Chain transfer substituent effects

Since transition states for insertion and chain transfer are all more or less central, approximately half of the substituent effect will be felt at the transition state. Thus, insertion of propene (a more stabilized olefin) has a higher barrier (by about 2 kcal/mol) than insertion of ethene stabilization is partly lost at the transition state (Fig. 4). 

The results show that the presence of bulky substituent on a polymer chain may effectively inhibit the termination proceeding by this mechanism. The results presented at this point may be summarized as follows chain transfer to polymer is a general feature of cationic ring-opening polymerization although for different systems the contribution of this reaction may vary only in some systems this process results in termination (These systems involve, e.g., cyclic amines (3- and 4-membered) and cyclic sulfides (3- and 4-membered) and the contribution of the reaction is reduced for substituted chains. 

As with cobaloximes, substituents on the equatorial ligand have only a moderate effect on the value of Cc for the complexes in Table 3. The same is true for substituents on cobalt porphyrins, 1 and 45—51 (Table 4). For tetrakis(pentafluoroethylphenyl)-porphyrin—Co11 the substituent effect is not clear. The fluorinated porphyrin works moderately for the polymerization of MMA in supercritical C02 with chain-transfer constant Cc = 550 at 60 °C.126 Unfortunately, no data on the chain-transfer constant in bulk polymerization are available, so that it is not clear whether this reduced value of Cc is the result of solvent or the presence of a strong EWG such as pentafluorophenyl in the porphyrin macrocycle. Similar experiments with 9c (Table 2) led to Cc = 378 000, which is 20 times higher than in bulk MMA or in organic solvents.30 We may conclude at this point that additional experiments are required with different catalysts to allow us to make reliable conclusions. 

Porphyrin molecules provide a good example of this effect.144 In Table 7, a clear connection between the size of the ortho substituents and the chain-transfer constant, Cc, is observed. The electronic effects of the substituents are less important because methyl and chloro have similar Cc but significantly different electronic effects. According to X-ray data, porphyrins such as 80 are so restricted in rotation of the phenyl rings that four atropo-isomers can be easily separated by preparative chromatography. 

Quantum chemistry thus provides an invaluable tool for studying the mechanism and kinetics of free-radical polymerization, and should be seen as an important complement to experimental procedures. Already quantum chemical studies have made major contributions to our understanding of free-radical copolymerization kinetics, where they have provided direct evidence for the importance of penultimate imit effects (1,2). They have also helped in our understanding of substituent and chain-length effects on the frequency factors of propagation and transfer reactions (2-5). More recently, quantum chemical calculations have been used to provide an insight into the kinetics of the reversible addition fragmentation chain transfer (RAFT) polymerization process (6,7). For a more detailed introduction to quantum chemistry, the interested reader is referred to several excellent textbooks (8-16). 

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