Controlled Synthesis of Linear Polyesters

The polymerizability of macrolides is expected to be much lower than that of CL or DXO because of the strain-free character of the large-sized rings. Only a few studies have been reported on polyesters derived from macrolactones by nonmethods, although in some of these no detailed description of the polymerization method has been provided. Skoglund and Fransson, for example, reported the thermophysical properties of polytridecanolactone and polypentadecanolactone, but failed to describe their method of preparation [23, 24]. 

The most effective initiators for anionic polymerization are alkali metals, alkali metal oxides (lithium, sodium and potassium alkoxides) and alkah metal naphtha-lenide complexes with crown ethers [2]. Anionic polymerizations are sometimes associated with termination and transfer reactions, while cycHc oHgomers are formed as a result of back-biting reactions. In CL polymerization initiated with potassium tertibutoxide, a huge amount of cyclic lactones was formed, yet in the presence of Hthium tertibutoxide in an apolar solvent the oligomer formation was significantly reduced. 

The anionic polymerization of 16-membered-lactone, co-pentadecalactone (PDL), initiated with potassium alkoxide under mild conditions was reported by Jedlinski et al. [26]. These authors showed that polymerization would occur under mild conditions (THF, 35 °C) to form high-molecular-mass polymers (M up to lOOOOOgmoT ), although the formation of cycHc oHgomers via a back-biting reaction was also noted. 

The aluminum alkoxide-initiated polymerization of lactones such as CL and DXO proceeds by a coordination-insertion mechanism, which involves acyl-oxygen bond cleavage of the monomer and insertion into the aluminum-oxygen bond of the initiator. The coordination of the exocycHc oxygen to the metal results in polarization, which in turn makes the carbonyl carbon of the monomer more 

The Hving ROP of CL is usually initiated by aluminum isopropoxide in toluene at 0-25 °C. The initiation rate is high compared to the rate of propagation, so that a narrow molecular weight distribution [M /M , polydisperisty index (PDl)] is obtained. There is no termination reaction and three polymer chains grow for each A1 atom [30]. A-B diblock copolymers of oxepan-2,7-dione and 2-oxepanone have been prepared by using this method [31]. 

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