Chemical reactions of polymer crosslinking and post-crosslinking at room and medium temperature

This review focuses on various strategies that enable the crosslinking and post-crosslinking of polymers, excluding crosslinking obtained by radiation (e.g., X-ray, UV, etc.) and that at high temperature. The review is divided into two main parts: systems enabling crosslinking at room temperature and those for which crosslinking occurs at intermediate temperatures (<150 degrees C). In the first part, various key functional groups can be used, such as (i) carboxylic acid involving reactions with compounds that bear carbodiimide or aziridine functions; (ii) acetoacetyl groups (with isocyanate, activated alkenes, aldehyde, amine functions); (iii) reactions involving activated amines with carbonyl functions (aldehydes, ketones, etc.); (iv) species bearing acetals as pH-sensitive crosslinking agents since they are stable in basic medium but they can self react under acidic conditions; (v) acrylamide functions which are able to self-crosslink; (vi) crosslinking agents able to react with water (such as species that bear a poly(alkoxy)silane for sol-gel process) and derivatives containing isocyanate functions and (vii) systems that require oxygen, for example polymers bearing double bonds, boranes for generating hydroperoxides and acetylenic functions which undergo acetylenic coupling. The second series of systems, used at higher temperatures (yet below 150 degrees C) involving the following key functions: (i) carboxylic acid that react with oxazoline, or epoxide function where specific catalysts are necessary; (ii) alcohols reacting with protected urethanes, azlactones and methylol amide (for coating applications); (iii) azetidines (obtained from a cyclic amine onto an activated double bond) which self-crosslink; (iv) reversible Diels-Alder reaction (such as furane/bismaleimide reaction), and (v) Huisgen reactions between azido and triple bond. Various examples are presented, along with a discussion of their properties and applications. (C) 2010 Elsevier Ltd. All rights reserved.