Propagation constants methacrylic derivatives

The elementary rate constants were calculated from ratio kp/kt, obtained from the polymerization rate and initiation rate and the ratio kp/kt, estimated from the lifetime of the radical determined by the rotating sector method. The mean lifetime of the propagating radical and derived rate constants for methacrylates are shown in Tables 7—8. The variation of the propagation rate constant for methyl methacrylate with solvents is in accordance with the result obtained by Bamford et al.2 at 25 °C. Since the largest and the smallest kp value for phenyl methacrylate differ by a factor of 1.6 and for methyl methacrylate by a factor of 1.4, the estimation of the rate constants must be performed under experimental conditions in which the accumulated error is so small as to permit a distinction of the difference. Therefore, particular attention was given to the constancy of the reaction temperature ( 0.001 °C), constancy of light source, purity of monomers and solvents, and reproducibility of observed values and to the retention of the square wave in the rotat-... 

The copolymerization equation is valid if all propagation steps are irreversible. If reversibility occurs, a more complex equation can be derived. If the equilibrium constants depend on the length of the monomer sequence (penultimate effect), further changes must be introduced into the equations. Where the polymerization is subjected to an equilibrium, a-methylstyrene was chosen as monomer. The polymerization was carried out by radical initiation. With methyl methacrylate as comonomer the equilibrium constants are found to be independent of the sequence length. Between 100° and 150°C the reversibilities of the homopolymerization step of methyl methacrylate and of the alternating steps are taken into account. With acrylonitrile as comonomer the dependence of equilibrium constants on the length of sequence must be considered. 

Table 7. Mean lifetime of the propagating radical of phenyl methacrylate and derived rate constants in various solvents at 30 °C15 ...

These equations are similar to those assumed for the reactivity ratio determination. In contrast to what has been observed for conventional styrene-MMA copolymers, however, these equations indicate that a substantial proportion of the (SMM+MMS)-type resonance appears to occur in the C-area. The proportion of methoxy resonance observed in the C-area, in fact, exceeds P(SMS) by a substantial amount for many of the copolymers. This can be due to the assumption of an inadequate model for the copolymerization reaction, to the use of incorrect reactivity ratios and cyclization constants for the calculations or to an inadequate understanding of the methoxy proton resonance patterns of S/MMA copolymers. It is possible that intramolecular reactions between propagating radicals and uncyclized methacrylic anhydride units present on propagating chains result in the formation of macrocycles. Failure to account for the formation of macrocycles would result in overestimation of rc and rc and in underestimation of the proportions of MMA units in SMS triads in the derived S./MMA copolymers. This might account for the results obtained. An alternate possibility is that a high proportion (>50%) of the M-M placements in the copolymers studied in this work can be expected to have meso placements (], J2), whereas only a small proportion of such placements ( 20%) are meso in conventional S/MMA copolymers. Studies with molecular models (20) have indicated that the methoxy protons on MMA units centered in structures such as the following can experience appreciable shielding by next nearest styrene units. 

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