Elsevier

Polymer

Volume 102, 12 October 2016, Pages 248-255
Polymer

Tuning the crystallinity and degradability of PCL by organocatalytic copolymerization with δ-hexalactone

https://doi.org/10.1016/j.polymer.2016.09.026Get rights and content

Highlights

  • Organocatalytic ROP of ε-caprolactone and δ-hexalactone yields random copolymers.

  • Copolymers present reduced crystallinity and enhanced enzymatic degradability.

  • δ-Hexalactone as a comonomer impacts more profoundly than trimethylene carbonate.

Abstract

The copolymerization of ε-caprolactone (CL) with its structural isomer, δ-hexalactone (HL), or trimethylene carbonate (TMC) catalysed by diphenyl phosphate was investigated. Copolymers with molar fraction of CL units ranging from 54% to 95% were obtained. Molecular characterization exhibited random distribution of the two monomeric units (CL/HL or CL/TMC), owing to the extensive occurrence of transesterification scrambling reactions at high temperature. Compared with poly(ε-caprolactone) (PCL), the copolymers showed reduced crystallinity and enhanced enzymatic degradability as revealed by the analysis with differential scanning calorimetry and quartz crystal microbalance with dissipation, respectively. Interestingly, the incorporation of HL units showed a more profound impact than TMC. It is therefore considered that organocatalytic copolymerization with HL, a bio-renewable lactone, can serve as an effective method to regulate the properties of PCL.

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.

References (59)

  • J. Zhao et al.

    Polymerization of 5-alkyl δ-lactones catalyzed by diphenyl phosphate and their sequential organocatalytic polymerization with monosubstituted epoxides

    Polym. Chem.

    (2015)
  • M. Schappacher et al.

    Study of a (trimethylenecarbonate-co-ε-caprolactone) polymer–Part 1: preparation of a new nerve guide through controlled random copolymerization using rare earth catalysts

    Biomaterials

    (2001)
  • Y. Hou et al.

    In situ investigations on enzymatic degradation of poly(ɛ-caprolactone)

    Polymer

    (2007)
  • K.W. Ng et al.

    Evaluation of ultra-thin poly(ε-caprolactone) films for tissue-engineered skin

    Tissue Eng.

    (2001)
  • N. Bölgen et al.

    In vivo performance of antibiotic embedded electrospun PCL membranes for prevention of abdominal adhesions

    J. Biomed. Mater. Res. Part B

    (2007)
  • W.J. Jia et al.

    Preparation of biodegradable polycaprolactone/poly(ethylene glycol)/polycaprolactone (PCEC) nanoparticles

    Drug Deliv.

    (2008)
  • C. Ma et al.

    Degradable polyurethane for marine anti-biofouling

    J. Mater. Chem.

    (2013)
  • T. Russell et al.

    An investigation of the compatibility and morphology of semicrystalline poly(ε-caprolactone)–poly(vinyl chloride) blends

    J. Polym. Sci. Polym. Phys. Ed.

    (1983)
  • Y.W. Cheung et al.

    Critical analysis of the phase behavior of poly(ɛ-caprolactone)(PCL)/polycarbonate (PC) blends

    Macromolecules

    (1994)
  • A. Arbaoui et al.

    Metal catalysts for ε-caprolactone polymerisation

    Polym. Chem.

    (2010)
  • S. Li et al.

    Structural characterization and hydrolytic degradation of a Zn metal initiated copolymer of L-lactide and ε-caprolactone

    J. Biomater. Sci. Polym. Ed.

    (1997)
  • C.G. Pitt et al.

    Aliphatic polyesters. I. The degradation of poly(ε-caprolactone) in vivo

    J. Appl. Polym. Sci.

    (1981)
  • Y. Kumagai et al.

    Enzymatic degradation of microbial poly(3-hydroxybutyrate) films

    Makromol. Chem.

    (1992)
  • R. Riva et al.

    Combination of ring-opening polymerization and “Click Chemistry”:  toward functionalization and grafting of poly(ε-caprolactone)

    Macromolecules

    (2007)
  • R.K. Srivastava et al.

    Microblock copolymers as a result of transesterification catalyzing behavior of lipase CA in sequential ROP

    Macromolecules

    (2007)
  • R.F. Storey et al.

    Hydroxy-terminated poly(ε-caprolactone-co-δ-valerolactone) oligomers: synthesis, characterization, and polyurethane network formation

    J. Polym. Sci. Part A Polym. Chem.

    (1991)
  • M. Hakkarainen et al.

    Tuning the release rate of acidic degradation products through macromolecular design of caprolactone-based copolymers

    J. Am. Chem. Soc.

    (2007)
  • E. Bat et al.

    Trimethylene carbonate and ε-caprolactone based (co)polymer networks: mechanical properties and enzymatic degradation

    Biomacromolecules

    (2008)
  • M. Hakkarainen et al.

    ESI-MS reveals the influence of hydrophilicity and architecture on the water-soluble degradation product patterns of biodegradable homo- and copolyesters of 1, 5-dioxepan-2-one and ε-caprolactone

    Macromolecules

    (2008)
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