Radical chain polymerization process conditions

From an industrial stand-point, a major virtue of radical polymerizations is that they can often be carried out under relatively undemanding conditions. In marked contrast to ionic or coordination polymerizations, they exhibit a tolerance of trace impurities, A consequence of this is that high molecular weight polymers can often be produced without removal of the stabilizers present in commercial monomers, in the presence of trace amounts of oxygen, or in solvents that have not been rigorously dried or purified, Indeed, radical polymerizations are remarkable amongst chain polymerization processes in that they can be conveniently-conducted in aqueous media. 

The majority of polymeric materials at operation contact to oxygen of air i.e. are in the oxidizing environment. Basically all reactions at ageing in natural conditions are characterized oxidizing necTpyKinm and represent radical - chain oxidizing process. This process is activated by various external factors - thermal, radiating, chemical, mechanical. 

The vast majority of azopolymers developed for optical storage are polyacrylates and polymethacrylates, which are generally prepared by free radical chain polymerization in solution using conventional experimental conditions. For example, azobisisobutyronitrile (AIBN) is used as a thermal initiator in dry organic solvents such as A(A-dimethylformamide (DMF), tetrahydrofuran (THF) or dioxane as the most common. Occasionally, the polymerization process of azobenzene (meth)acrylates can be limited by the radical transfer reaction promoted by the azo group, which seems to be associated with the formation of hydrazyl radicals (Nuyken and Weidner, 1986 Hallensleben andWeichart,1989). 

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. 

In this section, the reactions undergone by radicals generated in the initiation or chain transfer processes are detailed. Emphasis is placed on the specificity of radical-monomer reactions and other processes likely to take place in polymerization media under typical polymerization conditions. The various factors important in determining the rate and selectivity of radicals in addition and... 

The most important bulk plastics, e.g. the polyolefins, are produced using addition polymerization processes. The molecules of the starting materials contain double bonds which are broken with the help of initiators or catalysts. The resulting free radicals then undergo a chain reaction to form a macromolecule. In practice there are numerous processes with different reaction conditions. The start of chain reactions requires a radical produced as a rule by the disintegration of initiator substances, usually peroxide. 

The cation radical intermediate and the process of electron (hole) transfer have recently been shown to constitute the basis for a fundamentally new addition to the repertoire of polymerization methods [85], Both cation radical chain cyclobutana-tion polymerization (Scheme 44) and Diels-Alder polymerization have been demonstrated under the typical aminium salt conditions. 

Since the separate time scale condition is clearly valid for most of the polymerization process, one may say that each polymer chain is formed inside a particle of unchanging siz wherein all rate coefficients are constant and the distribution of free radicals has its steady-state value, for each volume V. Any residual effect of the PSD on the MWD would reside presumably in the effects of the PSD on the kinetic parameters (e.g., p. c, and to a lesser extent fc). Conversely, the MWD would possibly influence the PSD through its effects on the swelling of the particles by the monomer the effect, if it exists, is likely to ha small. 

Radical 12 is rather stable under polymerization conditions, but radical 11 decays into a triazole and the phenyl radical, which initiates new chains. Hence, the rate of polymerization is higher with 11 than with 12, because the decay prevents retarding of the buildup of large persistent radical concentrations such as an additional radical generation. This effect of the radical decay is equivalent to the rate enhancement by partial removal of nitroxides by appropriate additives, which was first applied by Georges et al.31 Interestingly, at 95 °C and in toluene solution, the lifetime of 11 is only about 15 min, whereas a reasonable control was found in polymerizations of styrene that lasted many hours at 120— 140 °C.120 Obviously, the radical moiety 11 is stable while it is coupled to the polymer chain. However, the different time scales raise the question of the upper limit of the conversion rate of the persistent radical to a transient one that can be tolerated in living radical polymerization processes (see section IV. C). 

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