The Inhomogeneity of Copolymers with Respect to Composition

Abstract

The inhomogeneity of composition (i.c.) which arises in copolymerisation not only influences the physical and physicochemical properties of the copolymers formed but also, when it is correctly assessed, makes it possible to arrive at certain conclusions concerning the copolymerisation mechanism. The review deals mainly with binary statistical copolymerisation. A critical analysis is given of the classical Mayo–Lewis concept, according to which the copolymerisation constants remain unchanged throughout the process. Regarding such systems as normal or standard, the authors have derived expressions for their distribution functions with respect to composition. Further it is noted that the case where r1 and r2 are constant is more likely to be an exception rather than the rule and possible causes of the departure from such constancy are indicated. The principles of the derivation of the integral and differential distribution functions with respect to composition for copolymerising systems are discussed in detail. Analytical expressions are proposed for numerical estimates of inhomogeneity with respect to composition. In their statistical significance the expressions obtained resemble equations of different degrees governing the molecular weight ratios in homopolymerisation. The deviations from the norm in the copolymerisation of ionogenic monomers in aqueous solution are considered as an example of the application of these expressions. In this case the copolymerisation constants depend on the conversion due to the changes in the degree of ionisation of the bound monomers compared with the free monomers and due to the overall change in ionic strength. In conclusion a critical analysis is given of the existing experimental methods for the determination of inhomogeneity of composition and the prospects for the further development of the statistical theory of copolymerisation are discussed. The bibliography includes 111 references.