Effect of Chain Transfer Reactions

Chain transfer reactions are considered here irrespective of the exact chemical mechanisms of transfer that are discussed in detail, for example, in the comprehensive review of Glasse [9]. It should be stressed that all calculations were performed bearing in mind anionic polymerization, so the main concern in all theoretical papers reviewed is the absence of kinetic termination rather than the anionic mechanism of chain propagation. Thus, the conclusions drawn are valid for all processes that satisfy this condition. 

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In the product, there should be a ladder-type blocks linked by segments composed of p-cresyl methacrylate units. This type of structure was confirmed by IR and NMR spectrometry. However, by preparation of such copolymers with labeled end-groups (using radioactive AIBN), and by fractionating and radiometric analysis, it was shown that copolymers obtained are slightly branched. There is slightly more branch points than in the case of copolymers with styrene. It could be an effect of chain transfer reaction. 

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-... 

Problem 6.17 What would be the effect of chain transfer reactions on the polymerization rate and polymer molecular weight in each of the following cases (Rudin, 1982 Odian, 1991) (a) kp s> ktT> kr — kp, (b)... 

The line of thought explained above is, however, not new and can be traced back to earlier papers where several people have dealt with the derivation of (approximate) closed expression for chain-length dependent kinetics. Mahabadi [151] was the first to explicitly postulate this simple identity between microscopic and macroscopic termination rate coefficient, later followed by more elaborate analyses by Olaj etal. [161, 162, 168]. Beside the geometric mean relation, Olaj s method has also been shown to hold for other power law relations for kt [174] and has been experimentally applied to styrene [175] and MMA [176]. Unfortunately, the effect of chain transfer reactions was ignored in their simulations. 

Studies of polymerization kinetics have a long history. Nevertheless, many problems in this field remain unresolved. Using computational methods, several of these problems are clarified in the chapter hy Litvinenko ( Computational Studies of Polymer Kinetics ). Special attention is paid to the effect of chain transfer reactions on polymer molecular weight and apphcations to different types of polymerization methods. 

It is apparent from these reactions how chain transfer lowers the molecular weight of a chain-growth polymer. The effect of chain transfer on the rate of polymerization depends on the rate at which the new radicals reinitiate polymerization ... 

The compound R X is a chain-transfer agent, with X usually H or Cl. The net effect of chain transfer is to kill a growing chain and start a new one in its place, thus shortening the chains. Mercaptan chain-transfer agents ate often used to limit molecular weight, but under appropriate conditions, almost anything in the reaction mass (solvent, dead polymer, initiator) can act as a chain-transfer agent to a certain extent. 

Bulk polymerisation is heterogeneous since the polymer is insoluble in the monomer. The reaction is autocatalysed by the presence of solid polymer whilst the concentration of initiator has little effect on the molecular weight. This is believed to be due to the overriding effect of monomer transfer reactions on the chain length. As in all vinyl chloride polymerisation oxygen has a profound inhibiting effect. 

The CCI3 radical usually goes on to start a new chain, so that there is no loss of radicals in the system, nor even any appreciable change in the rate of polymerization. The chief effect of such transfer reactions is found in the mean degree of polymerization. Since each transfer stops one chain and starts a new one, it will lower the mean chain length. Since the mean degree of polymerization n is the ratio of number of monomer units polymerized —d A)/dt to the number of polymer chains formed, we must modify our previous c(iuation (XVI. 10.9) to include chain transfer. If we assume termination by recombination... 

In Section 7.1 the effects of chain-transfer on polymerization rate were not considered. There is, however, evidence from the dependence of rate on monomer concentration to show that initiation reactions in olefin polymerization are slower than propagation. If the rate equation... 

If the effect of the transfer reactions and subsequent re-initiation of growing chains on monomer consumption and active centre concentration are taken into account the polymerization rate is given by [227]... 

Since chain transfer stops growing chain, it always results in a lower molecular weight than would occur in its absence. The effect of chain transfer on the rate of polymerization varies, however, and depends on the relative rates of the transfer [Eq. (6.135)] and reinitiation [Eq. (6.136)] compared to that of the normal propagation reaction [Eq. (6.6d)]. Several possible situations that may be encountered are sumniarized in Table 6.8. These are all instances of chain transfer, but they are usually given different names, as shown, depending on the net effects on polymerization rate and molecular weight. 

Equation (6.148), often referred to as the Mayo equation, shows the quantitative effect of various transfer reactions on the number average degree of polymerization. Note that the chain transfer constants, being ratios of the respective rate constants for chain transfer (Rtr) to the rate constant for propagation (kp), are dimensionless quantities dependent on the types of both the monomer and the material causing chain transfer as well as on the temperature of reaction. 

It should be noted that the occurrence of chain transfer reaction (6.158) does not change the number of monomer molecules which have been polymerized nor the number of polymer molecules over which they are distributed. Chain transfer to polymer thus has no effect on DP and so it is not included in Eq. (6.147). It, however, causes a change in the molecular weight distribution. The distribution becomes broader because the polymers which are already large are more likely to suffer transfer reactions and become yet bigger due to branching. 

This irreversible process affects the polymerization only if the rate of ionization of RX (i.e., ion-generation) is slow relative to that of propagation if initiation is instantaneous or very rapid the effect of chain transfer to initiator is negligible [1, 24, 45]. If the rate of ionization of HX is high relative to that of propagation, chain transfer to HX may be neglected. Since HM X and RMnX (see Scheme la) can be reactivated by the quasiliving equilibrium (see Sect. 2.1, and Scheme 1), chain transfer to initiator is by no means a termination reaction [see also 43]. 

In this article, two examples of the application ESR to conventional radical polymerizations, especially to both kinetics and mechanism, based on materials prepared by controlled/living radical polymerizations will be demonstrated. The first example is the estimation of the effect of chain length on propagating radicals. The second is the detection of chain transfer reactions on propagating radicals in the polymerization of rert-butyl acrylate. 

The effectiveness of a modifier depends on its chemical structure, concentration, temperature, and pressure. A concentration-independent measure for its effectiveness is the chain transfer constant, defined as the ratio of kinetic coefficients for the transfer reaction to this substance and radical chain propagation reaction. Usually the effectiveness of chain transfer agents is increased with rising temperature and reduced pressure. The chain transfer constant of modifiers falls from aldehydes, which are more effective than ketones or esters, to hydrocarbons. Unsaturated hydrocarbons typically have higher transfer constants than saturated hydrocarbons and a strong effect on polymer density must be considered because of the ability to copolymerize that give a higher frequency of short-chain branches in the polymer. 

On the contrary, it is suggested that in ionic liquid, the propagation of grafted chains from surface radicals formed by the thermal decomposition of azo groups effectively proceeded, because of stabilization of surface radicals and depression of chain transfer reaction. 

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