Copolymerization various monomer pairs

Reactivity ratios of a number of activated acrylates and methacrylates with different structural monomers are given in Table 2. Polymer compositions produced at low conversions from equimolar monomer fe compositions are also indicated in the Table. These values are back calculated from the reactivity ratios, and are hence equivalent to experimental data. The actual copolymeriz-ability patterns of three comonomer pairs are also shown in Fig. 4. These results show that, among the various monomer pairs, AOTcp with styrene, and AOTcp with 7V-vinylpyrrolidone, produce azotropic copolymers at about equimolar monomer feed compositions. The relatively small reactivity ratios of these two monomer pairs indicate that their equimolar copolymerization produces approximately alternating, rather than random or block, copolymers. 

The equilibrium constants for the charge-transfer complexes of MA with benzofuran (BF), benzothiophene (BT), and indole (table in appendix to this chapter) follow the order MA-BT > MA-indole MA-BF. This suggests that the copolymerization rates should follow the same order. This is not the case time-conversion curves show copolymerization rates of the three monomer pairs to be MA-BF > MA-BT > MA-indole, The same sequence was also found for maximum conversions at infinite time. The results suggest clearly that the reactivity of the comonomers to form copolymers with MA is predominately governed by resonance stabilization of the various monomer pairs. However, Goethals et suggest that the copolymeriz-... 

Another factor which can affect the soluble fraction is the relative reactivities of acrylic acid or sodium acrylate with the crosslinkers. As evidenced by reactivity ratios of various monomer pairs, methacrylate esters tend to be more reactive with acrylic acid and acrylate esters less reactive with acrylic acid. In a crosslinking copolymerization therefore, a methacrylate crosslinker will tend to be used up earlier in the reaction than will an acrylate crosslinker. As a result, use of methacrylate crosslinkers tends to give crosslinked polyacrylates with higher extractable fraction. 

Various mechanisms have been proposed to explain the initiation processes. The self-initiated copolymerizations of the monomer pairs S-MMA and S-AN proceed at substantially faster rates than pure S polymerization. For S-AN333 and S-MAHJJ the mechanism of initiation was proposed to be analogous to that of S homopolymerization (Scheme 3.62) but with acrylonitrile acting as the dicnophile in the formation of the Diels-Alder adduct (Scheme 3.66). 

A parallel situation is encountered for the copolymerization of 1,3-butadiene with isoprene. McGrath et al. 251) have shown that in homopolymerizations, under equivalent conditions, isoprene exhibits a rate constant which is more than five times larger than that observed for butadiene. However, butadiene is favored in the copolymeriza-tion. The available reactivity ratios for various diene and styrenyl monomer pairs in hydrocarbon solvents are listed in Table 24. 

Copolymerization studies demonstrated that aminimide, 1,1-dimethyl-l-(2-hydroxypropyl)amine methacrylimide (DHA) copolymerizes readily with 4-vinylpyridine (4VP) and N-vinylpyr-rolidone (NVP). These copolymers could he thermolyzed in solution to give soluble poly(4-vinylpyridine-co-isopropenyl isocyanate) and poly(N-vinylpyrrolidone-co-isopropenyl isocyanate) materials. The reactivity ratios of each monomer pair were determined, and the Alfrey-Price Q and e values for DHA were calculated for DHA (Mt)-4VP (M2), r, = 0.41, r = 0.77, Q = 0.68, and e = 0.58 and for DHA (Mt)-NVP (Mg), r2 = 0.15, r2 = 0.35, Q = 0.14, and e = 0.58. The DHA-4VP copolymers quaternized readily to give a new family of water-soluble polyelectrolyte materials. The various copolymers were examined as adhesion promoters for rubber-tire cord composites. 

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. 

Such contradictory behavior suggests that the copolymerization parameters are not only determined by the relative reactivities of the free macroions, but also by other parameters such as, on the one hand, the equilibria between free ions and the various ion pairs, and, on the other hand, the equilibria between one of the monomers and one of the growing macroions. Conventionally determined copolymerization parameters only provide mean reactivities if ion pairs are present. If a macroion-monomer equilibrium occurs, then the rate-determining step is the further reaction of the intermediary product, i.e.,... 

