There remains significant academic interest in the design of new catalyst systems capable of producing biodegradable polymers [1–3]. This interest stems in part from issues relating to the inertness of polyethylene and subsequent landfill issues, but also from the potential for such biodegradable polymers to be employed in other areas such as the biomedical arena. This is typified by
polycaprolactone (PCL), which has found application in tissue engineering and possesses drug permeability [4]. The formation of PCL via the ring opening polymerization (ROP) of ?-caprolactone using metal complexes as initiators, usually in the form of alkoxides, is a favoured synthetic route for PCL formation. However, despite the continued interest in such systems, the catalysts deployed in ROP
tend to be either based on main group species, primarily of aluminum, or are based on a select number of transition metals, lanthanides or, more recently, systems utilizing alkali/alkaline earth metals. By contrast, the more earth-abundant metals, have received far less attention. We were attracted to the potential use of molybdenum for the ROP of cyclic esters given its excellent track record over the last couple of decades in ring opening metathesis polymerization (ROMP), as well as its low cost and toxicity. Such ROMP studies have revealed the ability of the molybdenum complexes to promote living polymerizations, and to tolerate a wide range of functionalities, which bodes well for the proposed studies herein [5]. Central to such chemistry has been the use of high valent bis(imido) species, due to their ease of preparation and facile modification. The variety of precursor anilines available means that there is much scope for controlling both the steric and electronic properties of the resultant imido group at the metal, which in turn can influence both the catalytic activity and properties of the polymer products. With this in mind, we recently reported the use of molybdenum chelate complexes derived from the oxydianiline [(2-NH2C6H4)2O], and found that that for the ROP of ?-caprolactone, conversion rates
were good (>90%) at high temperatures (100 °C) [6]. As part of that study, a siloxide complex was also isolated and was found to be active without the need for the addition of external alcohol; for the chloride species the addition of benzyl alcohol was necessary to generate an alkoxide. Previous use of molybdenum species in the ROP of cyclic esters is somewhat limited [7–12]. Given this, we have now extended our studies to high-valent molybdenum imido phenolate chemistry, where again the expectation is that the addition of an external alcohol would not be necessary for ROP activity. We report the use of bulky di-phenols in combination with bulky organoimido groups which allows for the isolation of mono-nuclear four coordinate complexes, whilst variation of the imido group can lead to bridged di-nuclear species. The use of tri- and tetra-phenolates has also been explored, and in the case of the
latter, in the form of the para and meta pro-ligands, the possibility of possible cooperative effects has been investigated