Stabilizers for polyhydroxyalkanoates

Abstract

A composition comprising a polyhydroxyalkanoate (PHA) and a stabilizer comprising one or more of an aliphatic carboxylic acid, a cyclic anhydride, or a halocarboxylic acid having a hydroxyl or thiol group can be formed by contacting the PHA and the stabilizer and mixing them such that the stabilizer is incorporated into the PHA to stabilize the PHA. Typically, the PHA is melted and the stabilizer is added well before any degradation occurs (i.e., molecular weight reduction occurs). The aliphatic carboxylic acid having a hydroxyl group can be naturally occurring and can be particularly useful for human or food contact applications or applications with specific environmental constraints.

Description

[Technical field]

[0001] Polyester polymers have proven to be versatile materials with widespread use. Polyesters based on petroleum-derived aromatic monomers are among the most widely utilized polymers, e.g., polyethylene terephthalate (PET) is produced on a large scale to make water bottles, textiles, and other consumer goods. Unfortunately, PET is not biodegradable and therefore contributes primarily to the growing problem of environmental pollution, including harm to marine ecosystems, due to post-consumer plastic residual waste. In recent years, there has been growing interest in biodegradable polyesters, examples of which include polyhydroxyalkanoates (PHAs), such as polylactic acid (PLA) and poly-3-hydroxybutyrate (P3HB). PHAs are typically synthesized by engineering microbial enzymatic pathways that result in high costs and properties (e.g., high molecular weights that tend to make them difficult to process). These issues make it difficult to address mass-production applications to replace current bulk polymers. There is a need for high-performance biodegradable polymers and methods for producing such polymers from flexible feedstock sources that allow manufacturers to balance the cost and sustainability of their products.

[0002] While polyesters have useful properties, polyesters of lactones (herein "polylactones") have been prone to thermal degradation when they are processed (e.g., melt extrusion, blowing and molding) to form them into useful articles. Summary of the Invention [Problem to be solved by the invention]

[0003] It is therefore desirable to provide methods that overcome the limitations previously described to yield PHAs, particularly those derived at least in part from the ring-opening polymerization of lactones, and in certain polymers of beta-propiolactone that can be used in a wide variety of applications, including in the food and medical industries. [Means for solving the problem]

[0004] Summary of the Invention Applicants have discovered effective stabilizers / additives that may be incorporated into PHA during thermal processing that reduce degradation of the polymer at processing temperatures (i.e., substantially minimize the loss of molecular weight). For example, PHA powder and stabilizer powder may be dry mixed and then heated and melted to form the disclosed compositions. The stabilizer may be added to the molten PHA and formed into an article. The stabilizer may avoid the formation of harmful by-products or avoid wash solutions entirely. Food-safe, naturally occurring carboxylic acids (e.g., acids found in fruit) are particularly useful for food contact applications or other environmental constraints.

[0005] A first disclosed aspect is a composition including a polyhydroxyalkanoate and a stabilizer including one or more of an aliphatic carboxylic acid, a cyclic anhydride, or a halocarboxylic acid having a hydroxyl group or a thiol group.

[0006] (i) contacting a polyhydroxyalkanoate with one or more stabilizers including an aliphatic carboxylic acid, a cyclic anhydride, and a halocarboxylic acid having a hydroxyl group or a thiol group; and (ii) a method comprising mixing a polyhydroxyalkanoate and a polyhydroxyalkanoate-aliphatic carboxylic acid.

[0007] The compositions and methods may be used to produce PHAs with a wide range of weight average molecular weights (Mw). The ability to target Mw as well as control molecular weight distribution (polydispersity) using stabilizers allows for polymer properties that may be used in multiple applications (e.g., films, coatings, composites, fibers, and articles made therefrom). The compositions may be used in applications that utilize polymeric materials, including, but not limited to, industries such as healthcare, food production / storage and preparation, transportation (e.g., trucks, buses, RVs, trailers, bicycles, motorcycles, and automobiles), chemical, materials, and aerospace. [Brief description of the drawings]

