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Our investigations of the cationic ring-opening polymerization of oxetane via active chain end (ACE) mechanism have shown that the use of 1,4-dioxane as solvent can prevent intra-and intermolecular transfer reactions (Scheme 1, part a). Using 3-phenoxypropyl-1-oxonia-4-oxacyclohexane hexafluoroantimonate as a model of an initiator capable of yielding fast initiation, polymers with predictable number-average molecular weight (up to 160 000 g/mol), narrow molecular weight distribution (1.18 < M w /M n,GPC < 1.28) were produced with no cyclic oligomer formation. On the basis of the kinetic data, a mechanism of controlled and living polymerization has been proposed in which the rate of mutual conversion between "strain ACE species" (chain terminated by a tertiary 1-oxoniacyclobutane ion, A1) and "strain free ACE species" (chain terminated by a tertiary 1-oxonia-4-oxacyclohexane ion, T1) does not obey a quasi-steady-state assumption but depends on the rate at which the monomer converts the stable species T1 into a liVing "propagating" center A1(d[A1]/dt ) -d[T1]/dt * 0). With BF 3 ‚CH 3 OH (i.e., initiator yielding a slow initiation), a drift of the linear dependence M n,GPC vs conversion to lower molecular weight were observed together with the production of cyclic oligomers, ∼10% of the monomer consumed in 1,4-dioxane against ∼ 30% in dichloromethane.

Biodegradable, amphiphilic, diblock poly(ε‐caprolactone)‐block‐poly(ethylene glycol) (PCL‐b‐PEG), triblock poly(ε‐caprolactone)‐block‐poly(ethylene glycol)‐block‐poly(ε‐caprolactone) (PCL‐b‐PEG‐b‐PCL), and star shaped copolymers were synthesized by ring opening polymerization of ε‐caprolactone in the presence of poly(ethylene glycol) methyl ether or poly(ethylene glycol) or star poly(ethylene glycol) and potassium hexamethyldisilazide as a catalyst. Polymerizations were carried out in toluene at room temperature to yield monomodal polymers of controlled molecular weight. The chemical structure of the copolymers was investigated by 1H and 13C NMR. The formation of block copolymers was confirmed by 13C NMR and DSC investigations. The effects of copolymer composition and molecular structure on the physical properties were investigated by GPC and DSC. For the same PCL chain length, the materials obtained in the case of linear copolymers are viscous whereas in the case of star copolymer solid materials are obtained with low Tg and Tm temperatures. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 3975–3985, 2007

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Biodegradable and amphiphilic diblock copolymers [polylactide-block-poly-(ethylene glycol)] and triblock copolymers [polylactide-block-poly(ethylene glycol)block-polylactide] were synthesized by the anionic ring-opening polymerization of lactides in the presence of poly(ethylene glycol) methyl ether or poly(ethylene glycol) and potassium hexamethyldisilazide as a catalyst. The polymerization in toluene at room temperature was very fast, yielding copolymers of controlled molecular weights and tailored molecular architectures. The chemical structure of the copolymers was investigated with 1 H and 13 C NMR. The formation of block copolymers was confirmed by 13 C NMR and differential scanning calorimetry investigations. The monomodal profile of the molecular weight distribution by gel permeation chromatography provided further evidence of block copolymer formation as well as the absence of cyclic species. Additional confirmation of the block copolymers was obtained by the substitution of 2-butanol for poly(ethylene glycol); butyl groups were clearly identified by 1 H NMR as polymer chain end groups. The effects of the copolymer composition and lactide stereochemistry on the copolymer properties were examined. V V C 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: [2235][2236][2237][2238][2239][2240][2241][2242][2243][2244][2245] 2007

Poly(lactide) (PLA), poly(e-caprolactone) (PCL) and poly(trimethylene carbonate) (PTMC) homopolymers of high molecular weight were prepared using potassium-based catalyst. Polymerizations were carried out in toluene at room temperature. The chemical structure of the polymers was investigated by 1 H and 13 C NMR. The physical properties investigated by GPC and DSC for the polymers obtained are similar to those prepared using tin octanoate based catalyst. Using a sequential polymerization procedure, PLA-b-PCL, PLA-b-PTMC, and PCL-b-PTMC diblock copolymers were synthesized and characterized in terms of their composition and physical properties. The formation of diblock copolymers was confirmed by NMR and DSC measurements. In vitro cytotoxicity tests have been carried out using MTS assay and the results show the biocompatibility of these polymers in the presence of the fibroblast cells. V V C 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 5348-5362, 2008

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