User talk:MatChem121

Monomers

Monomer scope for cationic polymerization is limited to two main types: olefins and heterocyclic monomers. Cationic polymerization of both types of monomers occurs only if the overall reaction is thermally favorable. In the case of olefins this is due to isomerization of the monomer double bond and for heterocycles this is due to release of monomer ring strain and in some cases isomerization of repeating units. Monomers for cationic polymerization are nucleophilic and upon polymerization form a stable cation.3

Olefins

Cationic polymerization of olefin monomers occur with olefins that contain electron-donating aubstituents. 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. Some reactive olefin monomers are shown below in order of increasing reactivity with heteroatom groups being more reactive then alkyl or aryl groups. Note though that the reactivity of the carbenium ion formed is opposite of the monomer reactivity.3

Heterocyclic monomers

Heterocyclic monomers that are cationically polymerized are lactones, lactams, and cyclic amines. Upon addition of an initiatior cyclic monomers go on to form linear polymers. The reactivity of heterocyclic monomers depends on their ring strain. Monomers with large ring strain such as oxirane are more reactive then 1,3-dioxepane which has consideriable less ring strain. 6-membered rings and larger are less likely to polymerize due to this lower ring strain and must be carried out at low temperatures and usually capped to prevent depolymerization.5

Initiation

Initiation is the first step in cationic polymerization. During initiation, a carbenium ion is generated from which the polymer chain is made. The counterion should be non-nucleophilic otherwise the reaction is terminated instantaneously. There are a variety of initiators available for cationic polymerization and some of them require a co-initiator to generate the needed cationic species.

Classical protonic acids/acid surfaces

Strong protic acids can be used to form a cationic initiating species. 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 as to not cause early termination upon combination with the protonated olefin. Common acids used are phosphoric, sulfuric, fluro-, and triflic acids. Only formation of low molecular weight polymers are formed with these initiators.

Lewis acids/Friedel-Crafts catalysis

Lewis acids are the most common compounds used for initiation of cationic polymerization. The more popular lewis acids are SnCl₄, AlCl₃, BF₃, and TiCl₄. Although these lewis acids alone are able to induce polymerization they occur much faster with a suitable carbenium ion source. The carbenium ion source can be water, alcohols, or even a carbocation donor such as an ester or an anhydride. In these systems the lewis acid is referred to as a coinitiator while the carbenium ion source is the initiator. Upon reaction of the initiator with the coinitiator an intermediate complex is formed which then goes on to react with the monomer unit. The counterion produced by the initiator-coinitiator complex is less nucleophilic than that of the bronstead acid A^- counterion. Halogens such as chlorine, bromine and iodine can also initiate cationic polymerization upon addition of the more active lewis acids.

Carbenium ion salts

Stable carbenium ions are used to initiate chain growth of only the most reactive olefins and give well defined structures. These initiators are most often used in kinetic studies due to the ease of being able to measure the disappearance of the carbenium ions absorbance. Common carbenium ions are trityl and tropylium cations.

Ionizing Radiation

Ionizing radiation can form a radical-cation pair that can then react with a monomer to start cationic polymerization. Control of the radical-cation pairs are difficult and often depend on the monomer and reaction conditions. Formations of radical and anionic species are often observed.

Living Polymerization

In 1984 Higashimura and Sawamoto reported the first living cationic polymerization for alkyl vinyl ethers. This type of polymerization has allowed for the control of well defined polymers. A key characteristic of living cationic polymerization is that termination is essentially eliminated and thus the cationic chain growth continues until all monomer is consumed.1

Commercial Applications

The largest commercial application of cationic polymerization is for the production of polyisobutylene (PIB) products which includes polybutene and butyl rubber. These polymers have a variety of applications from adhesives and sealants to protective gloves and pharmaceutical stoppers. The reaction conditions for the synthesis of each type of isobutylene product vary depending on the desired molecular weight and what type(s) of monomer(s) is used. The conditions used most often to form low-molecular-weight (5-10 x 104 Da) polyiosbutylene are initiation with AlCl3, BF3, or TiCl4 at a temperature range of -40 to 10 ºC.4 These low-molecular-weight polyisobutylene polymers are used as sealants and for caulking. High-molecular-weight PIBs are synthesized at much lower temperatures of -100 to -90 ºC and in a polar medium of methylene chloride.3 These polymers are used to make uncrosslinked rubber products and are additives for certain thermoplasts. Another characteristic of high-molecular weight PIB is its low toxicity which allows it to be used as a base for chewing gum. The main chemical companies that produce polyisobutylene are Esso (ExxonMobil) and BASF.2

Butyl rubber in contrast to PIB is a copolymer in which monomers isobutylene (~98%) and isoprene (2%) are polymerized in a process similar to high-molecular weight PIBs. Butyl rubber polymerization is carried out as a continuous process with AlCl3 as the initiator. 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 was started during World War II and today roughly 1 billion pounds/year are produced in the U.S.4

Polybutene is another copolymers this time containing roughly 80% isobutylene and 20% other butenes (usually 1-butene). The production of these low-molecular weight polymers (300-2500 Da) are done between a large range of temperatures (-45 to 80 ºC) with AlCl3 or BF3. Depending on the molecular weight of these polymers they can be used as adhesives, sealents, plasticizers, additives for transition fluids and a variety of other applications. These materials are low-costing and made by a variety of different companies including BP chemicals, Esso, and BASF.3

Other polymers formed by cationic polymerization are homopolymers and copolymers of polyterpenes such as pinenes (plant-derived product) that are used as tackyfiers. In the field of heterocycles 1,3,5-trioxane is copolymerized with small amounts of ethylene oxide to form the highly crystalline polyoxymethylene plastic. Also, the homopolymerization of alkyl vinyl ethers are only obtained by cationic polymerization.4