Monomers solvent effects

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. 

Photoinitiation of polymerization of MMA and styrene by Mn(facac)3 was also investigated, and it was shown that the mechanism of photoinitiation is different [33] from that of Mn(acac)3 and is subject to the marked solvent effect, being less efficient in benzene than in ethyl acetate solutions. The mechanism shown in Schemes (15) and (16) illustrate the photodecomposition scheme of Mn(facac)3 in monomer-ethyl acetate and monomer-benzene solutions, respectively. (C = manganese chelate complex.)... 

One final point should be made. The observation of significant solvent effects on kp in homopolymerization and on reactivity ratios in copolymerization (Section 8.3.1) calls into question the methods for reactivity ratio measurement which rely on evaluation of the polymer composition for various monomer feed ratios (Section 7.3.2). If solvent effects arc significant, it would seem to follow that reactivity ratios in bulk copolymerization should be a function of the feed composition.138 Moreover, since the reaction medium alters with conversion, the reactivity ratios may also vary with conversion. Thus the two most common sources of data used in reactivity ratio determination (i.e. low conversion composition measurements and composition conversion measurements) are potentially flawed. A corollary of this statement also provides one explanation for any failure of reactivity ratios to predict copolymer composition at high conversion. The effect of solvents on radical copolymerization remains an area in need of further research. 

Very large solvent effects arc also observed for systems where the monomers can aggregate either with themselves or another species. For example, the apparent kp for polymerizable surfactants, such as certain vinyl pyridinium salts and alkyl salts of dimethylaminoalkyl methacrylates, in aqueous solution above the critical micelle concentration (cmc) are dramatically higher than they are below the cmc in water or in non-aqueous media.77 This docs not mean that the value for the kp is higher. The heterogeneity of the medium needs to be considered. In the micellar system, the effective concentration of double bonds in the vicinity of the... 

The effects of solvent on radical copolymerization are mentioned in a number of reviews.69 72 97,98 For copolymerizations involving monomers that arc ionizablc or form hydrogen bonds (AM, MAM, HEA, HEMA, MAA, etc.) solvent effects on reactivity ratios can be dramatic. Some data for MAA-MMA copolymerization are shown in Table 8.4.w... 

The solvent in a bulk copolymerization comprises the monomers. The nature of the solvent will necessarily change with conversion from monomers to a mixture of monomers and polymers, and, in most cases, the ratio of monomers in the feed will also vary with conversion. For S-AN copolymerization, since the reactivity ratios are different in toluene and in acetonitrile, we should anticipate that the reactivity ratios are different in bulk copolymerizations when the monomer mix is either mostly AN or mostly S. This calls into question the usual method of measuring reactivity ratios by examining the copolymer composition for various monomer feed compositions at very low monomer conversion. We can note that reactivity ratios can be estimated for a single monomer feed composition by analyzing the monomer sequence distribution. Analysis of the dependence of reactivity ratios determined in this manner of monomer feed ratio should therefore provide evidence for solvent effects. These considerations should not be ignored in solution polymerization either. 

Studies on the reactions of small model radicals with monomers provide indirect support but do not prove the bootstrap effect.111 Krstina et ahL i showed that the reactivities of MMA and MAN model radicals towards MMA, S and VAc were independent of solvent. However, small but significant solvent effects on reactivity ratios are reported for MMA/VAc111 and MMA S 7 copolymerizations. For the model systems, where there is no polymer coil to solvate, there should be no bootstrap effect and reactivities are determined by the global monomer ratio [Ma0]/[Mb0].1j1... 

Solvent Effects in the Sn Spectra of Poly(TBTM/MMA). Samples of poly(MMA/TBTM) synthesized by the free-radical copolymerization of the appropriate monomers were solutions in benzene with approximately 33% solids (weight to volume). The particular formulation chosen as representative of the class contained a 1 1 ratio of pendant methyl to tri-n-butyltin groups. In preparing the dry polymer, the benzene was removed in vacuo with nominally 5% by weight residual solvent. 

Reference Monomer Catalyst Temperature °C Solvent Effect of HX... 

Solutions in CH2C12, for which the complexing by the solvent is so strong, that the monomer is effectively excluded from the solvation shell of the cation, and therefore the propagation becomes predominantly a bimolecular reaction, i.e., the of (4.1) is negligible after a very small degree of dilution. 

Auto-acceleration was observed in the homopolymerization of methacrylic acid solutions over limited concentration ranges in methanol and in water. Perhaps under such conditions swelling of the polymer favors monomer diffusion leading to a larger amount of pre-oriented structures III. Alternatively, a monomer-solvent complex may arise which favors a pre-oriented structure and thus, may be responsible for the onset of a matrix effect (9). 

Apart from the chemical effects of solvent as hydrogen donor, being heterogeneous systems, an important role of solvent is to provide appropriate reaction sites on the balance of solvent interactions with base polymer and growing graft chain. For the study of solvent effects, AM is not an adequate monomer since it is scarcely soluble in non-polar solvents. Instead, acrylic acid(AA) was employed and photografting was conducted as shown in Figure 8. 

Ropson N, Dubois P, Jerome R, Teyssie P (1995) Macromolecular engineering of polylactones and polylactides. 20. Effect of monomer, solvent, and initiator on the ringopening polymerization as initiated with aluminum alkoxides. Macromolecules 28 7589-7598... 

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