Copolymerizing the styrene-MA monomer pair with various organometallic monomers, such as trimethyl, triethyl, or tributylmethacryloxy-stannane or vinyltriethoxysilane, in benzene at 60 C with BPO, provides... 

Using molecular orbital concepts,the 7r-extension model, and the various transition states in the copolymerization process, it was clearly established that the cross-propagation reactions were very much preferred over the homopropagation reactions.The rr-extension model was shown to be sufficient to account for the alternating copolymerization of the monomer pair to give 11. ° It would be of value to find out if the rr-extension model could also be used to account for other alternating copolymerizations of MA with electron-donor monomers. 

The monomers 4-vinylcyclohexene and 1,5,9-cyclododecantriene have also been copolymerized with MA. Copolymerization in both cases were run in benzene at 60°C with AIBN, using various molar ratios of the two different monomer pairs. In the case of 4-vinylcyclohexene, the low-molecular-weight copolymers for each run was essentially a 2 3 molar composition of vinyl-cyclohexene to MA. Copolymerization of 1,5,9-cyclo-dodecatriene with MA produced insoluble material. Copolymer composition studies showed these materials tended to be alternating. 

A number of additional studies have since shown that copolymerization of various mixtures of this monomer pair can also be achieved with standard free-radical initiatorsor ultraviolet radiationto obtain essentially 1 1 copolymer. 

Instractive example of the copolymerization involving monomer propagating reversibly comes from the L,L-lactide (LA)/ecopolymerization system, by means of the numerical integration method [183], revealed that the comonomers reactivity ratios can be controlled by the configuration of the active species [184]. Thus, using initiator of various stereochemical compositions a broad spectrum of copolymers... 

Various attempts have been made to place the radical-monomer reaction on a quantitative basis in terms of correlating structure with reactivity. Success in this area would give a better understanding of copolymerization behavior and allow the prediction of the monomer reactivity ratios for comonomer pairs that have not yet been copolymerized. A useful correlation is the Q-e scheme of Alfrey and Price [1947], who proposed that the rate constant for a radical-monomer reaction, for example, for the reaction of Mp radical with M2 monomer, be written as... 

Consider the following monomer reactivity ratios for the copolymerization of various pairs of monomers ... 

Statistical copolymerization occurs among ethylene and various a-olefins [Baldwin and Ver Strate, 1972 Cooper, 1976 Pasquon et al., 1967 Randall, 1978]. The reactivities of monomers in copolymerization generally parallel their homopolymerization behavior ethylene > propene > 1-butene > 1-hexene [Soga et al., 1989]. Table 8-7 shows monomer reactivity ratios for several comonomer pairs. 

A mixture of two monomers that can be homopo-lymerized by a metal catalyst can be copolymerized as in conventional radical systems. In fact, various pairs of methacrylates, acrylates, and styrenes have been copolymerized by the metal catalysts in random or statistical fashion, and the copolymerizations appear to also have the characteristics of a living process. The monomer reactivity ratio and sequence distributions of the comonomer units, as discussed already, seem very similar to those in the conventional free radical systems, although the detailed analysis should be awaited as described above. Apart from the mechanistic study (section II.F.3), the metal-catalyzed systems afford random or statistical copolymers of controlled molecular weights and sharp MWDs, where, because of the living nature, there are almost no differences in composition distribution in each copolymer chain in a single sample, in sharp contrast to conventional random copolymers, in which there is a considerable compositional distribution from chain to chain. Figure 26 shows the random copolymers thus prepared by the metal-catalyzed living radical polymerizations. 

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). 

We have given here an extensive treatment of the copolymerization with reversibility of propagation, because this phenomenon is still insufficiently recognized and there are still papers in which reversibility should have been considered schemes that ignore reversibility lead to wrong conclusions. Thus the r values given in these papers do not have the meaning they pretend to have, and various authors have obtained different values for the same pair of monomers for the supposed-to-be reactivity ratios 98). 

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