Acrylonitrile copolymerization solvent effects

In fact, recent theoreticaP and experimental studies of small radical addition reactions indicate that charge separation does occur in the transition state when highly electrophilic and nucleophilic species are involved. It is also known that copolymerization of electron donor-acceptor monomer pairs are solvent sensitive, although this solvent effect has in the past been attributed to other causes, such as a Bootstrap effect (see Section 13.2.3.4). Examples of this type include the copolymerization of styrene with maleic anhydride and with acrylonitrile. Hence, in these systems, the variation in reactivity ratios with the solvent may (at least in part) be caused by the variation of the polarity of the solvent. In any case, this type of solvent effect cannot be discounted, and should thus be considered when analyzing the copolymerization data of systems involving strongly electrophilic and nucleophilic monomer pairs. 

Solvents affect free-radical polymerization reactions in a number of different ways. Solvent can influence any of the elementary steps in the chain reaction process either chemically or physically. Some of these solvent effects are substantial, for instance, the influence of solvents on the gel effect and on the polymerization of acidic or basic monomers. In the specific case of copolymerization then solvents can influence transfer and propagation reactions via a number of different mechanisms. For some systems, such as styrene-acrylonitrile or styrene-maleic anhydride, the selection of an appropriate copolymerization model is still a matter of contention and it is likely that complicated copolymerization models, incorporating a number of different phenomena, are required to explain all experimental data. In any case, it does not appear that a single solvent effects model is capable of explaining the effect of solvents in all copolymerization systems, and model discrimination should thus be performed on a case-by-case basis. 

Copolymerization reactions are affected by solvents. One example that can be cited is an effect of addition of water or glacial acetic acid to a copolymerization mixture of methyl methacrylate with acrylamide in dimethyl sulfoxide or in chloroform. This causes changes in reactivity ratios [131]. Changes in r values that result from changes in solvents in copolymerizations of styrene with methyl methacrylate are another example [133, 134]. The same is true for styrene acrylonitrile copolymerization [132]. There are also some indications that the temperature may have some effect on the reactivity ratios [135], at least in some cases. 

It was reported by Barb in 1953 that solvents can affect the rates of copolymerization and the composition of the copolymer in copolymerizations of styrene with maleic anhydride [145]. Later, Klumperman also observed similar solvent effects [145]. This was reviewed by Coote and coworkers [145]. A number of complexation models were proposed to describe copolymerizations of styrene and maleic anhydride and styrene with acrylonitrile. There were explanations offered for deviation from the terminal model that assumes that radical reactivity only depends on the terminal unit of the growing chain. Thus, Harwood proposed the bootstrap model based upon the study of styrene copolymerized with MAA, acrylic acid, and acrylamide [146]. It was hypothesized that solvent does not modify the inherent reactivity of the growing radical, but affects the monomer partitioning such that the concentrations of the two monomers at the reactive site (and thus their ratio) differ from that in bulk. 

Copolymerizations of nonpolar monomers with polar monomers such as methyl methacrylate and acrylonitrile are especially comphcated. The effects of solvent and counterion may be unimportant compared to the side reactions characteristic of anionic polymerization of polar monomers (Sec. 5-3b-4). In addition, copolymerization is often hindered by the very low tendency of one of the cross-propagation reactions. For example, polystyryl anions easily add methyl methacrylate but there is little tendency for poly(methyl methacrylate) anions to add styrene. Many reports of styrene-methyl methacrylate (and similar comonomer pairs) copolymerizations are not copolymerizations in the sense discussed in this chapter. 

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. 

In some cases it is desirable to conduct a copolymerization in the presence of a solvent. Table III illustrates the effect of solvent concentration on the copolymerization of styrene and acrylonitrile. Higher concentrations of solvent produced a pronounced lowering of the rate without changing the molecular weight as measured by the solution viscosity of the copolymer. 

Both the polymerization rate and the composition of the copolymer also depend on the solvent. Solvents can determine the position of the complex equilibrium (see also Table 22-7). Thus, for example, the homopolymerization of a CT complex can be converted into a copolymerization of the CT complex with one of its two monomers, or even convert to a terpolymerization with both of its monomers when the solvent is changed. Such effects may, for example, be responsible besides the dilution effect for the variation in the acrylonitrile content of the terpolymer produced by the joint polymerization of acrylonitrile/p-dioxene/maleic anhydride when the kind and concentration of solvent used are changed (see Figure 22-10). 

Nitrile and Acrylic Rubber. Nitrile rubbers are made by the emulsion copolymerization of acrylonitrile (9-50%) and butadiene (21) and are abbreviated NBR (eq. 11). The ratio of acrylonitrile (ACN) to butadiene has a direct effect on the properties and the nature of the pol5nners. As the ACN content increases, the oil resistance of the poljnner increases (14). As the butadiene content increases, the low temperature properties of the polymer are improved. Nitrile rubber is much like SBR in its physical properties. It can be compoimded for physical strength and abrasion resistance using traditional fillers such as carbon black, silica, and reinforcing clays. The primary benefit of the polymer is its oil and solvent resistance. At a medium ACN content of 34% the volume swell in IRM 903 oil at 70°C is typically 25-30%. Nitrile rubber can be processed on conventional rubber equipment and can be compression, transfer, or injection molded. It can also be extruded easily. Nitrile rubber compoimds have good abrasion and water resistance. They can have compression set properties as low as 25% with the selection of a proper cure system. The temperature range for the elastomers is from -30 to 125°C. The compounds are also plasticized nsing polar ester plasticizers. 

Acrylic and methacrylic acids are common constituents in copolymer systems. If the copolymerizations are carried out in inert solvents, there are variations in reactivity ratios that are related to the solvent. One would also expect pH effects, if the processes were carried out in aqueous media. With comonomers that have limited solubilities in water, complications related to the distribution coefficients come into play. Some aspects of these situations in the copolymerization of acrylonitrile with methacrylic acid are reviewed in reference 31. 

The role of solvent alcohol in the photosensitized copolymerization or graft polymerization of styrene with cellulose has been investigated. Photo-induced grafting of poly(styrene-a/r-acrylonitrile) to cellulose yielded a product containing hetero- and homo-polymer chains. The effects of the matrix and concentration on the rate of reaction and the products have been investigated in persulphate-initiated grafting of poly(styrene-a/t-acrylonitrile) onto cellulose. ... 

Versions of the Bootstrap model have also been fitted to systems in which monomer-monomer complexes are known to be present, demonstrating that the Bootstrap model may provide an alternative to the MCP and MCD models in these systems. For instance, Klumperman and co-woikers have snccessfiilly fitted versions of the penultimate Bootstrap model to the systems styiene with maleic anhydride in butanone and toluene, " and styrene with acrylonitrile in varions solvents. This latter woik confirmed the earUer observations of Hill et alP for the behavior of styrene with aciylonitiile in bulk, acetonitrile and toluene. They had concluded that, based on sequence distribution data, penultimate unit effects were operating but, in addition, a Bootstrap effect was evident in the coexistent curves obtained when triad distribution was plotted against copolymer composition for each system. In the copolymerization of styrene with aciylonitiile Klumperman et alP a variable Bootstrap effect was required to model the data. Given the strong polarity effects expected in this system (see Section 12.2.2), part of this variation may in fact be caused by the variation of the solvent polarity and its affect on the reactivity ratios. In aity case, as this work indicates, it may be necessary to simultaneously consider a number of different influences (such as, for instance, penultimate unit effects. Bootstrap effects, and polarity effects) in order to model some copolymerization systems. 

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