Copolymerizations, radical, reactivity

The parameters rj and T2 are the vehicles by which the nature of the reactants enter the copolymer composition equation. We shall call these radical reactivity ratios, although similarly defined ratios also describe copolymerizations that involve ionic intermediates. There are several important things to note about radical reactivity ratios ... 

GopolymeriZation. The importance of VDC as a monomer results from its abiHty to copolymerize with other vinyl monomers. Its Rvalue equals 0.22 and its e value equals 0.36. It most easily copolymerizes with acrylates, but it also reacts, more slowly, with other monomers, eg, styrene, that form highly resonance-stabiHzed radicals. Reactivity ratios (r and r, with various monomers are Hsted in Table 2. Many other copolymers have been prepared from monomers for which the reactivity ratios are not known. The commercially important copolymers include those with vinyl chloride (VC),... 

Table 2 shows characteristic reactivity ratios for selected free-radical, ionic, and coordination copolymerizations. The reactivity ratios predict only tendencies some copolymerization, and hence some modification of physical properties, can occur even if and/or T2 are somewhat unfavorable. For example, despite their dissimilar reactivity ratios, ethylene and propylene can be copolymerized to a useful elastomeric product by adjusting the monomer feed or by usiag a catalyst that iacreases the reactivity of propylene relative to ethylene. 

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. 

Another characteristic feature of ionic copolymerizations is the sensitivity of the monomer reactivity ratios to changes in the initiator, reaction medium, or temperature. This is quite different from the general behavior observed in radical copolymerization. Monomer reactivity ratios in radical copolymerization are far less dependent on reaction conditions. 

The copolymerization of macromonomer with comonomer is governed by the general rules of copolymerization, the ability of any of the two polymerizable species present to participate in the process being determined by the radical reactivity ratios r. Let us denote the macromonomer as M and the comonomer as A. The well-known instantaneous composition law applies to the copolymer formed ... 

Kennedy 67,77 118) studied the ability of w-styryl-polyisobutene macromonomers to undergo free-radical copolymerization with either styrene or butyl or methyl methacrylate. Here, the macromonomers exhibited a relatively high molecular weight of 9000, and the reaction was stopped after roughly 20% of the comonomer had been converted. The radical reactivity ratios of styrene and methyl methacrylate with respect to macromonomer were found to be equal to 2 and to 0.5, respectively. From these results, Kennedy concluded that in the ra-styrylpolyisobutene/styrene system the reactivity of the macromonomer double bond is reduced whereas with methacrylate as the comonomer the polar effect is the main driving force, yielding reactivities similar to those observed in the classical system styrene/MMA. 

Another series of papers [296-298] should be mentioned, where low-molecular model compounds are used to prove the correctness of the penultimate model of copolymerization. Japanese scientists by means of the ESP-method [297-298] managed to observe a noticeable penultimate effect for the acrylate radical reactivity. 

These results suggest that of the two copolymerization parameters, and e, only the e values are aflFected appreciably by an increase in pressure. Calculations have been made obtaining relative Q and e values which bear out this thesis. The decreasing radical selectivity found in these experiments implies an increased radical reactivity. This offers further support to Walling s suggestion that the ally acetate radical increases in reactivity with increasing pressure. 

These results, of course, mean that the use of high pressures allows one to carry out copolymerizations which do not occur readily at atmospheric pressure because of widely different monomer-radical reactivities. For example, at several thousand atmospheres styrene and vinyl acetate, which will not appreciably copolymerize at 1 atm. (10), may be made to give copolymers at a reasonable rate. 

In a copolymerization of styrene and methyl methacrylate under CCT conditions, the fraction of unsaturated styrene end groups is proportional to the fraction of styrene in the monomer feed.368 Due to the stability of styrene radicals, the relative fraction of propagating styrene radicals is large over the whole range of monomer feed compositions.432 433 This feature complicates the determination of radical reactivity ratios but may be compensated for by measuring the average transfer rate coefficient as a function of monomer feed composition. 

All the above factors controlling monomer and radical reactivities contribute to the rate of polymerization, but in a manner which makes it difficult to distinguish the magnitude of each effect. Attempts to correlate copolymerization tendencies based on these factors are thus mainly of a semiempirical nature and can, at best, be treated as useful approximations rather than rigorous relations. However, a generally useful scheme was proposed by Alfrey and Price [23] to provide a quantitative description of the behavior of diferent monomers in radical polymerization, with the aid of two parameters, for each monomer rather than for a monomer pair. These parameters are denoted by Q and e and the method has been called the Q — e scheme. It allows calculation of monomer reactivity ratios r and T2 from properties of monomers irrespective of which pair is used. The scheme assumes that each radical or monomer can be classified according to its reactivity or resonance effect and its polarity so that the rate constant... 

Compilations of reactivity ratios for various pairs of monomers in radical polymerization have been provided by Eastmond [131] and Odian [132], The reactivity ratios for pairs of given monomers can be very different for the different types of chain-growth copolymerization radical, anionic, cationic, and coordination copolymerization. Although the copolymer equation is valid for each of them, the copolymer composition can depend strongly on the mode of initiation (see Figure 11.8). 

The analysis of copolymer composition in Section 4.6.4.1 has been carried out using the terminal model which assumes that radical reactivity is solely determined by the terminal unit on the free-radical chain. The terminal model has been successfully applied to represent the monomer and copolymer compositions for a wide variety of systems, mostly studied at ambient pressure. This model is, however, not capable of describing both copolymer composition and copolymerization kinetics with a single set of reactivity ratios [63]. 

The majority of copolymerization systems studied so far can by represented well by the implicit penultimate unit effect (IPUE) model, where the two radical reactivity ratios, Si = kau/km and S2 - ki22/ 222. are introduced as additional parameters, to account for the influence of the penultimate unit on homopropagation. Within the IPUE model, no penultimate unit effect is considered for the reactivity ratios r = r2i and r2i = rx2- Despite the remarkable... 

Reactivity of vinyl monomers is very often determined experimentally by studying copolymerizations. Values of many free-radical reactivity ratios have been tabulated for many different monomer pairs,... 

A terminal radical-complex model for copolymerization was formulated by Kamachi. He proposed that a complex is formed between the propagating radical chain and the solvent (which may be the monomer) and that this complexed radical has a different propagation rate constant to the equivalent uncomplexed radical. Under these conditions there are eight different propagation reactions in a binary copolymerization, assuming that the terminal unit is the only unit of the chain affecting the radical reactivity. These are as follows. 

In order to identify the chemical transformations due to the copolymerization, the reactivity of linseed oil with a radical initiator was studied and compared with the known behaviour of this oil in the usual drying process in the presence of air and light [16,26]. 

FRP leads to the formation of statistical copolymers, where the arrangement of monomers within the chains is dictated purely by kinetic factors. The most common treatment of free-radical copolymerization kinetics assumes that radical reactivity depends only on the identity of the terminal unit on the growing chain. The assumption provides a good representation of polymer composition and sequence distribution, but not necessarily polymerization rate, as discussed later. This terminal model is widely used to model free-radical copolymerization according to the set of mechanisms in Scheme 3.11. 

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