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Perfectly Alternating Copolymer of Lactic Acid and Ethylene Oxide As a Plasticizing Agent for Polylactide

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Perfectly Alternating Copolymer of Lactic Acid and Ethylene Oxide As a Plasticizing Agent for Polylactide Macromolecules 2001, 34, 8641-8648 8641 Perfectly Alternating Copolymer of Lactic Acid and Ethylene Oxide as a Plasticizing Agent for Polylactide Kevin Bechtold, Marc A. Hillmyer,* and William B. Tolman* Department of Chemistry, University of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota 55455-0431 Received August 20, 2001; Revised Manuscript Received October 1, 2001 ABSTRACT: The ring-opening polymerization of 3-methyl-1,4-dioxan-2-one (MDO) mediated by a catalytic system composed of Y[N(TMS)2]3 and benzyl alcohol (BnOH) led to a new polymer (PMDO) comprised of perfectly alternating lactic acid and ethylene oxide repeat units. Samples of PMDO were characterized by NMR spectroscopy, size exclusion chromatography (SEC), and matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS). In a series of MDO polymerizations ([MDO]0 ) 3.0Min toluene, between -30 and 60 °C), the equilibrium monomer concentrations were measured. The thermodynamic parameters for the polymerization reaction were determined: ∆Hp° )-12.1 ( 0.5 kJ -1 -1 -1 mol and ∆Sp° )-42 ( 2 J mol K . The glass transition temperature (Tg) of PMDO was determined to be ≈-24 °C by differential scanning calorimetry and found to vary little over the molecular weight range studied (3-21 kg/mol). Mixtures of low-molecular-weight PMDO and atactic polylactide (PLA) were prepared by solution casting and subsequent annealing significantly above the Tg’s of the individual components. Miscibility of PLA and PMDO was evinced by single Tg’s that were well-described by the Fox relationship for miscible blends. Because of its miscibility with PLA and low Tg, PMDO has potential as a macromolecular plasticizing agent for commercially relevant PLA. Introduction Scheme 1 Biodegradable and biorenewable polymers that do not rely on petrochemical-based feedstocks and decompose into nontoxic byproducts are environmentally friendly alternatives to current commodity polymers.1 One such material, polylactide (PLA, Scheme 1), is particularly significant since it is derived from fermentation of corn starch and other abundant naturally occurring biomass2 and ultimately may be degraded to carbon dioxide and water.3 PLA and its copolymers are currently used in biomedical applications,4-6 but a broad substitution of PLA for traditional plastics will require increased diversification beyond the parent polymer.7,8 The tensile properties of PLA are generally character- ized by a rather low elongation at break and moderate impact resistance.2 These features can restrict the use of the unmodified polymer for flexible blown and cast 3 film applications. Lowering the Tg of amorphous PLA exhibit biodegradable characteristics, and would be (≈60 °C), which lies above typical use temperatures, is minimally leached. a strategy used to toughen the parent polymer. Methods Poly--caprolactone,20 poly(3-hydroxy butyrate),18 ther- 17 8 to lower the Tg of PLA have generally involved synthesis moplastic starch, oligomeric PLA, and poly(ethylene of copolymers9-14 or physical blending with plasticizing oxide) (PEO)24 have all been investigated as potential agents15-20 (often an economical alternative to copoly- plasticizing agents for PLA. Of the miscible materials, merization schemes). While low-molecular-weight sub- PEO exhibits exceptional promise as a plasticizer due stances are typically the best plasticizing agents,21 in to excellent miscibility with PLA (even at high molecular applications such as food packaging, low-molecular- weights24) and drastic improvement in elongation at weight materials are often not appropriate because of break and impact resistance at low PEO loadings.16 problems with leaching.15,22 Polymeric plasticizing agents However, in one reported example, high PEO loadings can attenuate the leaching problem, but two-component (above 50%) in PLA led to some macrophase separation systems containing high-molecular-weight materials due to crystallization of the PEO.25 Furthermore, be- generally exhibit unfavorable thermodynamics of mix- cause of the hydrophilic nature of PEO, even mild ing, leading to macrophase separation, poor interfacial aqueous extraction can result in extensive leaching of adhesion, and inferior mechanical properties.23 The PEO from the host polymer.26 The crystallinity and ideal plasticizing agent for PLA packaging materials hydrophilicity of PEO are drawbacks to this otherwise would be miscible with PLA, significantly lower the Tg, useful plasticizer. To combat some of PEO’s deficiencies, block copoly- * To whom correspondence should be addressed. E-mail: mers of PLA and PEO have been synthesized and [email protected]; [email protected]. investigated as PLA plasticizers.27 The plasticization 10.1021/ma0114887 