Initiators for polymerization

AlkyUithium compounds are primarily used as initiators for polymerizations of styrenes and dienes (52). These initiators are too reactive for alkyl methacrylates and vinylpyridines. / -ButyUithium [109-72-8] is used commercially to initiate anionic homopolymerization and copolymerization of butadiene, isoprene, and styrene with linear and branched stmctures. Because of the high degree of association (hexameric), -butyIUthium-initiated polymerizations are often effected at elevated temperatures (>50° C) to increase the rate of initiation relative to propagation and thus to obtain polymers with narrower molecular weight distributions (53). Hydrocarbon solutions of this initiator are quite stable at room temperature for extended periods of time the rate of decomposition per month is 0.06% at 20°C (39). 

Uses. /-Butyl hypochlorite has been found useful in upgrading vegetable oils (273) and in the preparation of a-substituted acryflc acid esters (274) and esters of isoprene halohydrins (275). Numerous patents describe its use in cross-linking of polymers (qv) (276), in surface treatment of mbber (qv) (277), and in odor control of polymer latexes (278). It is used in the preparation of propylene oxide (qv) in high yield with Httle or no by-products (269,279). Fluoroalkyl hypochlorites are useful as insecticides, initiators for polymerizations, and bleaching and chlorinating agents (280). 

It should be emphasized that the above equations, which relate reaction temperatures to calculated reactant or product energies, are equivalent to the more conventional linear free energy relationships, which relate logarithms of rate constants to calculated energies. It was felt that reactant temperatures would be more convenient to potential users of the present approach -those seeking possible new free radical initiators for polymerizations. 

Yasuda, T., Aida, T., and Inoue, S., Reactivity of (por-phinato)aluminum phenoxide and alkoxide as active initiators for polymerization of epoxide and lactone. Bull. Chem. Soc. Jap.. 9, 3931-3934, 1986. 

The protecting group is removed by hydrogenation. The deprotected hydroxymethyl groups are used as initiators for polymerization of new e-CL. The process was repeated to the third generation to give a narrow molecular weight... 

Block copolymers of isobuylene with styrene, isoprene, and vinyl ethers have been synthesized, often requiring an appropriate adjustment of reaction conditions for the second stage, as described above for styrene-MVE. Another approach is the use of a telechelic polymer (containing the first block) as the initiator for polymerization of the second monomer (Sec. 5-4b). 

Combinations of a Lewis acid, protogen or cationogen, and a reactive cyclic ether (e.g., oxirane or oxetane) have been used to initiate the polymerization of less reactive cyclic ethers such as tetrahydrofuran [Saegusa and Matsumoto, 1968]. Initiation occurs by formation of the secondary and tertiary oxonium ions of the more reactive cyclic ether, which then act as initiators for polymerization of the less reactive cyclic ether. The reactive cyclic ether, referred to as a promoter, is used in small amounts relative to the cyclic ether being polymerized and increases the ability of the latter to form the tertiary oxonium ion. 

The yield of initiating radicals is, however, generally smaller than would be expected. In the case of AIBN this is because a certain amount of tetramethylsuc-cinic acid dinitrile is formed by combination of the primary radicals, while some methacrylonitrile and iso-butyronitrile are formed by disproportionation of the primary radicals. Azo compounds are especially suited as initiators for polymerization in bulk or in organic solvents. 

Benzoyl peroxide is used as an initiator for polymerization of acrylates (including dental cements and restoratives) and other polymers as a bleaching agent for flour, fats, oils, waxes and milk used in the preparation of certain cheeses in pharmaceuticals for the topical treatment of acne in rubber curing and as a finishing agent for some acetate yams (Anon., 1984 Lewis, 1993 Medical Economics Co., 1996 United States Food and Dmg Administration, 1997). 

Copolymers. Copolymers from mixtures of different bisphenols or from mixtures of dichlorosulfone and dichlorobenzophenone have been reported in the patent literature. Bifunctional hydroxyl-terminated polyethersulfone oligomers are prepared readily by the polyetherification reaction simply by providing a suitable excess of the bisphenol. Block copolymers are obtained by reaction of the oligomers with other polymers having end groups capable of reacting with the phenol. Multiblock copolymers of BPA-polysulfone with polysiloxane have been made in this way by reaction with dimethyl amino-terminated polydimethylsiloxane the products are effective impact modifiers for the polyethersulfone (79). Block copolymers with nylon-6 are obtained when chlorine-terminated oligomers, which are prepared by polyetherification with excess dihalosulfone, are used as initiators for polymerization of caprolactam (80). 

The first application of the ferrous ion-hydrogen peroxide initiation for polymerizing vinyl monomers on and into cellulose fibers has been reported by Landeias and Whewell (41) in three successive papers. They are apparently the first who applied the "anchored catalyst technique, which other people have termed "in situ polymerization to cellulose grafting. The authors internally deposited methyl methacrylate, acrylonitrile, styrene, methyl vinyl ketone and methacrylamide in amounts between 10 and 80%. No attempt had been made to determine if actual grafting had occurred. In 1961 Richards (42) studied this question in great detail. Products obtained by polymerization of acrylonitrile and of styrene in viscose rayon were acetylated. Fractionation of... 

Table V. Initiators for Polymerization with Hexacyanometalate Salt...

Perfluoro-M-alkyl diacyl peroxides decompose homolytically to give perfluoroalkyl radicals under mild conditions, and radicals formed in such a manner have been used synthetically or as radical initiators for polymerizations [66,67]. 

An example of a functional organolithium initiator is p-lithiophenoxide. It acts both as a carrier of hydroxyl functionality and as an initiator for polymerization (122). Unfortunately, p-lithiophenoxide has a low solubility, even in such polar solvents as THF, and up to 30% of the initiator is initially inactive toward polymerization. It is difficult to control polymer molecular weights when such initiators are used (123). Trepka (124) improved the solubility of p-lilhiophenoxide by the introduction of alkyl... 

Dendrimers with their multiple end-standing functional groups are ideally suited for the construction of star-shaped polymers. Indeed, the end-standing functional groups can be used as initiators for polymerization ( grafting from method) or as functional groups for grafting onto . They can also be used as redistribution centers in equilibrium polymerization. 

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