[0008] [Figure 1] FIG. 1 is a plot of molecular weight reduction of PHAs with and without the disclosed stabilizers as a function of time at elevated temperatures. [Diagram 2] Data for the phthalates and comparative samples tested at 120° C. are shown in FIG. [Diagram 3] Data for the phthalates and comparative samples tested at 140° C. are shown in FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] definition Definitions of specific functional groups and chemical terms are described in more detail below. For the purposes of the present invention, chemical elements are identified according to the Periodic Table of the Elements, CAS edition, "Handbook of Chemistry and Physics", 75th edition, inside cover, and specific functional groups are generally defined as described therein. In addition, general principles of organic chemistry, as well as specific functional moieties and reactivities, are described in "Organic Chemistry", Thomas Sorrell, University Science Books, Sausalito, 1999; Smith and March, "March's Advanced Organic Chemistry", 5th edition, John Wiley & Sons, Inc., New York, 2001; Larock, "Comprehensive Organic Transformations", VCH Publishers, Inc., New York, 1989; Carruthers, "Some Modern Methods of Organic Synthesis", 3rd edition, Cambridge University Press, Cambridge, 1987, the entire contents of each of which are incorporated herein by reference.

[0010] Certain disclosed polymers may contain one or more asymmetric centers and therefore may exist in various stereoisomeric forms, such as enantiomers and / or diastereomers. The polymers and compositions thereof may be in the form of individual enantiomers, diastereomers or geometric isomers, or may be in the form of a mixture of stereoisomers. The disclosed polymers may be enantiopure compounds. Mixtures of enantiomers or diastereomers are disclosed.

[0011] The term "beta-lactone" as used herein refers to a substituted or unsubstituted cyclic ester having a four-membered ring containing an oxygen atom, a carbonyl group, and two optionally substituted methylene groups. When unsubstituted, the beta-lactone is referred to as beta-propiolactone (bPL). The beta-lactone may be mono-, di-, tri-, and tetra-substituted. Such beta-lactones may be further optionally substituted as defined herein. The beta-lactone comprises a single lactone moiety. The beta-lactone may comprise two or more four-membered cyclic ester moieties. Examples of beta-lactones are shown in Table A (between paragraphs 65 and 66) of PCT Publication No. WO2020 / 033267, which is incorporated herein by reference.

[0012] The terms "halo" and "halogen" as used herein refer to an atom selected from fluorine (fluoro, -F), chlorine (chloro, -Cl), bromine (bromo, -Br) and iodine (iodo, -I). The terms "aliphatic" or "aliphatic group" as used herein refer to a hydrocarbon moiety that may be linear (i.e. unbranched), branched or cyclic (including fused, bridged and spiro-fused polycyclic), fully saturated or may not be aromatic but may contain one or more units of unsaturation. An aliphatic group may contain 1-40 carbon atoms, 1-20 carbon atoms, 2-20 carbon atoms, 1-12 carbon atoms, 1-8 carbon atoms, 1-6 carbon atoms, 1-5 carbon atoms, 1-4 carbon atoms, 1-3 carbon atoms, or 1 or 2 carbon atoms. Exemplary aliphatic groups include, but are not limited to, linear or branched alkyl, alkenyl, and alkynyl groups and hybrids thereof, such as (cycloalkyl)alkyl, (cycloalkenyl)alkyl, or (cycloalkyl)alkenyl. As used herein, the term "heteroaliphatic" refers to an aliphatic group in which one or more carbon atoms are independently replaced with one or more atoms selected from the group consisting of oxygen, sulfur, nitrogen, or phosphorus. One to six carbon atoms may be independently replaced with one or more of oxygen, sulfur, nitrogen, or phosphorus. Heteroaliphatic groups may be substituted or unsubstituted, branched or unbranched, cyclic or acyclic, and may include saturated, unsaturated, or partially unsaturated groups. As used herein, the term "unsaturated" means that the moiety has one or more double or triple bonds. The terms "alicyclic", "carbocycle" or "carbocyclic" used alone or as part of a larger moiety refer to a saturated or partially unsaturated cycloaliphatic monocyclic or polycyclic ring system having 3 to 12 members, as described herein, where the aliphatic ring system is optionally substituted as defined below and described herein. Alicyclic groups include, but are not limited to, cyclopropyl, cyclobutyl, cyclopentyl, cyclopentenyl, cyclohexyl, cyclohexenyl, cycloheptyl, cycloheptenyl, cyclooctyl, cyclooctenyl, norbornyl, adamantyl and cyclooctadienyl. Alicyclic groups may have 3 to 6 carbons.The terms "alicyclic," "carbocycle," or "carbocyclic" also include an aliphatic ring fused to one or more aromatic or non-aromatic rings, e.g., decahydronaphthyl or tetrahydronaphthyl, where the radical or point of attachment is on the aliphatic ring.

