Vinyl monomers, temperature-controlled free radical polymerization

Temperature-controlled free radical polymerization of vinyl monomers such as butyl acrylate, methyl methacrylate, and styrene has been carried out in the reactor shown in Figure 8.3. Following mixing of the initiator 2,2-azobis(isobutylronitrile)... 

A combination of variables controls the outcome of the copolymerization of two or more unsaturated monomers by CCT free-radical polymerization.382 Of course, all of the features that control the outcome of a normal free-radical polymerization come into effect.40 426 429 These include the molar ratio of monomers, their relative reactivity ratios and their normal chain-transfer constants, the polymerization temperature, and the conversion. In the presence of a CCT catalyst, the important variables also include their relative CCT chain-transfer constants and the concentration of the Co chain-transfer agent. The combination of all of these features controls the molecular weight of the polymer and the nature of the vinyl end group. In addition, they can also control the degree of branching of the product. 

Homopolymerization. The free-radical polymerization of VDC has been carried out by solution, slurry, suspension, and emulsion methods. Slurry polymerizations are usually used only in the laboratory. The heterogeneity of the reaction makes stirring and heat transfer difficult consequently, these reactions cannot be easily controlled on a large scale. Aqueous emulsion or suspension reactions are preferred for large-scale operations. The spontaneous polymerization of VDC, so often observed when the monomer is stored at room temperature, is caused by peroxides formed from the reaction of VDC with oxygen, fery pure monomer does not polymerize under these conditions. Heterogeneous polymerization is characteristic of a number of monomers, including vinyl chloride and acrylonitrile. 

Free radical polymerization of neat monomer in the absence of solvent and with only initiator present is called bulk or mass polymerization. Monomer in the liquid or vapor state is well mixed with initiator in a heated or cooled reactor as appropriate. The advantages of this method are that it is simple, and because of the few interacting components present, there is less possibility for contamination. However, vinyl-type polymerizations are highly exothermic so that control of the temperature of bulk polymerization may be difficult. Also, in the absence of a solvent viscosities may become very high toward the end of a polymerization, which could make stirring difficult, and add to the difficulty of heat removal from the system. The advantages of this system, however, are sufficiently attractive for this to be used commercially for the free radical polymerization of styrene, methyl methacrylate, vinyl chloride, and also for some of the polymerization processes of ethylene [7]. 

Exploration of the template controlled free-radical oligomerization of other activated olefins began with standard monomers utilized in bulk polymer synthesis and the template 63. Vinyl acetate and acrylonitrile led only to uncontrolled polymerization, while vinylene carbonate did not react under the standard experimental conditions. More exotic monomers, such as vinyl trifluoroacetate and rert-butyl acrylate, were also unsuccessful. Only methyl acrylate polymerization was arrested by template 64 to provide the macrocyclized product 96 in modest yield as a mixture of five diastereomers (Scheme 8-25). Subsequent studies with the more effective thiophenyl-bearing template 63 at lower temperatures improved this yield to 35%. The diastereomer distribution was reminiscent of the methyl methacrylate-derived product, although no stereochemical assignments were made in this case either. 

Free-radical polymerization reactions have recently been studied for different monomers, for example mono and disubstituted vinyl monomers and dienes. The bulk polymerization of vinyl monomers (e.g. vinyl acetate, styrene, methyl methacrylate, and acrylonitrile) has been investigated by Amorim et al. [10]. The reactions were conducted in the presence of catalytic amounts of AIBN (or benzoyl peroxide). It was found that the rate of polymerization depends on the structure of the monomers and the power and time of microwave irradiation. In a typical experiment 10.0 mL of each monomer and 50 mg AIBN was irradiated in a domestic microwave oven for 1 to 20 min to afford the polymers polystyrene, poly(vinyl acetate), and poly(methyl methacrylate) with weight-average molecular weights 48 400, 150 200, 176700 g mol, respectively (Scheme 14.1). The experiments were performed without temperature control. 

Stereo control can also be achieved in the homogeneous free-radical polymerization of vinyl monomers by varying the polymerization temperature. Some typical monomers that behave in this manner include methyl methacrylate,(39) vinyl acetate,(41) vinyl chloride,(42) isopropyl and cyclohexyl acrylates.(40) As the polymer temperature is lowered the crystallizabiUty of the polymers becomes more discernible. (46) This observation can be attributed to the fact that as the temperature is lowered there is a preference for units in the same configuration to be added to the growing chain. It has been found that in general there is a preference for syndiotactic sequences to develop as the temperature is lowered. As an example, the observed melting temperature of poly(vinyl chloride) increases from 285 °C to 310 °C as the polymerization temperature is lowered from —15 °C to —75°C, with a concomitant increase in the syndiotacticity.(47)... 