CCC: $20.00 © 2001 American Chemical Society Published on Web 11/06/2001 8642 Bechtold et al. Macromolecules, Vol. 34, No. 25, 2001 1 Figure 1. H NMR spectrum of MDO (500 MHz, CDCl3). behavior for these compounds is complicated due to a Scheme 2 dependence on the PEO block length; some samples exhibited microphase separation and crystallization of the PEO blocks, resulting in incomplete plasticization of the host polymer. In a separate strategy, the direct copolymerization of L-lactide with ethylene oxide was reported to yield copolymers having a multiblock struc- ture.28 Films formed from blends of these copolymers and PLA exhibited improved film moduli and yield mixture was treated directly with p-toluenesulfonic acid strengths as well as comparable elongations at break to complete the intramolecular transesterification. Pure relative to PLA/PEO homopolymer blends of identical racemic MDO was then isolated after chromatographic purification in an overall yield of ≈20%. The identifica- composition. Leaching of these copolymer plasticizing 1 13 agents was proposed to be greatly reduced compared to tion of MDO was accomplished by H and C NMR and FTIR spectroscopy as well as high-resolution mass the case of PEO. Although some control over the 1 individual block sizes was exerted through variation of spectrometry. Of particular note is the H NMR spec- the reaction conditions, all of the reported samples trum (Figure 1), which exhibits complex, yet well- exhibited two melting transitions, indicating that the resolved, splitting patterns for the diastereotopic ring blocks were of sufficient length to undergo crystalliza- protons (inset) in the chiral molecule. tion-induced microphase separation. For the polymerization of MDO we chose to use a yttrium alkoxide initiator.29-33 An easily prepared On the basis of the aforementioned considerations, we system with previously demonstrated high reactivity29 hypothesized that a polymer comprised of perfectly may be prepared “in situ” by the reaction of Y[N(TMS) ] alternating lactic acid and ethylene oxide units would 2 3 with alcohols. We successfully polymerized MDO by be an efficient PLA plasticization agent. To test this adding a solution of Y[N(TMS) ] in toluene to a solution notion, we developed a preparative route to this polymer 2 3 of BnOH and excess MDO in toluene with rapid stirring by the ring-opening polymerization of 3-methyl-1,4- at ambient temperature (Scheme 1; see Experimental dioxan-2-one (MDO, Scheme 1). The polymerization Section for details). Conversion of MDO was determined thermodynamics were examined, and the new polymer using 1H NMR spectroscopy (Figure S1). Quenching of (PMDO) was characterized. Finally, the potential utility the polymerization was accomplished in most cases by of PMDO as a macromolecular plasticizing agent for exposure to air, followed by dissolution in a minimum PLA was revealed through examination of the thermal amount of CHCl and precipitation from excess cold behavior of PMDO and PMDO/PLA blends. 3 pentane.34 Polymerizations were quite rapid; for the reaction of MDO (100 mg, 8.6 × 10-4 mol) with BnOH Results and Analysis (10 µL of 1.0 M toluene solution, 1 × 10-5 mol) and -6 Monomer and Polymer Synthesis. The prepara- Y[N(TMS)2]3 (20 µL of 0.05 M toluene solution, 1 × 10 tion of MDO was based on a previous method44 reported mol) conversions of 75, 78, and 80% were determined for the synthesis of analogues in which the methyl group for samples taken at 0.5, 2, and 24 h, respectively. In is replaced by longer alkyl chains (Scheme 2). In a key this example, the viscosity of the solution increased so difference from the reported sequence, we found that rapidly that stirring ceased less than 30 s after the the initial reaction of the monosodium salt of ethylene catalyst was added. glycol with racemic ethyl-2-bromopropionate did not The product polymer was characterized by NMR provide the intermediate ethyl-2-(2-hydroxyethoxy)- spectroscopy, size exclusion chromatography (SEC), and propionate cleanly and instead yielded a mixture of matrix-assisted laser desorption ionization mass spec- compounds, including some MDO (GC/MS). After sev- trometry (MALDI-MS). Shown in Figure 2 is the MALDI- eral failed attempts at purifying the intermediate, the MS spectrum for a low-molecular-weight PMDO sample. Macromolecules, Vol. 34, No. 25, 2001 Copolymer of Lactic Acid and Ethylene Oxide 8643 Table 1. Data for the Room Temperature Polymerization of MDOa b c d e e e entry M0/I0/C0 MN(theor) MN(NMR) MN MW PDI 1 260/6.4/1 3.5 3.0 3.4 7.2 2.08 2 260/3.2/1 7.0 6.5 5.4 12.6 2.36 3 260/1.6/1 14.0 10.0 6.7 18.6 2.80 4 260/0.8/1 27.9 16.4 6.3 22.2 3.52 5 260/0.4/1 55.8 20.8 6.6 31.8 4.80 a All samples at 74% conversion after 18 h; molecular weights -1 b are given in kg mol . Initial MDO/BnOH/Y[N(TMS)2]3 mole ratio. c At 74% conversion. d 1H NMR integration of benzyl ester end group to polymer, assuming one chain per end group, and no other polymeric species present.
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