[0013] The term "alkenyl" as used herein refers to a monovalent group derived from a straight or branched chain aliphatic moiety having at least one carbon-carbon double bond by the removal of a single hydrogen atom. The term "alkynyl" as used herein refers to a monovalent group derived from a straight or branched chain aliphatic moiety having at least one carbon-carbon triple bond by the removal of a single hydrogen atom. The term "alkoxy" as used herein refers to an alkyl group, as defined above, attached to the parent molecule by an oxygen atom. Examples of alkoxy include, but are not limited to, methoxy, ethoxy, propoxy, isopropoxy, n-butoxy, tert-butoxy, neopentoxy, and n-hexoxy. The term "acyl" as used herein refers to a carbonyl-containing functional group, such as -C(=O)R', where R' is hydrogen or an optionally substituted aliphatic, heteroaliphatic, heterocyclic, aryl, heteroaryl group, or is substituted (e.g., with hydrogen or an aliphatic, heteroaliphatic, aryl, or heteroaryl moiety) with an oxygen- or nitrogen-containing functional group (e.g., to form a carboxylic acid, ester, or amide functional group). The term "acyloxy" as used herein refers to an acyl group attached to a parent molecule by an oxygen atom. The term "aryl", used alone or as part of a larger moiety such as in "aralkyl", "aralkoxy" or "aryloxyalkyl", refers to monocyclic and polycyclic ring systems having a total of five to twenty ring members, where at least one ring in the system is aromatic and each ring in the system contains three to twelve ring members. The term "aryl" may be used interchangeably with the term "aryl ring," which refers to an aromatic ring system, including but not limited to phenyl, biphenyl, naphthyl, anthracyl, etc., which may bear one or more substituents. Also included within the scope of the term "aryl" as used herein are groups in which an aromatic ring is fused to one or more additional rings, such as benzofuranyl, indanyl, phthalimidyl, naphthymidyl, phenanthridinyl, or tetrahydronaphthyl, where the group or point of attachment is on the aryl ring.The terms "heteroaryl" and "heteroar-" used alone or as part of a larger moiety, e.g., "heteroaralkyl" or "heteroaralkoxy", refer to a group that shares 6, 10, or 14 pi-electrons in a cyclic arrangement; has 5 to 14 ring atoms, preferably 5, 6, or 9 ring atoms; and has 1 to 5 heteroatoms in addition to carbon atoms. The term "heteroaryl" may be used interchangeably with the terms "heteroaryl ring" and "heteroaryl group," either of which terms include optionally substituted rings. The term "heteroatom" refers to nitrogen, oxygen, or sulfur, and includes any oxidized form of nitrogen or sulfur, and any quaternized form of a basic nitrogen. Heteroaryl groups include, but are not limited to, thienyl, furanyl, pyrrolyl, imidazolyl, pyrazolyl, triazolyl, tetrazolyl, oxazolyl, isoxazolyl, oxadiazolyl, thiazolyl, isothiazolyl, thiadiazolyl, pyridyl, pyridazinyl, pyrimidinyl, pyrazinyl, indolizinyl, purinyl, naphthyridinyl, benzofuranyl, and pteridinyl. As used herein, the terms "heteroaryl" and "heteroar-" also include groups in which a heteroaryl ring is fused to one or more aryl, alicyclic, or heterocyclyl rings, where the group or point of attachment is on the heteroaryl ring. Non-limiting examples include indolyl, isoindolyl, benzothienyl, benzofuranyl, dibenzofuranyl, indazolyl, benzimidazolyl, benzthiazolyl, quinolyl, isoquinolyl, cinnolinyl, phthalazinyl, quinazolinyl, quinoxalinyl, 4H-quinolizinyl, carbazolyl, acridinyl, phenazinyl, phenothiazinyl, phenoxazinyl, tetrahydroquinolinyl, tetrahydroisoquinolinyl, and pyrido[2,3-b]-1,4-oxazin-3(4H)-one. Heteroaryl groups may be monocyclic or bicyclic. The term "heteroaralkyl" refers to an alkyl group substituted by a heteroaryl, where the alkyl and heteroaryl portions are independently optionally substituted.In certain embodiments, the term "5- to 10-membered heteroaryl" refers to a 5- to 6-membered heteroaryl ring having 1-3 heteroatoms independently selected from nitrogen, oxygen, or sulfur, or an 8- to 10-membered bicyclic heteroaryl ring having 1-4 heteroatoms independently selected from nitrogen, oxygen, or sulfur. The term "partially unsaturated" refers to a ring moiety that contains at least one double or triple bond. The term "partially unsaturated" is intended to encompass rings with multiple sites of unsaturation, but is not intended to include aryl or heteroaryl moieties as defined herein.

