Synthesis and characterization of poly(L‐lactide‐co‐ε‐ caprolactone) copolymers: influence of sequential monomer addition on chain microstructure

Abstract

A series of copolymers with various compositions were synthesized by one‐step and two‐step bulk ring‐opening polymerizations of L‐lactide (LA) and ε‐caprolactone (CL) using stannous octoate [Sn(Oct)2] and 1‐hexanol as the initiating system. For the sequential two‐step polymerization, a poly(ε‐caprolactone) (PCL) prepolymer was polymerized first to a percent conversion of approximately 70% and LA then added in order to produce a copolymer with a chain microstructure different from that obtained from the corresponding one‐step reaction. The resulting copolymers were characterized using a combination of nuclear magnetic resonance spectroscopy (1H‐ and 13C‐NMR), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and gel permeation chromatography (GPC). The average sequence lengths of the lactidyl ($l_{{\rm LL}}^{\rm e}$) and caproyl ($l_{{\rm cap}}^{\rm e}$) units, the degree of randomness (R) and the transesterification coefficient (T(II)) were calculated from the 13C‐NMR spectra. The appearance of a signal due to CapLCap sequences was directly attributable to transesterification of lactidyl (LL) units. It was found that both $l_{{\rm LL}}^{\rm e}$ and $l_{{\rm cap}}^{\rm e}$ values from the two‐step syntheses were significantly longer than from the corresponding one‐step syntheses, leading to different semi‐crystalline morphologies and chain microstructures. The copolymers all showed at least some blocky chain structure as a result of the significant difference in monomer reactivity (LA > CL) between LA and CL. Thermal properties including stability depended on both composition and chain microstructure which could be controlled by the method of synthesis. From their DSC curves, the two‐step copolymers were seen to be semi‐crystalline whereas the one‐step copolymers were mainly amorphous. A more blocky microstructure, as obtained from the two‐step method, appeared to result in a lower thermal stability. Copyright © 2007 John Wiley & Sons, Ltd.

Ring-opening polymerization of l-lactide and ε-caprolactone catalyzed by versatile tri-zinc complex: Synthesis of biodegradable polyester with gradient sequence structure

“…The TGA data illustrates that PCL moiety exhibits higher T 5% than PLA moiety, and consequently the incorporation of CL moieties into the copolymers can enhance the thermal stability copolymers, which is consistent with the recognized studies. 46 DSC thermograms of the copolymers consisting of varying LA/CL ratio are given in Fig. 9 It is rather difficult to detect the T g of neat PCL under adopted conditions owing to the crystalline nature.…”

Study of the chain microstructure effects on the resulting thermal properties of poly(l-lactide)/poly(N-isopropylacrylamide) biomedical materials

“…Unfortunately, the poor thermal stability of PLLA results in partially degraded parts during typically available thermoplastic melt conformation processes such as blow molding, extrusion or injection molding [15,16], which yields a reduction in functional properties and increased toxicity of degradation byproducts as compared with the starting material [17,18]. In addition, the relatively low glass transition temperature of PLLA [19] could be detrimental for some applications in which service temperature is above room temperature.…”

Physical and thermal properties of l‐lactide/ϵ‐caprolactone copolymers: the role of microstructural design

“…In addition to molecular weight, the microstructural design of biodegradable polymers is another key element for biomedical applications . It is well known that PLL is a brittle polymer of high modulus and low percentage elongation, while PCL is slow to biodegrade and is a flexible, low melting point polymer, properties which limit their respective applications as homopolymers .…”