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Unsaturated Carboxylic Acids and Related Compounds
The efficacy of the carboxylic anchor group has led to much interest in developing coupling-type modifiers based on it. The obvious way to do this would seem to be to replace the saturated hydrocarbon chain by an organic group containing some unsaturation. This has been extensively trialed but with limited success. The main candidates are unsaturated versions of fatty acids such as oleic acid and short chain species such as maleic acid and anhydride, acrylic acid, and methacrylic acid. The only commercial products of this type that the authors are aware of are based on unsaturated oligomeric carboxylic acids (Rothon et al. 2002b; Schofield et al. 2003).
As might be expected, the greatest success with the unsaturated systems has been in peroxide-cured polymers, such as unsaturated polyesters and some elastomers. Here the cure system is able to react with the unsaturation and lead to grafting. Less success has been obtained in sulfur-cured elastomers and especially in thermoplastics such as the polyolefins, where there is usually no added free- radical source.
While simple mechanical grafting is possible in polyolefins, this is usually limited by the presence of added antioxidant. This antioxidant interference can be overcome by the addition of a very small amount of peroxide.
Dimaleimides can be thought of as derivatives of maleic anhydride, produced by its reaction with aromatic or aliphatic diamines and thus containing two very reactive double bonds. They have been shown to be effective coupling agents in particulate- filled polyolefins (Khunova and Sain 1995; Xanthos 1983) but do not appear to be used commercially.
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