Solvent transfer, copolymerization

Polymerization and Spinning Solvent. Dimethyl sulfoxide is used as a solvent for the polymerization of acrylonitrile and other vinyl monomers, eg, methyl methacrylate and styrene (82,83). The low incidence of transfer from the growing chain to DMSO leads to high molecular weights. Copolymerization reactions of acrylonitrile with other vinyl monomers are also mn in DMSO. Monomer mixtures of acrylonitrile, styrene, vinyUdene chloride, methallylsulfonic acid, styrenesulfonic acid, etc, are polymerized in DMSO—water (84). In some cases, the fibers are spun from the reaction solutions into DMSO—water baths. 

The refined grade s fastest growing use is as a commercial extraction solvent and reaction medium. Other uses are as a solvent for radical-free copolymerization of maleic anhydride and an alkyl vinyl ether, and as a solvent for the polymerization of butadiene and isoprene usiag lithium alkyls as catalyst. Other laboratory appHcations include use as a solvent for Grignard reagents, and also for phase-transfer catalysts. 

Thiols react more rapidly with nucleophilic radicals than with electrophilic radicals. They have very large Ctr with S and VAc, but near ideal transfer constants (C - 1.0) with acrylic monomers (Table 6.2). Aromatic thiols have higher C,r than aliphatic thiols but also give more retardation. This is a consequence of the poor reinitiation efficiency shown by the phenylthiyl radical. The substitution pattern of the alkanethiol appears to have only a small (<2-fokl) effect on the transfer constant. Studies on the reactions of small alkyl radicals with thiols indicate that the rate of the transfer reaction is accelerated in polar solvents and, in particular, water.5 Similar trends arc observed for transfer to 1 in S polymerization with Clr = 1.4 in benzene 3.6 in CUT and 6.1 in 5% aqueous CifiCN.1 In copolymerizations, the thiyl radicals react preferentially with electron-rich monomers (Section 3.4.3.2). 

Free radical copolymerizations of the alkyl methacrylates were carried out in toluene at 60°C with 0.1 weight percent (based on monomer) AIBN initiator, while the styrenic systems were polymerized in cyclohexane. The solvent choices were primarily based on systems which would be homogeneous but also show low chain transfer constants. Methacrylate polymerizations were carried out at 20 weight percent solids... 

The free-radical nature of ATRP is well established through a number of studies [Matyjaszewski et al., 2001], The effects of inhibitors and retarders, solvents, and chain-transfer agents are the same in ATRP as in conventional radical polymerization. The regio-selectivity, stereoselectivity, and copolymerization behaviors are also the same. 

For any specific type of initiation (i.e., radical, cationic, or anionic) the monomer reactivity ratios and therefore the copolymer composition equation are independent of many reaction parameters. Since termination and initiation rate constants are not involved, the copolymer composition is independent of differences in the rates of initiation and termination or of the absence or presence of inhibitors or chain-transfer agents. Under a wide range of conditions the copolymer composition is independent of the degree of polymerization. The only limitation on this generalization is that the copolymer be a high polymer. Further, the particular initiation system used in a radical copolymerization has no effect on copolymer composition. The same copolymer composition is obtained irrespective of whether initiation occurs by the thermal homolysis of initiators such as AIBN or peroxides, redox, photolysis, or radiolysis. Solvent effects on copolymer composition are found in some radical copolymerizations (Sec. 6-3a). Ionic copolymerizations usually show significant effects of solvent as well as counterion on copolymer composition (Sec. 6-4). 

This article reports on the synthesis of photosensitive polymers with pendant cinnamic ester moieties and suitable photosensitizer groups by cationic copolymerizations of 2-(cinnamoyloxy)ethyl vinyl ether (CEVE) (12) with other vinyl ethers containing photosensitizer groups, and by cationic polymerization of 2-chloroethyl vinyl ether (CVE) followed by substitution reactions of the resulting poly (2-chloroethyl vinyl ether) (PCVE) with salts of photosensitizer compounds and potassium cinnamate using a phase transfer catalyst in an aprotic polar solvent. The photochemical reactivity of the obtained polymers was also investigated. 

