Study Of Lactide/Trimethylene Carbonate Comb-Shape Polymer

Poly(L-lactide) (PLLA) is a biodegradable and biocompatible aliphatic polyester. PLLA has been widely used in biomedical and pharmaceutical applications, such as surgical sutures, scaffolds and carriers for drug delivery etc. However, PLLA shows some disadvantages such as brittle nature, slow crystallization rate and poor processing performance. To tailor the properties of PLLA for more suitable applications, branching and copolymerization methods are performed. So far, most of the branched PLLA are produced with broad molecular weight distribution and uncontrollable branching degree. Due to the difficulty of preparing well-defined branched structure, the structure-properties relationship is unclear.In this work, well-defined linear-comb and star-comb PLLA homopolymers and PLLA/poly(trimethylene carbonate) (PTMC) copolymers were synthesized using linear/star hydroxylated polybutadiene as macroinitiators and non-metallic organocatalyst. The influence of graft length, graft density and topological structure on the properties of polylactide were systematic studied. Well, the influence of comonomer composition and sequence structure on the properties of PLLA/PTMC copolymer were also studied. Degradation properties of the polymers were evaluated. The main contents and results are as follows:Series of linear-comb PLLA (1cPLLA) were synthesized by ring-opening polymerization (ROP) of L-lactide (1-LA) using linear hydroxylated polybutadiene (1PB-OH) as a macroinitiator and 1,8-diazabicyclo[5,4,0]undec-7-ene (DBU) as a catalyst. Both standard first-order kinetics in monomer concentration and the linear relationship between molecular weight and conversion approve that the ROP of 1-LA in this macroinitiator system is a living chain-growth polymerization. The influence of graft length and graft density on properties were investigated:the increase of graft length and graft density both lead to an elevated value of enhanced complex modulus G*and increased complex viscosity ?*. Comparatively, graft density contributes more to the improvement of rheological property than graft length does; DSC results indicated that the increase of graft density resulted in a lower Tg, and also the less perfect crystals that were responsible for the lower Tm, Tm0 and ?Hm values observed experimentally; From the isothermal crystallization, the increase of graft density exhibits an improvement of spherulitic growth rate G, while the increase of chain length reduces G.Series of star-comb PLLA (scPLLA) were synthesized by ROP of L-LA using star hydroxylated polybutadiene (sPB-OH) as a macroinitiator and DBU as a catalyst. On the basis of systemic study of the scPLLA properties, to illustrate the topological structure-property relationship of 1PLLA, 1cPLLA and scPLLA. Linear-comb and star-comb graft PLLA were comparatively studied with linear PLLA. Intrinsic viscosity measurement confirms that the scPLLA displays the smallest hydrodynamic volumes in solution. Tg and Tm values of the graft PLLA were lowered owing to their non-linear architecture. Both DSC analysis and POM results indicated that the comb architectures didn't alter the structures of PLLA crystallites, but markedly improved the crystallization behavior. lcPLLA has the highest crystallinity, which indicates the more compacted comb structure of scPLLA presents steric hindrance of the graft points. Rheological measurements demonstrates that more closeness of the side chains, much lower complex viscosity and modulus the PLLA shows, which indicates the star-comb PLLA has the lowest complex viscosity and modules compared with its linear-comb and linear analogues.Series of linear-comb copolymers with block (1cP(TMC-b-LLA)), gradient (1cP(LLA-grad-TMC)) and random (1cP(LLA-ran-TMC)) side chains have been synthesized upon sequential or random ROP of 1-LA and TMC, using 1PB-OH as a macroinitiator and DBU/TBD as organocatalyst. The copolymerization kinetics were studied to prove that the polymerization rates of the comonomer were similar in this system (kp, LLA= 0.17 min-1, kp, TMC= 0.16 min-1). By increasing the content of TMC in the block side chain, both Young's modulus (E) and ultimate tensile strength (?b) decreased while the elongation at break (?b) increased. The Tg of the 1cP(LLA-ran-TMC) changed with the composition and could just fit in the Fox equation. The properties of these three topology copolymers with same molar composition were investigated comparably and yield unique properties:Two Tg values are observed in 1cP(TMC-b-LLA)4; 1cP(LLA-grad-TMC) copolymer can possess distinctively broad ?Tg around 34?; the 1cP(LLA-ran-TMC) uniform copolymer behaves like typical rubber with a significant high ? value (?b= 1797%) even higher than the pure lcPTMC (?b= 865%).Degradation properties of the above polymers were systemic evaluated to illustrate the influences of composition, sequence structure and topological structure. The weight loss, water absoption and degradation rate (k) were inceased with 1-LA content. The k value decreased as, 1cP(LLA-grad-TMC)> 1cP(LLA-ran-TMC)> 1cP(TMC-b-LLA). Compared with linear polymer, the comb-shape polymer had much lower k value. Therefore, copolymerization of TMC and 1-LA is an effective method to adjust the degradation rate of PLLA, and for illustrating the structure-degradation property relationship.