Recombination, vinyl polymerization

The gel or Trommsdorff effect (11) is the striking autoacceleration of the vinyl polymerization reaction as the viscosity of the monomer-polymer solution increases. Chain termination involving the recombination of two free radicals becomes diffusion controlled and this results in a decrease in the rate of termination. The concentration of active free radicals therefore increases proportionally. To sum up the gel effect the rate of Vazo catalyst initiation increases with temperature the rate of propagation or polymerization increases with the viscosity and the rate of termination of the growing polymer chains decreases with the viscosity. This of course also results in an increase in the molecular weight of linear polymers, but this has no practical significance when crosslinking is part of the reaction. 

The effect of various additives on a styrene polymerization reinforces the tentative conclusion that vinyl polymerization is not taking place. Even though yields were increased by halogenated additives they were not decreased by additives expected to act as scavengers for free-radical species (benzophenone) or ionic species (butylamine or water). Under an assumed mechanism of fragmentation and rapid recombination to condensed products, halogenated compounds additives may serve to increase the efficiency of energy transfer from the electric field to the monomer. 

Due to the lower reactivity of oxonium ions and the basic character of monomers, chain transfer (other than chain transfer to polymer) and/or termination is less critical in CROP of cyclic ethers than in vinyl polymerization (either cationic or anionic). The best documented system not involving any termination is polymerization of five-membered THF in the presence of stable counterions (SbFg", AsFg", and PFg") or noncomplex anions like CFsSOs" for which recombination occurs but is fully reversible. For these systems, the DP is equal to the calculated value up to the high DP values. " ... 

An example of a commercial semibatch polymerization process is the early Union Carbide process for Dynel, one of the first flame-retardant modacryhc fibers (23,24). Dynel, a staple fiber that was wet spun from acetone, was introduced in 1951. The polymer is made up of 40% acrylonitrile and 60% vinyl chloride. The reactivity ratios for this monomer pair are 3.7 and 0.074 for acrylonitrile and vinyl chloride in solution at 60°C. Thus acrylonitrile is much more reactive than vinyl chloride in this copolymerization. In addition, vinyl chloride is a strong chain-transfer agent. To make the Dynel composition of 60% vinyl chloride, the monomer composition must be maintained at 82% vinyl chloride. Since acrylonitrile is consumed much more rapidly than vinyl chloride, if no control is exercised over the monomer composition, the acrylonitrile content of the monomer decreases to approximately 1% after only 25% conversion. The low acrylonitrile content of the monomer required for this process introduces yet another problem. That is, with an acrylonitrile weight fraction of only 0.18 in the unreacted monomer mixture, the low concentration of acrylonitrile becomes a rate-limiting reaction step. Therefore, the overall rate of chain growth is low and under normal conditions, with chain transfer and radical recombination, the molecular weight of the polymer is very low. 

Azonitriles are not susceptible to radical-induced decompositions (56) and their decomposition rates are not usually affected by other components of the environment. Cage recombination of the alkyl radicals occurs when azo initiators are used, and results in the formation of toxic tetrasubstituted succinonitrile derivatives (56). This can be a significant drawback to the use of azo initiators. In contrast to some organic peroxides, azonitrile decomposition rates show only minor solvent effects (54—56) and are not affected by transition metals, acids, bases, and many other contaminants. Thus azonitrile decomposition rates are predictable. Azonitriles can be used as thermal initiators for curing resins that contain a variety of extraneous materials since cure rates are not affected. In addition to curing of resins, azonitriles are used for polymerization of commercial vinyl monomers. 

The charged end of a polymer and its counter-ion may recombine and form a stable covalent bond thus terminating the propagation of polymerization. Such a termination is frequently observed in carbonium ion polymerizations. For example, polymerization of a vinyl monomer if initiated by hydrochloric acid produces a carbonium ion and a chlorine-counter-ion. These two ions recombine readily forming a stable covalent C-Cl bond which does not propagate further the polymerization and forms, therefore, the dead end of a polymeric molecule. Actually, the recombination of carbonium ion with Cl- ion is such a rapid reaction that usually it follows immediately the formation of the relevant carbonium ion. This prevents the formation of a polymeric molecule and gives instead an addition product of HC1 to the reactive C=C double bond. A polymeric product can be obtained if the ions recombination is slowed down by sufficiently powerful solvation. For example, a solution of styrene in nitromethane, but not in a hydrocarbon, can be polymerized by HC1 (2), since the recombination of the solvated ions is sufficiently slow to permit the formation of a polymer. 

In the chain growth free radical polymerization of a vinyl monomer (conventional polymerization), the growth reaction is the repeated reaction of a free radical with numbers of monomer molecules. According to the termination by recombination of growing chains, 2 free radicals and 1000 monomer molecules leads to a polymer with the degree of polymerization of 1000. In contrast to this situation, the growth and deposition mechanisms of plasma polymerization as well as of parylene polymerization could be represented by recombinations of 1000 free radicals (some of them are diradicals) to form the three-dimensional network deposit via 1000 kinetic... 