Copolymerizations initiated by lithium metal should give the same product as produced from lithium alkyls. Usually the radical ends produced by electron transfer initiation have so short a lifetime they can have no influence on the copolymerization. This is true for instance in the copolymerization of isoprene and styrene (50). The product is identical if initiated by lithium metal or by butyllithium. With the styrene-methylmethacrylate system, however, differences are observed (79,80,82). Whereas the butyllithium initiated copolymer contains no styrene at low conversions, the one initiated by lithium metal has a high styrene content if the reaction is carried out in bulk and a moderate one even in tetrahydrofuran. These facts led O Driscoll and Tobolsky (80) to suggest that initiation with lithium occurs by electron exchange and that in this case the radical ends are sufficiently long-lived to produce simultaneous radical and anionic reactions at opposite ends of the chain. Only in certain rather exceptional circumstances would the free radical reaction be of importance. Some of the conditions required have been discussed by Tobolsky and Hartley (111). The anionic reaction should be slow. This is normally true for lithium based catalysts in hydrocarbon solvents. No evidence of appreciable radical participation is observed for initiation by sodium and potassium. The monomers should show a fast radical reaction. If styrene is replaced by isoprene, no isoprene is found in the copolymer for isoprene polymerizes slowly by free radical initiation. Most important of all, initiation should be slow to produce a low steady concentration of radical-anions. An initiator which produces an almost instantaneous and complete electron transfer to monomer produces a high radical concentration which will ensure their rapid mutual termination. 

The initiation of radical polymerizations, various transfer, as well as termination reactions all lead to a variety of products and the makeup of the mixture can only be slightly influenced by varying the reaction conditions or the monomer concentration, the initiator or the solvent. Furthermore, radical block copolymerization leads inevitably to more or less homopolymer so that the products require careful separation before the block copolymer can be characterized. Nevertheless, the synthesis of block copolymers via a radical mechanism has several important advantages ... 

Lacroix-Desmazes and Guyot [13] applied Paine s model to the dispersion copolymerization of amphiphilic macromonomers and re-discussed this model in terms of possible incorporation of a new parameter - the chain transfer parameter (Css the chain transfer constant for transfer to solvent-alcohol). The relations for the rate of dead chains (kj) and chain length (CL) are as follows ... 

A zwitterionic tetramethylene initiates ionic homopolymerization, while a diradical tetramethylene initiates free radical copolymerization. As initiating species, zwitterions are likely to remain in the coiled gauche-conformation and collapse to small molecules. Diradicals, on the other hand, are easily transferred to the trans-conformation. Accordingly, diradicals are more effective initiators and more radical copolymerizations occur than ionic homopolymerizations. Addition of solvent will also influence the reaction of polar tetramethylene. A polar non-donor solvent may permit carbenium ion polymerization, while a polar donor solvent impedes it. 

The polycarbonate oligomers were prepared by solution or interfacial techniques (10,17,18). Methylene chloride and tetraethyl ammonium chloride served as the solvent and phase transfer catalyst, respectively. The block copolymerizations were performed essentially under interfacial reaction conditions. In the case of copolymerizations using the Bis-S polysulfone oligomers, it was necessary to use tetrachloroethane as the organic solvent. 

Copolymers are produced by aqueous copolymerization of the monomers in a manner similar to the homopolymerizations. Nonaqueous polymerizations of TFE with PPVE may be conducted in water or in fluorinated solvents at lower temperatures (30-60 °C) using a soluble organic initiator such as perfluoro-propionyl peroxide. The molecular weight is controlled by addition of chain transfer agents such as methanol in nonaqeuous polymerizations or hydrogen in aqeous polymerizations. 

---