Mechanism of copolymerization

The various copolymerization models that appear in the literature (terminal, penultimate, complex dissociation, complex participation, etc.) should not be considered as alternative descriptions. They are approximations made through necessity to reduce complexity. They should, at best, be considered as a subset of some overall scheme for copolymerization. Any unified theory, if such is possible, would have to take into account all of the factors mentioned above. The models used to describe copolymerization reaction mechanisms arc normally chosen to be the simplest possible model capable of explaining a given set of experimental data. They do not necessarily provide, nor are they meant to be, a complete description of the mechanism. Much of the impetus for model development and drive for understanding of the mechanism of copolymerization conies from the need to predict composition and rates. Developments in models have followed the development and application of analytical techniques that demonstrate the inadequacy of an earlier model. 

Although the mechanism of copolymerization is similar to that discussed for the polymerization of one reactant (homopolymerization), the reactivities of monomers may differ when more than one is present in the feed, i.e., reaction mixture. Copolymers may be produced by step-reaction or by chain reaction polymerization. It is important to note that if the reactant species are Mi and M2, then the composition of the copolymer is not a physical mixture or blend, though the topic of blends will be dealt with in this chapter. 

From the above studies, it is clear that a copolymer of butadiene and styrene can be prepared from lithium-nitrogen bond initiators. The styrene content of the copolymer is highly dependent on the type of initiator and the polymerization conversion. These lithium-nitrogen bond initiators do not yield a randomized copolymer even with the presence of built-in polar modifier. This may be due to the heterogeneous nature of the initiator. In order to understand the mechanism of copolymerization with lithium-nitrogen bonded initiator. More work along these lines is needed. 

A comparison of the three binary copolymerizations of 1,6-anhydro-/S-D-gluco-, -galacto-, and -manno-pyranose derivatives gives some insight into the mechanism of copolymerization, if it is assumed on this evidence that the per-p-xylyl and perbenzyl derivatives can be used interchangeably.107... 

Our attention was concentrated mainly on the kinetics and mechanisms of copolymerization, the effect of epoxide and anhydride structure in copolymerization, the effect of type and structure of initiatior on the rate, and the course of copolymerization. The probable mechanisms are discussed. The copolymerization in the presence of proton-donor compounds as well as the effect of proton-donors are also considered. For a better understanding of the processes, data and theoretical views on the non-catalyzed reaction are included. 

In this paper, we try to review results obtained from anionic copolymerization of cyclic ethers with cyclic anhydrides. For a better understanding data and theoretical views on non-catalyzed copolymerizations are also included. We concentrate mainly on the kinetics and mechanism of copolymerization and the effect of the type and character of the initiator used. The influence of the epoxide and anhydride structure on copolymerization, of proton donors on the rate and course of copolymerization, and on the molecular weight of the resulting polyesters are also discussed. 

Therefore, a similarity between the mechanisms of copolymerization of epoxides with anhydrides initiated either by phosphonium salts or alkali metal or ammonium salts can be expected and copolymerization then proceeds according to Hamann s mechanism illustrated by Eqs. (16M23). 

Another mechanism of copolymerization was reported by Feltzin et al.73). These authors also considered activation of tertiary amine by a compound with active hydrogen but with the formation of a quaternary ammonium base (Eq. (45)), being regarded as the real initiator of copolymerization. Initiation, propagation and termination reactions according to Feltzin are illustrated by schemes (46)-(49). 

In our opinion, the area of this type of copolymerization is still open to discussion and research, and this suggestion concerns mainly the mechanism of copolymerization, formation of active centres in the initiation by Lewis bases, the influence of proton donors on the course of copolymerization, and tbe effect of the structure of the initiator on the rate of copolymerization. It is necessary, however, to study the copolymerization on model compounds. 

An investigation into the initiation mechanism of copolymerization of ethyl vinyl ether and acrylonitrile by /-butoxyl radicals lias shown that the reaction between the two monomers competes successfully with radical trapping by the nitroxide radical trap (5).37 The /-butoxyl radicals react 3-6 times faster with ethyl vinyl ether than acrylonitrile the authors proposed that this is due to selective interaction of one monomer with the radical species rather than a solvent polarity effect. 

