Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
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Versatile Ring-Opening Copolymerization and Postprinting Functionalization of Lactone and Poly(propylene fumarate) Block Copolymers: Resorbable Building Blocks for Additive Manufacturing Shannon R. Petersen, James A. Wilson, and Matthew L. Becker* Department of Polymer Science, University of Akron, Akron, Ohio 44325, United States
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ABSTRACT: Additive manufacturing has the potential to change medicine, but clinical applications are limited by a lack of resorbable, printable materials. Herein, we report the first synthesis of polylactone and poly(propylene fumarate) (PPF) block copolymers with well-defined molecular masses and molecular mass distributions using sequential, ring-opening polymerization and ring-opening copolymerization methods. These new copolymers represent a diverse platform of resorbable printable materials. Furthermore, these polymers open a previously unexplored range of accessible properties among stereolithographically printable materials, which we demonstrate by printing a polymer with a molecular mass nearly 4 times that of the largest PPF homopolymer previously printed. To further demonstrate the potential of these materials in regenerative medicine, we report the postprinting “click” functionalization of the material using a copper-mediated azide− alkyne cycloaddition.
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INTRODUCTION Since the advent of additive manufacturing, regenerative medicine has undergone a paradigm shift toward anatomically correct and patient specific treatment options. Many injuries and defects pose significant challenges to surgeons because they are geometrically complex, anatomically variable, and often require mechanical and structural support to ensure proper healing. The use of additive manufacturing techniques in conjunction with advanced medical imaging to preoperatively render uniform, high-fidelity, and defect specific scaffolds offers the potential to improve healing, significantly reduce operating costs, and fundamentally alter how surgeons approach these complicated reconstructive procedures.1 However, widespread clinical application of these treatment options is highly dependent on the development of specialized materials that possess the requisite properties for 3D printing, while maintaining the structural and biological properties necessary for specific regenerative applications. Degradable polyesters have been heavily investigated in this application as a consequence of their low toxicity and facile hydrolytic degradation into resorbable byproducts.2−5 While many polyesters are synthesized via step-growth polymerization methods, chain-growth methods offer facile control of molecular mass and molecular mass distribution, which facilitates uniform properties and degradation.6,7 One of the most common chain-growth methods for obtaining polyesters is the ring-opening polymerization (ROP) of lactones or lactides, but limited chemical and structural diversity within the lactone and lactide monomer families restrict these polymers to a narrow range of accessible degradation, © XXXX American Chemical Society
mechanical, rheological, and thermal properties, which limit their utility in some tissue engineering applications.5,7−11 An alternative chain-growth polymerization strategy to expand the range of accessible polyesters is the ring-opening copolymerization (ROCOP) of epoxides and anhydrides.5,13 The introduction of two distinct monomer species allows for synthesis of materials with properties and functionalities not accessible by the strict ROP of lactones. The technique has also improved the biomedical viability of promising polymers like poly(propylene fumarate) (PPF), which was previously hindered by the high energy input, low yields, and lack of control over molecular mass associated with its step-growth polycondensation synthesis.13−15 PPF has been heavily investigated for use in biological scaffolds as a consequence of its hydrolytic degradation into resorbable byproducts, moduli comparable to that of human cancellous bone, and an unsaturated backbone that can be used for photochemical cross-linking reactions in stereolithographic printing.13−17 Stereolithographic printing techniques, such as continuous digital light processing (cDLP), utilize a digital projector to selectively cross-link fluid resins. The resolution of these printers is much higher than extrusionbased printing techniques and ideal for producing tissue engineering scaffolds.15,16,20,21 Despite its promise, PPF is a solid at molecular weights greater than 4000 Da, so only oligomeric resins with an equal weight percent of reactive diluent have been printed.15 Large sol fractions will continue to Received: June 28, 2018 Revised: July 6, 2018
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DOI: 10.1021/acs.macromol.8b01372 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
poly(propylene maleate) (PPM), and the BnOH initiator. No resonances corresponding to the methylene protons of homopolymerized PO were observed (δ = 3.3−3.5 ppm), confirming the preference for the alternating copolymerization of MAn and PO afforded with the Mg(BHT)2(THF)2 that was previously reported (Figure 1A).14 End-group analysis from 1H NMR spectroscopy was used to determine the number-average molecular mass of the resulting polymer (Mn). Size exclusion chromatography (SEC) was used to determine that the molecular mass distribution (ĐM) of the recovered polymer; ĐM = 1.21 (Figure 1B). The ĐM value is consistent with PPF homopolymers synthesized using ROCOP and significantly narrower than those obtained for PPF and PCL copolymers synthesized via step-growth or transesterification methods.14,42 This clearly indicates that transesterification side reactions are limited during the polymerization. The structure and regioregularity of the recovered polymer were investigated further using quantitative 13C NMR spectroscopy (Figure 1C) and diffusion-ordered NMR spectroscopy (DOSY) (Figure 1D). The 13C NMR spectra revealed a prominent carbonyl diad resonance at δ = 173.4 ppm, corresponding to an εCL carbonyl adjacent to another εCL repeat unit (εCL*−εCL, in which the asterisk denotes the observed carbonyl), and two prominent diad resonance peaks at δ = 164.57 ppm and δ = 164.19 ppm, which correspond to MAn*−PO. Two very small resonances are also present at δ = 173.2 ppm and δ = 164.9 ppm, which correspond to εCL*− MAn and MAn*−εCL, respectively. Integration of the carbonyl diad resonance peaks denote blocklike sequencing, in which one block is a PCL homopolymer and the other block is PPM. DOSY NMR spectroscopy revealed a single diffusing species, confirming the synthesis of the block copolymer poly(ε-caprolactone-block-propylene maleate) (P(CL-b-PM)) as opposed to individual PCL and PPM chains. To exploit the alkene functionality present in the PPM block for stereolithographic printing, isomerization of P(CL-b-PM) into poly(ε-caprolactone-b-propylene fumarate) (P(CL-b-PF)) without cleavage or side reactions must occur. Hence, a 0.5 M solution of P(CL-b-PM) in CHCl3 with diethylamine (0.15 mol equiv per alkene) was heated at reflux overnight.15 The diethylamine was removed by washing with 0.5 M sodium phosphate buffer solution prior to solvent removal via rotary evaporation. A complete reduction of the cis-alkene resonance (δ = 6.2 ppm) and a new resonance corresponding to the trans-alkene protons (δ = 6.7 ppm) were shown using 1H NMR spectroscopy (Figure S2). The kinetics of the ROCOP of MAn and PO onto a PCL block were studied under the same conditions with a targeted degree of polymerization (DP) of 50 repeat units for both PCL and PPM blocks. Aliquots were withdrawn every 24 h over a period of 6 days after the injection of propylene oxide and maleic anhydride into the polymerization solution. 1H NMR spectroscopic analysis of the crude mixture was used to determine MAn conversion (Figure 2A). 1H NMR spectroscopic analysis and SEC of the recovered material were used to determine molecular mass and ĐM, respectively (Figure 2B). Similar to the ROCOP of MAn and PO from a primary alcohol initiator, the polymerization follows pseudo-first-order kinetics, but the rate of monomer conversion is hindered. This is likely a consequence of the lactone coordinating with the metal center of the catalyst, which has been observed in previous systems that utilize this catalyst with a lactone macroinitiator.43 Highly
be a barrier to distributed manufacturing and regulatory approval. Thus, the clinical translation of PPF is dependent on the development of methods and formulations to tailor PPF properties while maintaining a viscosity low enough for stereolithographic printing (
