Cationic chain polymerization retardation

Solomon (3, h, 5.) reported that various clays inhibited or retarded free radical reactions such as thermal and peroxide-initiated polymerization of methyl methacrylate and styrene, peroxide-initiated styrene-unsaturated polyester copolymerization, as well as sulfur vulcanization of styrene-butadiene copolymer rubber. The proposed mechanism for inhibition involved deactivation of free radicals by a one-electron transfer to octahedral aluminum sites on the clay, resulting in a conversion of the free radical, i.e. catalyst radical or chain radical, to a cation which is inactive in these radical initiated and/or propagated reactions. 

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. 

With formic and carbonic acid as potential chain transfer agents, termination would involve the proton of the undissociated acid, but HCOO" and HOCOO were very poor nucleophiles and did not reinitiate the polymerization. This amounted to an overall inhibition or retardation of the formaldehyde polymerization. 2 mole % of COj inhibited formaldehyde polymerization completely [29], as did other inorganic or organic acids which gave poorly nucleophilic anions. Higher organic acid concentration may cause cationic polymerization. 

The free-radical polymerization of NVP is rather complex but its liability to be polymerized by its own peroxide is well documented as well as its strong tendency for chain transfer during polymerization However, retardation by oxygen has also been claimed Alternatively, the formation of a donor-acceptor complex has been proposed which could yield anionic and cationic species,... 

In other studies [307, 308], this same group produced bionanocomposites by melt intercalation of PCL and MMT modified by various alkylammonium cations. Depending on whether the ammonium cations contain nonfunctional alkyl chains or chains terminated by carboxylic acid or hydroxyl functions, microcomposites or nanocomposites were produced. The layered silicate PCL nanocomposites exhibited some improvement in mechanical properties and increased thermal stabihty as well as enhanced flame retardancy. The authors concluded that formation of PCL-based nanocomposites, not only depended on the nature of the ammonium cation and its functionaHty, but also on the selected synthetic route, that is, melt intercalation versus in situ intercalative polymerization. 

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