M.B. Lachinov

On the gel-effect mechanism in radical polymerization☆

Abstract An explanation of the gel-effect is proposed based on a representation of the change of the type of diffusion in macromolecules in moderately concentrated solution with increasing conversion by taking into account the non-equivalence of contacts of macromolecules of different length. Attention was mainly devoted to the threshold of gel-effect since it can be analyzed separately from other stages. The dependence of this threshold on the macromolecular mass distribution was considered for polymer being formed up to the onset of gel-effect. The conditions estimated from the suggested model for onset of gel-effect correspond with the experimental data. On the basis of this model some conclusions are discussed concerning auto-acceleration in systems containing polymer with bimodal distribution, which can be used for experimental verification and correcting the model.

Radical polymerization of β-[N,N-dimethyl-N-(β-methacryloyloxyethyl)] propiobetaine in aqueous solutions

The kinetic features of the radical polymerization of an unsaturated β-betaine, namely, β-[N,N-dimethyl-N-(β-methacryloyloxyethyl)] propiobetaine, have been studied in aqueous solutions in the pH range 2–6. The polymerization rate is found to pass through an extreme value as a function of pH and this is caused by a change in the conformational condition of the propagating chains. It has been shown that in the pH region in which the monomer molecules and also the links of the propagating chains are positively charged, chain propagation occurs principally at macroradicals with terminal ion pairs.

Synthesis of block copolymers by non-fissile radical polymerization in the presence of o-phosphoric acid☆

It was shown that during polymerization of methyl methacrylate (MMA) or methyl α-chloracrylate (MCA) in the presence of o-phosphoric acid, complex forming agent, long life radicals are formed at room temperature in an external homogenous system, which after excluding the source of initiation, can effect non-fissile polymerization. If, after all the monomer had reacted, further monomer is added, the reaction continues as a result of extension of macroradicals formed. MMA-MCA block copolymers were sythesized. Block copolymers were obtained both by grafting MCA on PMMA macroradicals and by grafting MMA on PMCA macroradicals. Fractionation, sedimentation and turbidimetric titration showed that the polymers synthesized only contain two types of macromolecule, one of which is a block copolymer and the other PEC is observed over a sufficiently wide composition range 2.5<PVPB:PMMA<5, and does not involve dissociation of PEC into components. The mechanism of formation of water-soluble polycomplexes is considered, and a model of such polymcomplexes is suggested.

On kinetic regularities of the radical polymerization of methyl methacrylate with high degrees of conversion in the presence of ZnCl2

The radical polymerization of methyl methacrylate (MMA) in the presence of ZnCl2 has been investigated at high degrees of conversion to determine the influence of ZnCl2 on the polymerization rate and the molecular weight (MW) of the resulting polymer. It was found that ZnCl2 displaces characteristic conversions on kinetic curves of the polymerization process: degrees of conversion for the onset of auto-acceleration and for the onset of auto-inhibition are displaced towards lower conversions, as is the maximum degree of conversion. Where the ZnCl2 concentration in the reaction mixture is high, the onset of auto-acceleration is displaced all the way down to zero conversions. A characteristic feature of the introduction of ZnCl2 is the resulting change in the shape of curves for MW of the polymer versus degree of conversion. It was found in particular that in the region of auto-acceleration absolute values of MW and the rise in the latter with conversion are lower than in the absence of ZnCl2, which is connected with chain transfer in the system. The experimental results can all be accounted for in the light of the proposed crosslinking effect of ZnCl2 on PMMA macromolecules formed during the reaction. The mechanism of the action of ZnCl2 gives rise to a conformational transition and alters the character of intermolecular interactions in the system.

