Bulk polymerization rates, acrylated

Bulk Polymerization. Tlie bulk polymerization of acrylic monomers is characterized by a rapid acceleration in the rate and the formation of a cross-linked insoluble network polymer at low conversion (90,91). Such network polymers are thought to tonu by a chain-transfer mechanism involving abstraction of the hydrogen alpha to the ester carbonyl in a polymer chain followed by growth of a branch radical. Ultimately, two of these branch radicals combine (91). Commercially, the bulk polymerization of acrylic monomers is of limited importance. 

Free-radical bulk polymerizations of acrylate esters exhibit rapid-rate accelerations at low conversions. This often results in formation of some very high molecular weight polymer and some crosslinked material. The crosslinking is a result of chain transferring by abstractions of labile tertiary... 

Usually, free-radical initiators such as azo compounds or peroxides are used to initiate the polymerization of acrylic monomers. Photochemical (72—74) and radiation-initiated (75) polymerizations are also well known. At a constant temperature, the initial rate of the bulk or solution radical polymerization of acrylic monomers is first order with respect to monomer concentration and one-half order with respect to the initiator concentration. Rate data for polymerization of several common acrylic monomers initiated with 2,2 -azobisisobutyronittile (AIBN) [78-67-1] have been determined and are shown in Table 6. The table also includes heats of polymerization and volume percent shrinkage data. 

There is also a more general consequence of Rp varying as kplktl/1. If a monomer is polymerized in dilute solution, the kinetics follows the predicted first-order dependence on monomer concentration. In concentrated solutions, or in bulk polymerizations, where a pot full of monomer undiluted by solvent is polymerized, there is often a big acceleration in the polymerization rate at some point For monomers such as methyl acrylate, this can occur after conversions of just 1% and can result in an explosion (as illustrated schematically in Figure 4-18) ... 

Of a large number of possible fluorinated acrylates, the homopolymers and copolymers of fluoroalkyl acrylates and methacrylates are the most suitable for practical applications. They are used in the manufacture of plastic lightguides (optical fibers) resists water-, oil-, and dirt-repellent coatings and other advanced applications [14]. Several rather complex methods to prepare the a-fluoroalkyl monomers (e.g., a-phenyl fluoroacrylates, a-(trifluoromethyl) acrylic and its esters, esters of perfluoromethacrylic acid) exist and are discussed in some detail in [14]. Generally, a-fluoroacrylates polymerize more readily than corresponding nonfluorinated acrylates and methacrylates, mostly by free radical mechanism [15], Copolymerization of fluoroacrylates has been carried out in bulk, solution, or emulsion initiated with peroxides, azobisisobutyronitrile, or y-irradiation [16]. Fluoroalkyl methacrylates and acrylates also polymerize by anionic mechanism, but the polymerization rates are considerably slower than those of radical polymerization [17]. 

The bulk polymerization of allyl acrylate with benzoyl peroxide as initiator to 10% conversion gave a brittle, glassy polymer which was considered partially cyclized with a Kc (ratio of rate of cyclization to the rate of bimolecular propagation, cf., Table IV) of 0.41 moles/liter [72]. The polymer was described as soluble in both toluene and in carbon tetrachloride. 

Most effective polymerizations were achieved in the absence of solvent and would proceed almost to completion without difficulty. For example, with use of benzoyl peroxide as an initiator, the conversion rate of DHCD-DA would reach 90% in 40 h. Moreover, the variation in molecnlar weight, which was represented by the number-average degree of polymerization and the dispersity, showed that the bulk polymerization of these monomers was similar to that of acrylic esters (Ballard et al. 1994). 

The choice of the proper peroxy initiator largely depends on its decomposition rate at the reaction temperature of the polymerization. BPO is the major initiator for bulk polymerization of polystyrene or acrylic ester polymers, where temperatures from 90°C to 220°C are encountered. Dilau-royl, dicaprylyl, diacecyl, and di- err-butyl peroxides are also used. In the case of suspension polymerization of styrene, where temperatures between 85°C and 120 C are applied, the initiators also range in activity from BPO to di-tm-butyl peroxide. In suspension polymerization of vinyl chloride (reaction temperatures of 45-60°C for the homopolymer), thermally very labile peroxides such as diisopropyl peroxydicarbonate and rm-butyl peroxy-pavilate are used. 

Figure 1.6 Chain-length dependent termination rate coefficients determined via the SP-PLP-NIR-RAFT technique for methyl acrylate and dodecyl acrylate bulk polymerization at 60 °C and 1000 bar.

The continuous bulk polymerization of methyl methacrylate was used as an example in Section 5.2. A stirred bulk polymerization like that used for styrene (Section 5.4) could be adapted for methyl methacrylate. A suspension process for poly(methyl methacrylate) was described in Section 5.4. The polymerization of ethyl acrylate most often is carried out in emulsion. A process such as that used for vinyl acetate is suitable (Section 16.4). Like vinyl acetate, the monomer is slightly water soluble, so true emulsion polymerization kinetics are not followed. That is, there is initiation of monomer dissolved in water in addition to that dissolved in growing polymer particles. Ethyl acrylate is distinguished by its rapid rate of propagation. Initiation of a 20% monomer emulsion at room temperature by the redox couple persulfate-metabisulflte can result in over 95% conversion in less than a minute. As with vinyl acetate polymerization, a continuous addition of monomer at a rate commensurate with the heat transfer capacity of the reactor is necessary in order to control the temperature. 

Acryl amide is an important bulk chemical used in coagulators, soil conditioners and stock additives. The chemical synthesis has several drawbacks because the rate of acryl amide formation is lower than the formation of the by-product acrylic acid [54]. Further, the double bonds of the reactants and products cause by-product formations as well as formation of polymerization products. As a result of optimization with methods of molecular engineering, a very high activity of the biocatalyst nitrile hydratase at low temperature is yielded, enabling a successful biotransformation that is superior to the chemical route. Here, the synthesis is carried out at a low temperature of about 5°C, showing a conversion of 100%. 

Methacrylic acid also polymerizes in bulk under precipitating conditions. It forms molecular associations very similar to those of acrylic acid. However, the conversion curves were found to be linear under a variety of experimental conditions temperatures of 16.5 to 60°C and broad ranges of initiation rates and monomer concentration in numerous solvents (7). It was assumed that structures of type III do arise but owing to steric hindrance and to the rigidity of the poly(methacrylic acid) molecule the monomer cannot align to form a "pre-oriented" complex as in the case of acrylic acid and propagation is not favored. 

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