Tuning the crystallinity and degradability of PCL by organocatalytic copolymerization with δ-hexalactone
Graphical abstract
Introduction
Aliphatic polyesters such as polyhydroxyalkanoates, polylactide, polyglycolide, poly(ε-caprolactone) (PCL) and the corresponding copolymers have been a compelling class of materials for various biomedical applications including tissue engineering, drug release, wound dressing, and post-operative anti-adhesion due to their desirable properties such as crystallinity, biocompatibility and (bio)degradability [1], [2], [3], [4], [5]. Especially, it has been shown recently that such degradable polyesters have great potential for the fabrication of anti-biofouling surfaces because their decomposition and erosion under the action of enzymes and/or seawater in marine environments lead to self-renewal of the surface which prevents biofouling by sedimentation or growth [6], [7].
PCL is one of the most widely applied (bio)degradable polymers because of its good processibility (high solubility and low melting point) and blend-compatibility with a broad range of other polymers, e.g. poly(vinyl chloride) [8], polycarbonate [9] and poly(vinyl methyl ether) [10], as well as the low cost and high polymerizability of the monomer from which it is usually derived, ε-caprolactone (CL) [11], [12]. However, the application of PCL is still limited in many cases due to low degradation rate related to its hydrophobicity and crystallinity [13], [14], [15]. It has been demonstrated that the (bio)degradation of a crystalline polyester usually starts from the amorphous regions where the degradation rate is much higher than that in the crystalline regions [16]. Therefore, attenuating the crystallinity of polyesters has been regarded and employed as an efficient method to refine their (bio)degradation behaviour [17].
So far, several routes have been developed to regulate the crystallinity and, consequently, the (bio)degradation rate of PCL-based materials, among which copolymerization and blending with other polymers are the most frequently used [18], [19], [20], [21]. In general, copolymerization (constitution of CL-based copolymers) shows several advantages over the blending method, including lower amounts of heterogenous component needed, better compositional uniformity and stability, as it changes the chemical structure of PCL and thus modifies the properties intrinsically.
Commonly, PCL-derived copolymers are synthesized through the ring-opening copolymerization of CL and comonomers. It has been reported that copolymerization with 1,5-dioxepan-2-one, lactide and trimethylene carbonate (TMC) can increase the overall hydrophilicity and water uptake of the system and therefore enhance the degradation rate [22], [23], [24], [25], [26], [27]. In addition, PCL-based (linear) statistical copolymers as well as topological structures including block, multiblock, star-shaped copolymers and cross-linked networks have been synthesized and utilized to finely tune the crystallinity and consequently the (bio)degradability [28], [29], [30], [31]. Catalysts such as tin (Ⅱ) octoate are frequently applied for the copolymerization process [32], [33], [34]. As the poor solubility of metal catalysts may pose difficulties in the purification of the resultant copolymers and there has been an increasing concern about the bio- and environmental toxicity of metallic residues in polymeric materials, organocatalysts have been regarded as better alternatives for the synthesis of such (bio)degradable copolymers [35], [36].
Substituted δ-valerolactones, such as δ-hexalactone (HL, Scheme 1), are bio-renewable resources [37], [38] which have recently shown to undergo (co)polymerization in the presence of organocatalysts giving rise to homopolymers, block copolymers and statistical copolymers [39], [40], [41], [42], [43]. The polymerizability varies with the size and position of the substituent, and such polyesters are usually non-crystalline due to the existence of pendent alkyl groups. It has been reported that the incorporation of HL units in poly(ω-pentadecalactone) can reduce its crystallinity [42], [43]. Therefore, in the present study we have focused on the organocatalytic copolymerization of CL and HL, as the pendent methyl groups was expected to reduce the crystallinity of PCL and accelerate the enzymatic degradation process. For the purpose of comparison, the copolymers of CL and TMC, a frequently employed comonomer [44], [45], were also synthesized and investigated.
Section snippets
Chemicals
CL (Aladdin 99%) and HL (TCI 99%) were dried over calcium hydride and distilled under vacuum. Trimethylene carbonate (TMC; 99%) was purchased from Daigang Biomaterial Co. (Jinan), recrystallized from toluene and dried to a constant weight prior to use. Toluene, to be used as the polymerization solvent, was dried successively by calcium hydride and n-butyllithium. Diphenyl phosphate (DPP; Aldrich 99%) was dried by azeotropic distillation of toluene and dissolved in purified toluene to prepare a
Copolymerization of CL and HL or TMC
DPP has shown to be an efficient organocatalyst for CL, HL and TMC for both bulk polymerization and solution polymerization in toluene [40], [50], [51]. In the present study, DPP-catalysed copolymerization of CL and HL was performed in bulk at 100 °C (Table 1 and Scheme 1) with BA being the initiator and the molar ratio of ([CL]0+[HL]0)/[BA]0/[DPP] being 300/1/1. The feed ratio of [CL]0/[HL]0 was varied in order to obtain poly(CL-co-HL) copolymers with different comonomer compositions. 1H NMR
Conclusions
Ring-opening copolymerization of CL with HL or TMC at high temperature catalysed by an acidic organocatalyst, DPP, yields poly(CL-co-HL) or poly(CL-co-TMC) (statistical or random) copolymers and thus can be used as a facile method for compositional modification of PCL. The copolymers present reduced crystallinity and correspondingly enhanced enzymatic degradability compared to PCL homopolymer. Poly(CL-co-HL) shows good thermostability and enzymatic degradability with HL content being as low as
Acknowledgements
The financial support of National Natural Science Foundation of China (21504024), Ministry of Science and Technology of China (2012CB933802) and Fundamental Research Funds for Central Universities is acknowledged.
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2019, European Polymer JournalCitation Excerpt :δ-hexalactone shows identical structure to δ-valerolactone but possessing a methyl pendant group, which is sufficient to disrupt crystallinity of the resulting polymer, resulting in amorphous (Tg = −51 °C) viscous liquids at room temperature with limited applicability [49]. However, its copolymerization with other comonomers (e.g., ε-caprolactone [50], ω-pentadecalactone [51], ethylene brassylate [52]) has been proven a satisfactory strategy to tune the crystallinity, degradability and mechanical properties of the resulting copolymers. x = 5.
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2017, Progress in Polymer ScienceCitation Excerpt :Catalyzed by methanesulfonic acid (MSA), the copolymerization of εCL and TMC in toluene at 30 °C yields gradient copolymers with εCL units enriched at the initiating ends [55]. On the other hand, diphenyl phosphate (DPP)-catalyzed copolymerization conducted at much higher comonomer concentration and temperature (80 °C) results in completely random distribution of εCL and TMC units in the product due to extensive occurrence of transesterification reactions [56]. The bifunctional organocatalytic systems of TBD or N-methyl-TBD (MTBD) with TU have shown good control and selectivity for the OC-ROP of εCL, δ-valerolactone (δVL) and lactides (LA) (the reactivity of LA >> δVL > εCL) [89].