The measured data of the polymerization rate using a molar ratio of ethene/ propene = 1 1 are four times higher than the calculated data. A clear increase in activity by the comonomer is observed. The results of the sequence analysis of the copolymer samples suggest no change in the mechanism of copolymerization. One explanation for this effect lies in the increase in the insertion rate due to an electronic influence of the comonomer. 

The mechanism of particle formation at submicellar surfactant concentrations was established several years ago. New insight was gained into how the structure of surfactants influences the outcome of the reaction. The gap between suspension and emulsion polymerization was bridged. The mode of popularly used redox catalysts was clarified, and completely novel catalyst systems were developed. For non-styrene-like monomers, such as vinyl chloride and vinyl acetate, the kinetic picture was elucidated. Advances were made in determining the mechanism of copolymerization, in particular the effects of water-soluble monomers and of difunctional monomers. The reaction mechanism in flow-through reactors became as well understood as in batch reactors. Computer techniques clarified complex mechanisms. The study of emulsion polymerization in nonaqueous media opened new vistas. 

Scheme 18 Plausible mechanism of copolymerization of (a) monosubstituted and (b) 1,1-disubstituted allenes with CO by Rh complexes...

Scheme 19 Mechanism of copolymerization of 2-phenyl-1-methylenecyclopropane with CO by Pd complexes...

The possible relationship between the rate and degree of aggregation cannot be ignored. The differences in aggregation may also be the key to the mechanism of copolymerization. The so-called "inversion" behavior in copolymerizations of diene and styrene may well be caused by the differences in degrees of aggregation, which in turn, control the cross-propagation rates. 

Several studies on the reactivities of small radicals with donor-acceptor monomer pairs have been carried out to provide insight into the mechanism of copolymerizations of donor-acceptor pairs. Tirrell and coworkers " reported on the reaction of n-butyl radicals with mixtures of N-phcnylmalcimidc and various donor monomers e.g. S, 2-chloroethyl vinyl ether),. lenkins and coworkers have examined the reaction of t-butoxy radicals with mixtures of AN and VAc. Both groups have examined the S-AN system (see also Section 7.3.1.2). In each of these donor-acceptor systems only simple (one monomer) adducts are observed. Incorporation of monomers as pairs is not an important pathway i.e. the complex participation model is not applicable). Furthermore, the product mixtures can be predicted on the basis of what is observed in single monomer experiments. The reactivity of the individual monomers (towards initiating radicals) is unaffected by the presence of the other monomer i.e. the complex dissociation model is not applicable). Unless propagating species are shown to behave differently, these results suggest that neither the complex participation nor complex dissociation models apply in these systems. 

Copolymerization of ionic-type MCMs, alkali and alkaline earth metal salts, with various comonomers has been studied quite well. A detailed review of the basic studies has been given in a monograph [4] and they will not be analyzed in this book. Studies of the mechanism of copolymerization of such types of MCM were carried out in the case of organotin and -lead monomers. The widest recognition among these MCMs used as comonomers has been gained by trimethyl-, tributyl-, and triphenyltin methacrylates (Table 4-7) [4 and references therein]. 

Determining the composition and sequence of comonomer units is essential in the case of copolymers, since both parameters influence the physical and chemical properties of these materials. Furthermore, comonomer sequence is related to the mechanism of copolymerization, and to the reactivity ratios of the comonomers. Among the techniques developed for polymer characterization. Mass Spectrometry (MS) is one of the most powerful. The mass spectrum of a polymer contains plenty of information on polymer properties such as the structure, the repeat units which constitute the mac-romolecular backbone, the length of macromolecular chains, the end groups which terminate the chains, the chemical heterogeneity, the sequence of copolymers and their composition heterogeneity. 

In constrast, for synthetic copolymers (with 2,3, or more components), tire sequence of comonomer units present in each copolymer chain is no more constant. Furthermore, we can define only an average sequence lengtir of alike monomers for these syntiretic materials. Chain statistics quantities, such as the probability matrices (average number of identical repeating units) that are related to composition, to mechanism of copolymerization, and to reactivity ratios are useful in order to deal wifh fhis average copolymer sequence. 

The Refs. [190-199] are devoted to the problem of synthesis of statistical copolymers of polysulfones, production of mixes on the basis of polysulfones and study of their properties as well to the mechanism of copolymerization. 

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