Copolymerization polarity effects

For conventional free-radical copolymerizations, polar effects of growing polymer radicals on the approaching monomer is expressed by the Alfrey-Price Q — e scheme, where the copolymerization tendency, i.e., product of monomer reactivity ratios, may be expressed, Eq. (20), in terms of e values. 

Alfrey and Price proposed a means of predicting monomer reactivity in copolymerization from two parameters, (a measure of resonance) and e (a measure of polar effects) (8). These parameters have been related to the reactivity ratios by equations 15—17. 

Steric effects similar to those in radical copolymerization are also operative in cationic copolymerizations. Table 6-9 shows the effect of methyl substituents in the a- and 11-positions of styrene. Reactivity is increased by the a-methyl substituent because of its electron-donating power. The decreased reactivity of P-methylstyrene relative to styrene indicates that the steric effect of the P-substituent outweighs its polar effect of increasing the electron density on the double bond. Furthermore, the tranx-fl-methylstyrene appears to be more reactive than the cis isomer, although the difference is much less than in radical copolymerization (Sec. 6-3b-2). It is worth noting that 1,2-disubstituted alkenes have finite r values in cationic copolymerization compared to the values of zero in radical copolymerization (Table 6-2). There is a tendency for 1,2-disubstituted alkenes to self-propagate in cationic copolymerization, although this tendency is low in the radical reaction. 

Because the addition steps are generally fast and consequently exothermic chain steps, their transition states should occur early on the reaction coordinate and therefore resemble the starting alkene. This was recently confirmed by ab initio calculations for the attack at ethylene by methyl radicals and fluorene atoms. The relative stability of the adduct radicals therefore should have little influence on reacti-vity 2 ). The analysis of reactivity and regioselectivity for radical addition reactions, however, is even more complex, because polar effects seem to have an important influence. It has been known for some time that electronegative radicals X-prefer to react with ordinary alkenes while nucleophilic alkyl or acyl radicals rather attack electron deficient olefins e.g., cyano or carbonyl substituted olefins The best known example for this behavior is copolymerization This view was supported by different MO-calculation procedures and in particular by the successful FMO-treatment of the regioselectivity and relative reactivity of additions of radicals to a series of alkenes An excellent review of most of the more recent experimental data and their interpretation was published recently by Tedder and... 

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. 

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. 

Penultimate and similar kinetic models are used nowadays principally for the treatment of the experimental data, obtained from the copolymerization of certain monomers like fumaronitrile or maleic anhydride which are characterized by rather strong steric and polar effects. 

Soga and co-workers examined the relation between the Hammett s a value of each substituent and the reactivities in copolymerization (Figure 17.8). It is observed that the monomer reactivity is enhanced by electron-releasing substituents in the aromatic ring. Even />-ter/-butylstyrene with a substituent of large steric hindrance shows a high reactivity. This indicates that there is a strong polar effect of the substituent on the rate of addition [28]. Similar effects were observed by Ishihara et al. in the homopolymerization of these monomers [27]. 

The high polar group tolerance of co-catalyst-free ylide nickel catalysts makes them interesting candidates for fhe polymerization of polar monomers. In fact, quite a number of polar vinyl monomers can be homo- and copolymerized quite effectively. The mechanisms of initiation and chain propagation have not been elucidated yet. Especially, acrylic monomers are well suited. It is fhus possible to produce, for example, poly(methyl methacrylate), poly(efhyl acrylate) and poly-(butyl acrylate) in high yield [Eq. (15)]. 

Problem 7.9 Explain the following copolymerization results or observed behaviors considering the influence of resonance, steric, and polar effects on monomer reactivity ... 

In practice, (f) can be calculated by inserting experimental copolymerization rates into Eq. (7.64). The values of (j> thus obtained are frequently greater than unity, and these deviations are ascribed to polar effects that favor cross-termination over homotermination. However, this is not always unambiguous, since the apparent cross-termination factor may vary with monomer feed composition in a given system [25,26]. It is clear also that termination reactions are at least partially diffusion controlled [27,28]. A dependence of segmental diffusivity on the structure of macroradicals is to be expected and dependence of diffusion controlled termination on copolymer composition seems reasonable. It is therefore plausible that the value of the overall termination rate constant ku in copolymerizations should be functions of fractions F and Fi) of the comonomers incorporated in the copolymer. An empirical expression for ku has thus been proposed [27] ... 

Considering resonance and polarity effects what type of monomers would you choose to copolymerize with vinyl ethers ... 

In the g - e scheme, Q characterizes the resonance stabilization of a monomer during copolymerization and e is the factor reflecting the degree of the polar effect of substituents at a multiple bond. These parameters are associated with the constants of relative activity of monomers by the known... 

Broadly speaking, an efficient copolymerization tends to take place when the comonomers are either both reactive or both relatively unreactive, but not when one is reactive and the other umeactive. As with most generalizations, this is rather an extreme statement and cannot be treated too rigorously, especially when one realizes that resonance is not the only factor contributing to copolymerization behavior, and that both steric and polar effects have to be considered. 

The extreme case of azeotropic copolymerization is rA = rn = 0 and always produces perfectly alternating copolymers, irrespective of the value of /a (i.e. Fa = 0.50 for 0 < /a < 1), because the homopiopagation reactions do not occur (see Figure 1.2). An example is the alternating copolymerization of stilbene with maleic anhydride, a copolymerization which also demonstrates that strong polar effects can overcome the inhibiting effects of steric hindrance in polymerization of 1,2-disubstituted monomers. 

Traditionally, catalysts for polyolefin production are based on early transition metal complexes, which are highly oxophilic. A point of concern for polymerization in SCCO2 is the compatibiHty of the catalyst with the mildly acidic CO2. The acidity of CO2 poisons the early transition metal catalysts used for conventional olefin polymerizations, and therefore these catalysts cannot be used in SCCO2. Since late transition metal-based catalysts are less sensitive to hetero-atom functional groups [10-12], they are more likely to be effective polymerization catalysts in SCCO2. Additionally, these catalysts are interesting for their high tolerance toward impurities and their ability to copolymerize polar monomers [12-14]. 

According to this equation, the dielectric constant of the solvent is expected to influence the copolymerization. In actual fact, such an effect is very seldom observed. An exception is found, for example, in the copolymerization of vinyl monomers with oxygen to polyperoxides. Thus, equation (22-46) is not applicable in the general case, probably because the partial charges are not localized at all. In addition, the polarity effects are small in comparison to the resonance effects (see Section 22.4.1). 

Cross termination n. In free radical copolymerization, termination by reaction of two radicals terminated by monomer units of the opposite type, i.e., termination, by combination or disproportionation with rate constant /cab Crosstermination is often favored over termination by reaction between two like radicals due to polar effects. 

Copolymerization. Acrylic and methacrylic acids readily copolymerize free radically with many vinyl monomers. This versatility results from a combination of their highly reactive double bonds and their miscibility with a wide variety of water- and solvent-soluble monomers. Reactivity ratios derived from copolymerizations with many monomers are tabulated in many books on polymerization, for example in Wiley s Polymer Handbook (14) (see also Wiley s Database of Polymer Properties). Q and e values are parameters that may be established for a monomer based on a large number of reactivity ratios with other monomers. These parameters are associated with interactions between the monomer and the growing chain via resonance (Q) and polar effects (e). 

Polarity effects can also result in solvent effects in free-radical copolymerization. Recent theoretical studies (43,55-57) of small-radical addition reactions suggest that in a wide range of cross-propagation reactions, the transition structure is stabilized by the contribution of charge-transfer configurations. When this is the case, the extent of stabilization (and hence the propagation rate) will be... 

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