Cross termination

The resulting prepolymer can then be chain extended with water, glycols or amines by linking cross terminal isocyanate groups (Figure 27.4). 

Unsymmetrical azo-compounds find application as initiators of polymerization in special circumstances, for example, as initiators of living radical polymerization [e.g. triphenylmethylazobenzene (30) (see 9.3.4)], as hydroxy radical sources [e.g. a-hydroperoxydiazene (31) (see 3.3.3,1)1, for enhanced solubility in organic solvents [e.g. f-butylazocyclohexanecarbonitrile (32)J, or as high temperature initiators [e.g. t-butylazoformamide (33)]. They have also been used as radical precursors in model studies of cross-termination in copolymerization (Section... 

In evaluating the kinetics of copolymerization according to the chemical control model, it is assumed that the termination rate constants k,AA and A,Br are known from studies on homopolymerization. The only unknown in the above expression is the rate constant for cross termination (AtAB)- The rate constant for this reaction in relation to klAA and kmB is given by the parameter . 

However, recent work based on the assumption of the implicit penultimate model suggests a value of 0 for S-MMA copolymeri/ation to be in the range 2-3.3"161 T his value is in remarkably good agreement with that suggested by experiments with simple model radicals. These experiments also indicate that cross termination is 2-3 times faster than either homotermination reaction (Section 7.4.3.1). 

The rate constants for the cross termination terms are approximated as the geometric mean of the corresponding homotermination terms. Thus ... 

Even when only the terminal monomer unit is considered, radical-radical termination in binary copolymerization involves at least seven separate reactions (Scheme 7.12). There are two homoterminalion processes and one cross termination process to consider. In the case of cross termination, there arc two pathways for disproportionation. There are then at least three pieces of information to be gained ... 

Perhaps because of this complexity, few studies on determining kld/ktt, in cross termination in copolymerization have been reported and most of the available data come from model studies, it is also usually assumed, without specific justification, that penultimate unit effects are unimportant in determining which reactions occur and that values of k klt for the homotermination reactions are similar to those in the corresponding homopolymerizations. 

A second approach has been to use an unsymmetrical initiator which allows the two radicals of interest to be generated simultaneously in equimolar amounts.175 In this case, analysis of the cage recombination products provides information on cross termination uncomplicated by homotermination. Analysis of products of the encounter reaction can also give information on the relative importance of cross and homotermination. However, copolymerization of unsaturated products can cause severe analytical problems. 

Both S polymerization initiated by AlBMe176 180 (i.e. PS + 4) and MMA polymerization initiated by 1 J -azobis-l-phenylethane176 (i.e. PMMA + 1-phenylethyl radical) are reported lo give predominantly combination. Ito,7e has concluded that cross termination is not particularly favored over homotermination in S-MMA copolymerization. 

Several experimental studies on S-MMA copolymcrization have appeared all suggest predominant combination.181" 83 Ohtani el al.m analyzed the end groups of PSMMA (60°C, AIBN, chloroform) by pyrolysis-gas chromatography to find values for the number of end groups per molecule of between 1.56-1.77 (increasing with polymer M ) which corresponds lo ail overall kjkw of between 0.39 and 0.21. Estimation of kjktc for cross termination requires knowledge of the... 

Analysis of the products from the thermal decomposition of the mixed azo compound 6 showed that in the cross-reaction of radicals 5 and 7 ld/A tt(90oC) is 0.61.179 This study also found that in disproportionation, hydrogen transfer from 5 to 7 is ca 2.2 times more frequent than transfer from 7 to 5. Both self-reactions involve predominantly combination (Scheme 7.14). The values of Ar1j/Aru.(80°C) are 0.16 and 0.05 for radicals 5 (Section 5.2.2.1.1) and 7 (Section 5.2.2.1.3) respectively. It is clear that values of kJkK for homotermination cannot be used as a guide to the value for kjkyt in cross-termination. 

The value of ,<)// ,- 80°C) in the cross-reaction between radicals 4 and 8 has been examined.175 This system is a model for cross-termination in MMA-BMA eopolymeri/alion. The value of kjkw (1.22) is similar to that found for the self-reaction of 8(1.17) and much larger than that for the self-reaction of 4 (0.78). There is a small preference (m 1.4 fold) for the transfer of hydrogen from the butyl ester (8) to the methyl ester (4). 

The polymerizations (a) and (b) owe their success to what has become known as the persistent radical effect."1 Simply stated when a transient radical and a persistent radical are simultaneously generated, the cross reaction between the transient and persistent radicals will be favored over self-reaction of the transient radical. Self-reaction of the transient radicals leads to a build up in the concentration of the persistent species w hich favors cross termination with the persistent radical over homotermination. The hoinolermination reaction is thus self-suppressing. The effect can be generalized to a persistent species effect to embrace ATRP and other mechanisms mentioned in Sections 9.3 and 9.4. Many aspects of the kinetics of the processes discussed under (a) and (b) are similar,21 the difference being that (b) involves a bimolecular activation process. 

A characteristic of free radicals is the bimolecular radical-radical reaction which in many cases proceeds at the diffusion-controlled limit. These radical-radical reactions can occur either between two identical radicals or between unlike radicals, the two processes being known as self-termination and cross-termination reactions, respectively. 

The H and 13C CIDNP studies have shown that not only the sulfone 14, but also the sulfmic ester 15, is generated as cage products from the phenyl/p-toluenesulfonyl radical pair during the thermal decomposition of phenylazo aryl sulfone (13)54 (Scheme 2). The cross-termination of arenesulfonyl and triethylgermyl radicals was found to occur exclusively via the formation of germyl sulfinate, ArS(0)0GeEt333. 

If cross termination reactions represented by (65) were to assume importance, the rate of consumption of inhibitor would not be independent of its concentration. The almost universal observation of proportionality between the length of the inhibition period and the amount of inhibitor available confirms our expectation that such processes are unimportant. 

Table XXIII.—Cross Termination Rate Ratios...

Actually, /r should be compared with . On this basis the greater sensitivity of the cross termination reaction is even more marked. 

Although the basic mechanisms are generally agreed on, the difficult part of the model development is to provide the model with the rate constants, physical properties and other model parameters needed for computation. For copolymerizations, there is only meager data available, particularly for cross-termination rate constants and Trommsdorff effects. In the development of our computer model, the considerable data available on relative homopolymerization rates of various monomers, relative propagation rates in copolymerization, and decomposition rates of many initiators were used. They were combined with various assumptions regarding Trommsdorff effects, cross termination constants and initiator efficiencies, to come up with a computer model flexible enough to treat quantitatively the polymerization processes of interest to us. 

The last reaction occurs much rapidly than the disproportionation of two cumylperoxyl radicals and accelerates chain termination in oxidized cumene [15]. The addition of cumene hydroperoxide helps to avoid the influence of the cross termination reaction Me2PhCOO + CH302 on the oxidation of cumene and to measure the pure disproportionation of cumylperoxyl radicals [15]. 

In addition to cross-propagation reactions, one cross-termination reaction is introduced in addition to two self-termination reactions, viz,... 

Equation (5.10) can be used for the additional estimation of the values of the rate constants 2ktn (cross-termination) and kexc (exchange between R 02 and R OOH) from experimental measurements. The values of kexc are given in Table 5.3. 

The values of 2kt for secondary peroxyl radicals lies in the range 106 10s L mol-1 s 1 and that of tertiary peroxyl radicals are sufficiently less, i.e., in the range of 103—105 L mol-1 s 1. Cross-termination by the reaction of tert-R02 + vcr-R02 predominantly occurs according to the mechanism of disproportionation of secondary peroxyl radicals. 

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