Functionalization of aliphatic polyketones
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Introduction Polyketones exhibit many advantageous properties, including good chemical resistance toward solvents, acids and bases, impermeability to hydrocarbons, and high rigidity and impact strength, and therefore have a wide range of possible applications.1–3 Furthermore, many of these copolymers exhibit high degrees of crystallinity due to the keto functionality and are even photo- and biodegradable.4–5 The use of carbon monoxide as a co-monomer for copolymerization with ethylene and other olefins is attractive not only because it is a cheap and easily accessible monomer, but also because incorporation of CO into a polyolefin chain allows tailoring of the resulting aliphatic polyketone properties in a very broad and versatile manner. The copolymerization of olefins and carbon monoxide can be performed in three different ways. Two of these, namely polymerization mediated by γ-rays and by radicals, have not found widespread application because of the harsh reaction conditions needed and the poor control of the structure and properties of resulting copolymers.4,6 By contrast, a third approach based on catalytic conversion of suitable alkenes and CO to high molecular weight copolymers with late transition metals (Scheme 1) was implemented in industrial pilot plants by Shell and BP in the early 1990s.7,8 This process was realized due to the development of highly active catalytic systems, which afforded excellent control over the copolymerization process through systematic catalyst design.
Catalytic copolymerization of olefins with CO of this type results in copolymers that show an exclusively alternating structure. The generally accepted mechanism for the polymer chain propagation reaction has been widely discussed in the literature, revealing that the catalytic cycle initiates most probably with the insertion of ethene in a Pd–H bond, followed by insertion of CO. Chain propagation then proceeds with the strictly alternating insertion of ethene and CO. The main chain termination reaction is via methanolysis. 9 It was demonstrated that the consecutive insertion of two CO monomers is thermodynamically unfavorable, whereas the sequential incorporation of two olefin monomers is unlikely on kinetic grounds. For example, in the absence of CO, a (P^P) chelating palladium bis -phosphine catalyst does not homopolymerize ethene but only dimerizes it to form butenes.10 However, neutral phosphine-sulfonate (P^O)Pd catalysts were shown to incorporate higher levels of ethene (∼30% relative to the alternating polyketone composition) under increased ethene pressure in copolymerization reactions.11 An increased ethene content in the polymer leads to materials with lower stiffness and lower melting transition temperatures, which may be preferred for some applications. Other ways to modulate these important characteristics are based on terpolymerization with higher olefin substrates as well as on regio- and stereo-control of the copolymerization of CO and 1-alkenes.12
Philip C. Zehetmaier, Technical University of Munich, Germany; philip.
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