(ethyleneimine), one of the most simple heterochain polymers exists in the form of two different chemical structures one of them is branched, which is a commercially available and the other one linear which is synthesized by cationic polymerization of oxazoline monomers and subsequent hydrolysis of polyf(/V acylimino)cthylcne]. The most salient feature of poly(ethyleneimine) is the simultaneous presence of primary, secondary, and tertiary amino groups in the polymer chain which explains its basic properties and gives access to various modification reactions. A great number of synthetic routes to branched and linear poly(ethyleneimine)s and polymer-analogous reactions are described. In addition, the complexation of polyfethyleneimine) and its derivatives with metal ions is investigated. Homogeneous and heterogeneous metal separation and enrichment processes are reviewed. 

The scope of the living cationic polymerizations and synthetic applications of these functionalized monomers will be treated in the next chapter on polymer synthesis (see Chapter 5, Section III.B). One should note that the feasibility of living processes for these polar monomers further attests to the formation of controlled and stabilized growing species. Conventional nonliving polymerizations, esters, ethers, and other nucleophiles are known to function as chain transfer agents and sometimes as terminators. In addition, the absence of other acid-catalyzed side reactions of the polar substituents, often sensitive to hydrolysis, acidolysis, etc., demonstrates that these polymerization systems are free from free protons that could arise either from incomplete initiation (via addition of protonic acids to monomer) or from chain transfer reactions (/3-proton elimination from the growing end). 

This characteristic feature of cationic polymerization of THF allows the important synthetic application of this process for preparation of oli-godiols used in polyurethane technology and in manufacturing of block copolymers with polyesters and polyamides (cf., Section IV.A). On the other hand, the cationic polymerization of THF not affected by contribution of chain transfer to polymer is a suitable model system for studying the mechanism and kinetics of cationic ring-opening polymerization. 

Equilibrium polymerization, which can be anionic or cationic, is utilized to convert cyclic organosiloxanes into polydiorganosiloxane polymer chains. In the chemical industry octamethylcyclotetrasiloxane is preferred as such, or as a mixture with other siloxanes for chain termination and/or production of copolymers for specific applications. Particularly indu.strially important is anionic polymerization with basic catalysts such as alkali hydroxides, whereby the activity falls off in the order Cs > Rb > K > Na > Li. KOH is most frequently used e.g. as a suspension in octamethylcyclotetrasiloxane at 140°C, the catalyst being active from a concentration of several ppm. According to the assumed mechanism of this catalytic process, potassium siloxanolate is initially formed, which leads to cleavage of the Si-O-Si bonds and chain formation ... 

It has been noted (Odian, 1991) that the steady-state approximation cannot be applied in many cationic polymerizations because of the extreme rate of reaction preventing the attainment of a steady-state concentration of the reactive intermediates. This places limitations on the usefulness of the rate expressions but those for the degree of polymerization rely on ratios of reaction rates and should be generally applicable. The molar-mass distribution would be expected to be very narrow and approach that for a living polymerization (with a poly-dispersity index of unity), but this is rarely achieved, due to the chain-transfer and termination reactions discussed above. Values closer to 2 are more likely. 

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