Synthesis of graft polymers and block-copolymers of acrylic and methacrylic monomers by a “living” radical polymerization☆

It was shown that in the presence of a complex-forming agent—o-phosphoric acid radical graft polymerization of methyl metahacrylate or methyl-α-chloracrylate on a γ-irradiated cellophane film takes place by a mechanism of “living” chains. At the initial stage of the reaction with a practically constant number of graft chains under given conditions graft polymer yield and the molecular weight of graft chains increases in direct proportion to the time of contact of a γ-irradiated film with the reaction system. A study was made of the dependence of graft polymer yield, molecular weight and the number of PMMA graft chains, on the composition of the H3PO4-MMA system. The rate constant of chain propagation was evaluated when carrying out graft post-polymerization of MMA on a γ-irradiated cellophane film. It was shown that the existence of “live” chains in the grafting of methyl methacrylate and methyl-α-chloracrylate in the presence of H3PO4 on to γ-irradiated cellophane film enables graft block-copolymers and graft tri-block copolymers of these monomers to be obtained with control of the molecular composition and the length of each unit. The block structure of individual graft chains was confirmed by turbidimetric titration. Block-copolymers of methyl methacrylate and methyl-α-chloracrylate practically free from homopolymers were obtained by hydrolysis of this modified cellophane film.

Features of the selective sorption of ZnCl2 by polybutyl methacrylate macromolecules

Abstract The methods of light scattering and equilibrium dialysis have been used to investigate processes of selective sorption of ZnCl 2 by polybutyl methacrylate macromolecules in butyl methacrylate-ZnCl 2 systems differing in their composition. It is shown that up to a certain ZnCl 2 concentration the average amount of ZnCl 2 in the macromolecular coil volume is equal to the average concentration of the complexing agent in solution, i.e. the selective sorption parameter β =0. Subsequently, as ZnCl 2 increases, parameter β turns negative. Above a “treshold” concentration of ZnCl 2 /butyl methacrylate=0·15, the degree of selective sorption in this range of ZnCl 2 concentration is increased. Interaction between ZnCl 2 and polybutyl methacrylate macromolecules is accompanied by a change in the steric parameter and in the thermodynamic quality of the solvent. A possible mechanism of interaction between ZnCl 2 and the polymer coils is discussed.

Features of radical polymerization in alcohols of vinylpyridinium salts, varying the hydrophilic/phobic balance☆

A kinetic study has been made of the radical polymerization of 2-methyl-5-vinylpyridine salts with ethyl and octyl bromoacetic esters in methanol, ethanol and butanol. Increases in the polymerization rate occur in this order and also in the case of the more bulky substituent. Elementary rate constants kp and kt for chain propagation and termination have been calculated. It was found that kp is constant for all the systems, which is associated with a single form of the monomers in an alcoholic medium with slightly dissociated ionogenic groups. The kt values decrease in the order methanol-ethanol-butanol on account of increasing solvent viscosities. In addition, kt decreases with increasing size of the aliphatic substituent in a monomer. The effect is explained by inhibition of rotational diffusion of the propagating radicals. A lower rate of rotation of the growing chain end in the case of the 2-methyl-5-vinylpyridine salt with octyl bromoacetic ester was detected, using a “spin trap” method.

Study of the reactivity of polymethyl methacrylate radicals combined in a complex in chain transfer to CBr4

A study was made of the effect of ZnCl2 on chain transfer to CBr4 in radical polymerization of methyl methacrylate (MMA). It was found that the chain transfer constant Ct and the rate constant for chain transfer kt decrease in the presence of ZnCl2. These effects are caused by a reduction in the reactivity of radicals combined in a complex in extension of PMMA as a result of a change of polarization and conjugation. The dependence of kt on the content of ZnCl2 in the reaction system enabled us to evaluate the equilibrium constant . The value of K>1 proves a greater affinity of ZnCl2 complexes with radicals than with the monomer. The value of K determined in this study agrees with that obtained previously from results of copolymerization in the MMA-vinylidene chloride-ZnCl2 system. Study of the effect of ZnCl2 on Ct in copolymerization of MMA with styrene indicates that active centres responsible for chain extension in alternating copolymerization in the presence of ZnCl2 are radicals, of which the activity does not markedly differ from that of free radicals responsible for statistical copolymerization of MMA with styrene in the absence of ZnCl2.