The first reported controlled polymerization based on the OMRP-RT principle appears to have been presented by Minoura in a series of articles starting in 1978, where the redox initiating system BPO/Cr was used for the polymerization of vinyl monomers.Not only were the kinetics different than in free-radical polymerization (very low reaction orders in Cif and BPO), but also the polymerization was observed to continue after all Cr had been converted by the peroxide to Cr and the degree of polymerization was found to increase with monomer conversion at low temperatures (<30 0). These studies included the report of a block copolymer (PMMA-b-PAN). Polydispersity indexes were not reported for these studies. Minoura formulated the mechanistic hypothesis of the formation of a metal complex with the free radical and stated that "the recombination of free radicals formed by the dissociation of the complexed radicals competes with a disproportionation of free radicals". However, these studies did not have a great impact in the polymer community, being cited only a handful of times before 1994. A few subsequent contributions reported the application of similar conditions to other metals but well-controlled polymerizations were not found."- " ... 

Monomer and initiator must be soluble in the liquid and the solvent must have the desired chain-transfer characteristics, boiling point (above the temperature necessary to carry out the polymerization and low enough to allow for ready removal if the polymer is recovered by solvent evaporation). The presence of the solvent assists in heat removal and control (as it also does for suspension and emulsion polymerization systems). Polymer yield per reaction volume is lower than for bulk reactions. Also, solvent recovery and removal (from the polymer) is necessary. Many free radical and ionic polymerizations are carried out utilizing solution polymerization including water-soluble polymers prepared in aqueous solution (namely poly(acrylic acid), polyacrylamide, and poly(A-vinylpyrrolidinone). Polystyrene, poly(methyl methacrylate), poly(vinyl chloride), and polybutadiene are prepared from organic solution polymerizations. 

The polymerization of vinyl monomers is an exothermic reaction and a considerable amount of heat is released, about 18 kCal per mole. In both the catalyst-heat and gamma radiation processes the heat released during polymerization is the same for a given amount of monomer. The rate at which the heat is released is controlled by the rate at which the free radical initiating species is supplied and the rate at which the chains are growing. As pointed out above, the Vazo and peroxides are temperature dependent and the rate of decomposition, and thus the supply of free radicals, increases rapidly with an increase in temperature. Since wood is an insulator due to its cellular structure, heat flow into and out of the wood-monomer-polymer material is restricted. In the case of the catalyst-heat process heat must be introduced into the wood-monomer to start the polymerization, but once the exothermic reaction begins the heat flow is reversed. 

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. 

Some pure monomers undergo initiation when heated. The subsequent polymerization is free radical in character. Styrene exhibits significant thermal initiation at temperatures of 100°C or more. Methyl methacrylate also self-initiates but at a slower rate. Low-molecular-weight vinyl polymers can often be made simply by heating the monomers, but the molecular weight control is not very close and initiation in some cases at least may be from thermal homolysis of impurities in the reaction mixture. 

Since the transfer reaction rate is much larger than the termination reaction rate, the degree of polymerization is practically independent of the initiator concentration. So, industrially, the degree of polymerization is controlled by variation of the polymerization temperature. The monomer free radicals start the polymerization of vinyl chloride and produce unsaturated end groups... 

Emulsion polymerizations are carried out in one liquid phase dispersed within another. The monomer or a solution of the monomer is dispersed with the aid of an emulsifier in the homogeneous phase and polymerized, for example, with free radical initiators. The product is a colloidal dispersion of the polymer. Since dispersions have lower viscosity than the melt, they can be handled much better. Also, the temperature control is easier. Typical emulsion polymers are poly(methyl methacrylate), poly(methacrylic acid), polystyrene, and poly(vinyl chloride). Two special applications of emulsion polymerization are the making of well-defined dispersion particles that may contain only one or few polymer molecules, and the possibility to make better defined molecular sizes by controlling the growth periods. 

N-Vinyl-2-pyrrolidone-acrylamide copolymers were prepared by aqueous homogeneous solution polymerization in either distilled water or synthetic seawater. The initial monomer level in the polymerization solutions was either 9.1% or 20% (w/w). The solutions were initiated at either room temperature or 50 C by either 0.50phm or O.lOphm commercial free radical initiators. A temperature controlling water bath was used with the 50 C polymerizations. 

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