Solvent Effect on Homopropagation Rate Constants for VAc at

The heterogeneity of the reaction medium is also important in determining the molecular weight and in solution polymerization of macromonomers. The magnitude of the effect varies according to the solvent quality. PS macromonomer chains in good solvents e.g. toluene) have an extended confonnation whereas in poor solvents (e.g. methylcyclohexane) chains are tightly coiled. As a consequence, the radical center may see an environment tliat is medium dependent (see also Sections 7.6.5 and 8.3.7). 

Tacticity of MAA is influenced by solvent, the presence of amines (Table 

3) and complexation. PMAA appears more isotactic when formed in a non-bydrogen-bonding solvent. Polymerization of MAA in CHCl in the presence of 14 or 15 also yields a more isotactic polymer. Polymerization of zinc complexes of MAA also yields more isotaclic polymers.  

The effects of solvent on radical copolymerization are mentioned in a number of reviews.For copolymerizations involving monomers that arc ionizablc or form hydrogen bonds (AM, MAM, HEA, HEMA, MAA, etc.) solvent effects on reactivity ratios can be dramatic. Some data for MAA-MMA copolymerization are 

For MMA-MAA copolymerizations carried out in the more hydrophobic solvents (toluene, dioxane), MAA is the more reactive towards both propagating species while in water MMA is the more reactive. In solvents of intermediate polarity (alcohols, dipolar aprotic solvents), there is a tendency towards alternation. For these systems, ehoiee of solvent eould offer a means of controlling copolymer structure. 

For copolymerizations between non protie monomers solvent effects are less marked. Indeed, early work concluded that the reactivity ratios in copolymerizations involving only non-protic monomers (eg. S, MMA, AN, VAe, etc.) should show no solvent dependence.100101 More recent studies on these and other systems (e.g. AN-S,102-105 E-VAc,106 MAN-S,107 MMA-S,10s ° MMA-VAc1 ) indicate small yet significant solvent effects (some recent data for AN-S copolymerization are shown in Table 8.5). However, the origin of the solvent effect in these cases is not clear. There have been various attempts to rationalize solvent effects on copolymerization by establishing correlations between radical reactivity and various solvent and monomer properties.71,72 97 99 None has been entirely successful. 

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Table 8.1 Solvent Effect on Homopropagation Rate Constants for VAc at 30°C7 f...

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