Cationic polymerization
[2][3] The types of monomers necessary for cationic polymerization are limited to alkenes with electron-donating substituents and heterocycles.
Specifically, the ability of a solvent to form free ions will dictate the reactivity of the propagating cationic chain.
These electron-donating groups make the olefin nucleophilic enough to attack electrophilic initiators or growing polymer chains.
At the same time, these electron-donating groups attached to the monomer must be able to stabilize the resulting cationic charge for further polymerization.
[5] Heterocyclic monomers that are cationically polymerized are lactones, lactams and cyclic amines.
High concentrations of the acid are needed in order to produce sufficient quantities of the cationic species.
The counterion (A−) produced must be weakly nucleophilic so as to prevent early termination due to combination with the protonated alkene.
Although these Lewis acids alone are able to induce polymerization, the reaction occurs much faster with a suitable cation source.
Upon reaction of the initiator with the coinitiator, an intermediate complex is formed which then goes on to react with the monomer unit.
Halogens, such as chlorine and bromine, can also initiate cationic polymerization upon addition of the more active Lewis acids.
[2] Stable carbenium ions are used to initiate chain growth of only the most reactive alkenes and are known to give well defined structures.
These initiators are most often used in kinetic studies due to the ease of measuring the disappearance of the carbenium ion absorbance.
[5] Ionizing radiation can form a radical-cation pair that can then react with a monomer to start cationic polymerization.
[5][6] As the temperature is raised, the energy barrier for the termination reaction is overcome, causing shorter chains to be produced during the polymerization process.
[2][6] The association is strongest as a covalent bond and weakest when the pair exists as free ions.
However, in this polymerization reaction, the monomer units are cyclic in comparison to the resulting polymer chains which are linear.
[5][6][12] For simplicity, counterions are not shown in the above reaction equations and only chain transfer to monomer is considered.
becomes[6][12] In 1984, Higashimura and Sawamoto reported the first living cationic polymerization for alkyl vinyl ethers.
These polymers have a variety of applications from adhesives and sealants to protective gloves and pharmaceutical stoppers.
The conditions most commonly used to form low molecular weight (5–10 x 104 Da) polyisobutylene are initiation with AlCl3, BF3, or TiCl4 at a temperature range of −40 to 10 °C.
[2] High molecular weight PIBs are synthesized at much lower temperatures of −100 to −90 °C and in a polar medium of methylene chloride.
Another characteristic of high molecular weight PIB is its low toxicity which allows it to be used as a base for chewing gum.
Its low gas permeability and good resistance to chemicals and aging make it useful for a variety of applications such as protective gloves, electrical cable insulation, and even basketballs.
Large scale production of butyl rubber started during World War II, and roughly 1 billion pounds/year are produced in the U.S.
The production of these low molecular weight polymers (300–2500 Da) is done within a large range of temperatures (−45 to 80 °C) with AlCl3 or BF3.
Depending on the molecular weight of these polymers, they can be used as adhesives, sealants, plasticizers, additives for transmission fluids, and a variety of other applications.
These materials are low-cost and are made by a variety of different companies including BP Chemicals, Esso, and BASF.
[5] Other polymers formed by cationic polymerization are homopolymers and copolymers of polyterpenes, such as pinenes (plant-derived products), that are used as tackifiers.
In the field of heterocycles, 1,3,5-trioxane is copolymerized with small amounts of ethylene oxide to form the highly crystalline polyoxymethylene plastic.
