From classical to living/controlled statistical free-radical copolymerization

Abstract

Solvent effects on monomer reactivity ratio values along with the determination of radical reactivity ratios, their possible origins as well as their importance in conventional free-radical copolymerization is described first. Predictions from data obtained at low conversion of cumulative composition and stereochemical distribution in copolymer chain along with glass transition temperature and the lumped overall copolymerization parameter $\text{k}{}_{\mathrm{<mtext>p</mtext>}}\text{/}\text{k}{}_{\mathrm{<mtext>t</mtext>}}{}^{\mathrm{1/2}}$ at high conversion are described. Transfer reactions in free-radical copolymerization using classical chain transfer agent as well as the new catalytic chain transfer agent and the effect on the copolymer structure and properties are also studied. Finally the new living/controlled free-radical polymerization process and the preparation of well-controlled statistical copolymers are described to stand out the subtle mechanisms differences between living/controlled and conventional free-radical copolymerization.

Introduction

Free-radical copolymerization reactions are important from both practical and theoretical point of view. Theoretically, it is interesting to focus on the effect of the chemical structure on the reactivity of monomers and radicals. Practically, radical copolymerization is the most recommendable process to prepare an unlimited number of polymer compounds by changing the nature and relative proportions of monomers in the reaction medium.

The kinetics and mechanisms involved in the free-radical copolymerization of vinyl monomers have been a research topic of interest for many years. The copolymerization of two monomers, A and B, give rise to the formation of a copolymer chain having composition and sequence distribution, which are dependent on the relative proportion of applied monomers as well as the monomers and radical reactivities. Depending on the species considered different important kinetic models have been developed.

A great impulse of the copolymerization studies emerges from the Symposium on “Free Radical Polymerization: Kinetics and Mechanism” held in Santa Margherite Ligure, Italy, May 1987. Sessions on copolymerization are devoted to point out the importance of the copolymer composition, model discrimination and copolymerization rate.

In the field of free-radical copolymerization, great emphasis has been always put on copolymer composition, appropriately considered as the most important aspect of the polymerization reaction, since it influences the properties of the resultant materials. Then, it is important to estimate the monomer reactivity ratios in a statistical correct manner. O'Driscoll and Reilly [1] based on the Mayo-Lewis Terminal Model (MLTM) analyze different linear and nonlinear regression approaches to estimate the monomer reactivity ratios.

Although the estimation of parameters is important, the copolymer composition is often inadequate to discriminate among models such as MLTM, penultimate unit model (PUM) or complex participation model (CPM). Hill and O'Donnell [2] described a general method based on the nonlinear least-square analysis of copolymer composition or sequence distribution variation with the monomer feed composition. The latter is much more sensitive to probe the mechanism, which best represents a particular copolymerization.

Though before the Santa Margherite Ligure Congress (SMLC) the copolymerization rate had been much less studied, its relevance was well recognized since accurate kinetic modeling, suitable to describe the whole course of the copolymerization reaction, was necessary for any reaction engineering scheme. Russo [3] analyzed the initiation, propagation and termination step in copolymerization. The termination step can be described on the basis of chemical control [4], [5], diffusion control [6] and the segmental diffusion model [7]. Furthermore, Russo's lecture include the first results obtained by Fukuda et al. [8], [9] in which the termination step is well fit by chemical or ideal diffusion models and the propagation step does not follow the terminal model, but seems to fulfill the PUM. Furthermore analyzing the propagation step, Russo [3] pointed out that according to literature data the values of monomer reactivity ratios are, in some cases, dependent on the reaction medium. Consequently all the quantities dependent on the reactivity ratios in these cases will also be dependent on the reaction medium, e.g. copolymer composition, average propagation rate constant, etc.

Harwood [10] resolved these apparent anomalies by comparing the copolymer microstructure as a function of copolymer composition instead of comparing the copolymer microstructure as a function of monomer feed composition: the ‘bootstrap model’ emerged.

Considering that the previous knowledge on copolymerization reaction until 1987 is collected in references and the Lectures of SMLC, this review starts from the data with the new development in conventional copolymerization, such as solvent effects on monomer reactivity ratios, radical reactivity ratios, high conversion reaction and transfer reaction in copolymerization. Furthermore, considering the new living/controlled processes of radical polymerization, which allows preparing polymers with controlled architecture, narrow molecular weight distributions (MWD) and predetermined molecular weights, a general overview on the mechanism and a description of the statistical copolymers obtained from different systems will be also described.

It is also necessary to have in mind the previous and well documented reviews [11], [12], [13], [14]. Therefore, the present review does not try to give an exhaustive list of references, but simply to raise a general overview of the current main developments in the statistical free-radical copolymerization.