Acrylonitrile polymerization chain transfer

Chain transfer is an important consideration in solution polymerizations. Chain transfer to solvent may reduce the rate of polymerization as well as the molecular weight of the polymer. Other chain-transfer reactions may iatroduce dye sites, branching, chromophoric groups, and stmctural defects which reduce thermal stabiUty. Many of the solvents used for acrylonitrile polymerization are very active in chain transfer. DMAC and DME have chain-transfer constants of 4.95-5.1 x lO " and 2.7-2.8 x lO " respectively, very high when compared to a value of only 0.05 x lO " for acrylonitrile itself DMSO (0.1-0.8 X lO " ) and aqueous zinc chloride (0.006 x lO " ), in contrast, have relatively low transfer constants hence, the relative desirabiUty of these two solvents over the former. DME, however, is used by several acryhc fiber producers as a solvent for solution polymerization. 

The concept of PO macroinitiators centers on the introduction of an initiation moiety into an olefinic polymer chain for polymerization. The most effective route for preparing PO macroinitiators is by employing functional polyolefins containing hydroxyl groups or other reactive groups. These functional POs are prepared by copolymerization of olefins with functional monomers and post-polymerization reaction, as mentioned above. In the case where an initiation moiety was at the chain-end of the polyolefins, a block type copolymer is produced. It has been reported that thiol-terminated PP was used as polymeric chain transfer agent in styrene and styrene/acrylonitrile polymerization to form polypropylene-b/odc-polystyrene (PP-b-PS) and polypropylene-btock-poly(styrene-co-acrylonitrile) (PP-b-SAN) block copolymer [19]. On the other hand, polymer hybrids with block and graft structures can be produced if initiation moieties are in the polymer chain. 

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. 

A low-molecular-weight, polyfimctional polymer can be formed by polymerizing a vinyl monomer in the presence of a mercaptan chain transfer agent [1861]. The vinyl monomer may be an imsaturated acid, acrylonitrile. 

Polymerization of a monomer in a solvent overcomes many of the disadvantages of the bulk process. The solvent acts as diluent and aids in the transfer of the heat of polymerization. The solvent also allows easier stirring, since the viscosity of the reaction mixture is decreased. Thermal control is much easier in solution polymerization compared to bulk polymerization. On the other hand, the presence of solvent may present new difficulties. Unless the solvent is chosen with appropriate consideration, chain transfer to solvent can become a problem. Further, the purity of the polymer may be affected if there are difficulties in removal of the solvent. Vinyl acetate, acrylonitrile, and esters of acrylic acid are polymerized in solution. 

The claimed copolymerizability of a-ethylthioacrylonitrile H2C = C(SEt)CN with acrylonitrile at 70 °C in the patent literature is however doubtful [67]. a-Ethylthioacrylonitrile acts as a chain transfer agent during the polymerization... 

Similarly, acrylonitrile, methyl acrylate and acrylamide a-substituted with a benzyloxy group act as chain transfer agents during the polymerization of MMA, St, MA, and VA, which is due to the following fragmentation reaction [94] ... 

Zilkha, Feit, and Frankel (106), in their study of the anionic polymerization of acrylonitrile and methacrylonitrile with quaternary ammonium hydroxides, found the molecular weight of polyacrylonitrile to be independent of monomer and catalyst concentration while that of polymethacrylonitrile was not. The infrared spectra of the polyacrylonitrile indicated terminal CH2= groups. They suggested that termination by chain transfer to monomer was the explanation. 

Economically produced block copolymers containing acrylonitrile or methacrylo-nitrile as the principal component have been prepared that are heat resistant, weath-erable, and oil and flame resistant. These materials were prepared using reversible addition-fragmentation chain transfer polymerization. 

Tlie rubber latex is usually produced in batch reactors. The rubber can be polybutadiene [9003-17-2] or a copolymer of 1,3-butadiene [106-99-0] and either acrylonitrile [107-13-1] or styrene [100-42-5]. The latex normally has a polymer content of approximately 30 to 50% most of the remainder is water. In addition to the monomers, the polymerization ingredients include an emulsifier, a polymerization initiator, and usually a chain-transfer agent for molecular weight control. 

If equal concentrations of acrylonitrile and methyl methacrylate were each polymerized at 60°C with equal concentrations of the same initiator, which polymer would have the higher DP and by how much Assume polyacrylonitrile undergoes termination only by radical combination and poly(methyl methacrylate) by disproportionation, that no chain transfer occurs, and that initiator efficiencies are the same in both reactions. (Use Table 4-1 below.)... 

The mechanism proposed by Jaacks, Eisenbach, and Kern for chain transfer in the polymerization of acrylonitrile may well apply here. 

In general, an alternating eopolymer is formed over a wide range of monomer compositions. It has been reported that little chain transfer occurs, and in some cases, conventional free radical retarders are ineffective. Reaction occurs with some combinations, like styrene-acrylonitrile, when the monomers are mixed with a Lewis acid, but addition of a free-radical source will increase the rate of polymerization without changing the alternating nature of the copolymer. Alternating copolymerizations can also be initialed photochemically and electrochemically. The copolymerization is often accompanied by a cationic polymerization of the donor monomer. 

Acrylonitrile is not conveniently polymerized in bulk except on a scale of only a few grams. The reaction is rapid, highly exothermic (18 kcal per mole), and difficult to control. Polymer is insoluble in the monomer, and heat is not easily removed from the resulting sluny or semisolid mass. Polymerization in homogeneous solution is limited by the fact that only a few liquids dissolve the polymer. Reactions in some of these liquids lead to polymers of low molecular weights because of chain transfer, or are objectionable because of toxicity, cost, or difficulty in solvent recovery. 

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