The term free radical is often used in the context of a reactive intermediate, as in the case of polymerization of vinyl monomers, but the same structure (unpaired electron) can and does exist in a kind of immobilized environment. For example, a bulk-polymerized (monomer and initiator only in the polymerization system) poly(methyl methacrylate) (PMMA) contains an appreciable number of free radicals that can be detected by electron spin resonance (ESR) [1]. When the polymerization system becomes highly viscous toward the end of the bulk polymerization, gel formation occurs and immobilizes the growing end of free radical chain growth polymerization, preventing recombination of two free radical ends of growing chains. 

Polymerization by ionic initiation is much more limited than that by free-radical initiation with vinyl monomers, but there are monomers such as carbonyl compounds that may be polymerized ionically but not through free radicals because of the high polarity. The polymerization is much more sensitive to trace impurities, especially water, and proceeds rapidly at low temperature to give polymers of narrow molar-mass distribution. The chain grows in a living way and, unlike in the case of free-radical polymerization, is generally terminated not by recombination but rather by trace impurities, solvent or, rarely, the initiator s counter-ion (Fontanille, 1989). 

It is very clear that if the initiator has hydroxyl groups, and if the termination takes place exclusively by recombination then a polymeric diol is obtained [2, 3], which is ideal for polyurethane. If the termination takes place by disproportionation, only monofunctional compounds are obtained, which cannot be used in PU. The vinylic and dienic monomers used in practice have various termination mechanisms. Some monomers give only recombination reactions, such as styrene, acrylates (methyl acrylate, ethyl acrylate, butyl acrylate, 2-ethyl hexyl acrylate), acrylonitrile and butadiene. Other monomers give both mechanisms of termination, around 65-75% disproportionation and 25-35% recombination, such as methacrylates (methyl methacrylate, ethyl methacrylate, butyl methacrylate etc.), substituted styrenes and other monomers [2, 3, 4]. 

The initiator not only influences the rate of polymerization, however it can, under certain conditions, also influence the constitution of the polymer produced. For example, AIBN produces linear polymer at low monomer conversions, but lightly branched products at higher conversions when used to polymerize / -vinyl benzyl methyl ether. If this monomer is initiated with dibenzoyl peroxide, cross-linking occurs at high yields, and the monomer produces cross-linked polymer even at low yields if diacetyl is used as photoinitiator. What happens with these free radical initiators is that transfer to polymer occurs the polymer free radicals produced can add on monomer molecules, whereby branched products are produced, or recombination resulting in cross-linking can occur ... 

Head-to-Head (H-H) and tail-to-tail (T-T) linkages may also be present in polymer structures. They have been identified in addition polymers prepared by radical or coordination polymerization (polypropylene). Introduction of H-H linkages into addition polymers by radical polymerization can proceed by two mechanisms (a) terminations of radical polymerization (recombination or disproportionation of the polymer radicals) (3). The recombination introduces one H-H linkage, disproportionation of a vinyl and a saturated polymer end. Recombination and disproportionations are not always equally important termination reactions what type of termination depends on the type of monomer and polymerization conditions, (b) H-H linkages are also introduced into polymers by reverse addition of the monomer in radical polymerization. This behavior is particularly noticeable in radical polymerizations of hal-... 

This example demonstrates that free-radical polymerization could be the preferred mechanism for many vinyl monomers since, unlike ionic polymerization, it is tolerant of trace impurities and monomer functionality. However, one of its major drawbacks is the lack of control over the molecular weight distribution due to intrinsic termination reactions. Moreover, the efficiency factor of the initiator decreases by the so-called cage effect, for example by recombination of the primary fi-ee radicals, with increasing molecular weight of the macroinitiator [28]. This normally prevents the synthesis of block copolymers with controlled architectures, narrow molecular weight distributions and well-defined molecular weights. 

The problem of incorporation of chains with a terminal double bond (TDB) exists in polymerizations discussed above, such as radical polymerization of vinyl acetate and olefin polymerization with a constrained-geometry metallocene catalyst (CGC). Tobita [15] has developed an MC algorithm for this problem for the PVAc case. It is assumed that TDBs are created by transfer to monomer only, while recombination is absent, which results in a maximum of one TDB per chain. We largely follow Tobita s explanation, but differ in that we will assume that disproportionation is the termination mechanism, while transfer to solvent and to polymer are not yet being accounted for. Later we will address the real PVAc problem, which in fact has two branching mechanisms TDB propagation and transfer to polymer. 

Akutin and co-workers [159] made block and graft copolymer of fluorine-containing polymers or polysiloxanes as one component and poly(methyl methacrylate), poly(vinyl chloride), and ethylcellulose as the second. During the irradiation, samples were withdrawn at regular intervals in order to determine the extent of reaction by the decrease in solution viscosity, rj. In other cases, a recombination of macroradicals into new polymeric species caused an increase in the viscosity. The sign and rate of change of rj was thus different for different systems. 

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