Cationic Polymerization
As mentioned, in cationic polymerizations the reactive portions of the chain ends carry positive charges during the process of chain growth. These active centers can be either unpaired cations or they can be cations that are paired and associated closely with anions (counterions). The initiations result from transpositions of electrons, either as pair or as single one. A two-electron transposition takes place when the initiating species are either protons , or carbocations. Ion generations take place through heterolytic bond cleavages or through dissociations of cationic precursors. Such initiating systems include Lewis acid/Bronsted acid combinations, Bronsted acids by themselves, stable cationic salts, some organometallic compounds, and some cation forming substances. When, however, initiations take place by one electron transposition, they occur as a direct result of oxidation of free radicals. They can also take place through electron transfer interactions involving electron donor monomers. The carbon cations can form from olefins in a variety of ways. One way is through direct additions of cations, like protons, or other positively charged species to the olefins. The products are secondary or tertiary carbon cations:
When the cations add to conjugated dienes, the charges can be distributed over several centers in the products
The charge may also be distributed in polar monomers like, for instance, in vinyl ethers:
Chemical considerations indicate that the more diffuse the charges the more stable are the ions. Cationic polymerizations are not affected by common inhibitors of free-radical polymerizations. They can , however be greatly influenced by impurities that can act as ion scavengers .These can be water, ammonia, amines, or any other compounds that can be basic in character, affecting rates and molecular weights of the products. Typical cationic polymerization sproceed at high rates even at low temperatures, as low as-100C[8]. In the literature one can find many reports of cationic polymerizations of many different monomers with many different initiators. Often, however, such initiators are quite specific for individual monomers and their activities are strongly influenced by the solvents. The initiation processes can be summarized as follows:
I. Initiation by chemical methods
A. Two electron transposition (heterolytic) initiation by reactions with:
1. Protonic acids
2. Lewis acids
3. Stable carbon cations
4. Certain metal alkyls
5. Cation forming substances
B. One electron transposition (homolytic) initiations by reactions of:
1. Direct oxidation of radicals
2. Cation-radicals formed through charge-transfers
II. Initiation by physical methods
A. Photochemically generated cations
B. High-energy irradiation C. Electro initiation
