Cationic polymerization chain transfer reaction

In general, the activation energies for both cationic and anionic polymerization are small. For this reason, low-temperature conditions are normally used to reduce side reactions. Low temperatures also minimize chain transfer reactions. These reactions produce low-molecular weight polymers by disproportionation of the propagating polymer ... 

Chain growth occurs through a nucleophilic attack of the carbanion on the monomer. As in cationic polymerizations, lower temperatures favor anionic polymerizations by minimizing branching due to chain transfer reactions. 

What species, in addition to a dead polymer, is produced in a chain transfer reaction with a macrocarbocation in cationic chain polymerization ... 

For PIB, a method resulting in end-reactive polymers, however, based on chain transfer reactions during polymerization, was addressed as the cationic inifer method (54). 

As discussed in the preceding sections of this chapter, the key to living cationic polymerization is to reduce the effect of chain transfer reactions (Scheme 4) because termination is much less important in the cationic polymerization of vinyl monomers. The primary reason for frequent chain transfer reactions of the growing carbocation (1) is the acidity of the /3-H atoms, next to the carbocationic center, where a considerable part of the positive charge is localized. Because of their electron deficiency, the protons can readily be abstracted by monomers, the counteranion (B ), and other basic components of the systems, to induce chain transfer reactions. It is particularly important to note that cationically polymerizable monomers are, by definition, basic or nucleophilic. Namely, they have an electron-rich carbon-carbon double bond that can be effectively poly-... 

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). 

The main advantages of cationic photoinitiators is that they have high reaction rates and require a low energy. They can operate at a low temperature, they are not inhibited by oxygen, they do not promote the polymerization of epoxy groups in the dark, and they are often stable at elevated temperatures. Some disadvantages exist that is, inhibition by bases, chain-transfer reaction by water, and the presence of acids in cured products. 

With regard to kinetics of chain breaking, the various chain-transfer reactions are analogous to those in free-radical polymerization (see Section 10.3.1) and need not be described again in detail. In cationic polymerization, the most common chain transfer is to monomer and leaves the polymer with a double bond while generating a monomeric chain carrier of the same structure as the original one [93]. 

For obtaining a cationic polymerization, the new carbocation generated between R and the monomer should have enough stability to be relatively easily formed and to continue the polymerization (for example, CH2=CH2 is not polymerized using a cationic initiator, while (CH3)2C=CH2 can be polymerized because the species RCH2-C(CH3)2 is stable enough to be formed). The stability of the carbocation increases as the chain length increases. Chain transfer reactions are common in carbocation polymerization. The termination reactions typically occur because of the combination of the cationic component with a counterion. 

Scheme 9.5 Chain-transfer reactions as side reactions in cationic polymerization...

For a long time, cationic polymerization has been considered to be very difficult to control to obtain polymers of narrow molecular-weight distribution. Usually, molecular weights are unpredictable and M /M is far from one. An active species at the propagating polymer end is a carbocation or an onium ion, which reacts with olefin monomers extremely rapidly. The active polymer end is also highly unstable and readily participates in chain-transfer reactions by loss of p-protons leading to uncontrollable molecular-weight distributions. 

In the cationic polymerization of styrene in aromatic solvents such as toluene, the chain transfer reaction to a solvent molecule usually occurs as follows 45-49... 

Cationic polymerization is initiated by acids such as perchloric acid, boron trifluoride, or aluminum trichloride. High molecular weight polystyrene is difficult to make cationically because of chain transfer reactions that occur with the monomer and with the commonly used solvents. Thus, molecular weight and molecular weight distributions can be controlled by the polymerization conditions and the method of polymerization. Examples of desired molecular weights are shown in Figure 2. 

While termination of chain growth in cationic polymerization may take place in various ways, many of the termination reactions are, in fact, chain transfer reactions in which the termination of growth of a propagating chain is accompanied by the generation of a new propagating species. 

Cationic polymerization can be terminated by loss of a proton or by addition of a nucleophile that reacts with the propagating site. The chain can also be terminated by a chain-transfer reaction with the solvent (XY). 

The chain can be terminated by a chain transfer reaction with the solvent or by reaction with an impurity in the reaction mixture. If the solvent cannot donate a proton to terminate the chain and if all impurities that can react with a carbanion are rigorously excluded, chain propagation will continue until all the monomer has been consumed. At this point, the propagating site will still be active, so the polymerization reaction will continue if more monomer is added to the system. Such nonterminated chains are called living polymers because the chains remain active until they are killed. Living polymers usually result from anionic polymerization because the chains caimot be terminated by proton loss from the polymer, as they can in cationic polymerization, or by disproportionation or radical recombination, as they can in radical polymerization. 

Analogously to water, alcohols (used in the form of polyols) also cause a chain-transfer reaction during the cationic polymerization of epoxides. Polyols (polyvalent alcohols with an average molecular weight from several hundred to several thousand grams/mole) are often utihzed in technical formulations. These serve to reduce the network density of the polymerized epoxide and therefore make the material less brittle. At the same time the reactivity also is influenced and the susceptibiHty of the polymerization rate and polymer properties to the influence of air humidity is reduced. This is important, as the influence of air humidity is difficult to reproduce in technical appHcations. 

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