[0014] As described herein, the disclosed compounds may include "optionally substituted" moieties. The term "substituted," whether preceded or not by the term "optionally," means that one or more hydrogens of the specified moiety are replaced with a suitable substituent. Unless otherwise specified, an "optionally substituted" group may have a suitable substituent at each substitutable position of the group, and when multiple positions in a given structure may be substituted with multiple substituents selected from a specified group, the substituents may be the same or different at all positions. The combinations of substituents envisioned are those that result in the formation of stable or chemically feasible compounds. The term "stable," as used herein, refers to compounds that remain substantially unchanged when subjected to conditions that allow for their preparation, detection, and in some embodiments, their recovery, purification, and use for one or more purposes disclosed herein. As used herein, the term "alkoxylated" means that one or more functional groups of a molecule (usually, but in no way limited to, functional groups are alcohols, amines, or carboxylic acids) have been added to the molecule with a hydroxy-terminated alkyl chain. Alkoxylated compounds may contain a single alkyl group, or they may be oligomeric moieties such as hydroxyl-terminated polyethers. Alkoxylated materials can be derived from a parent compound by treating a functional group with an epoxide. Unless otherwise specified, "a," "an," "the," and "at least one" are used interchangeably and mean one or more than one. Electron-withdrawing groups include halogen groups, nitro groups, ketones, cyano groups, carboxylic acids, and carboxylates.

[0015] The composition may be prepared by contacting the PHA with a stabilizer, which may be one of three types: (1) an aliphatic carboxylic acid having a hydroxyl or thiol group, (2) a cyclic anhydride, or (3) a halocarboxylic acid.

[0016] By way of example, the carboxylic acid includes those that are naturally occurring or derived from naturally occurring carboxylic acids. Naturally occurring refers to those that can be found and isolated from organisms in which they occur. The acid may be saturated or unsaturated. The naturally occurring carboxylic acid may be a naturally occurring carboxylic acid, such as abietic acid, gallic acid, alginic acid, azelaic acid, caffeic acid, malic acid, pyruvic acid, niacin, citric acid, biotin, abietic acid, cholic acid resin, pectin, alginic acid, gum rosin (a mixture of naturally occurring natural acids), or a combination thereof. The fatty acid may be any fatty acid derived from any animal fat or vegetable oil, and may be saturated or unsaturated. More preferably, the carboxylic acid with a hydroxyl group is present in fruit. The carboxylic acid may be malic acid, citric acid, their thiol analogs, or a mixture thereof.

[0017] Carboxylic acids typically have at least one hydroxyl or thiol in the α, β, γ position relative to the carbonyl of the acidic group in the carboxylic acid. Desirably, at least one hydroxyl or thiol is in the α or β position. Carboxylic acids may be substituted with hydroxyl or thiol in any useful amount along their aliphatic backbone. Typically, the amount of substitution is equal to or less than the carboxylic acid groups present in the carboxylic acid (i.e., the equivalent ratio of COOH / [OH+SH]). Carboxylic acids may have hydroxyl or thiol located between two carbonyls in different α, β, γ positions, for example, in dibasic or tribasic acids. For example, the positions may be α to one carbonyl and β to another carboxylic acid group, but if only the dibasic acid has a hydroxyl group in the α position, the thermal stabilization is reduced (e.g., tartaric acid). The hydroxyl or thiol groups may each be in the beta position. Carboxylic acids may be substituted with a mixture of thiol and hydroxyl groups, or may be singly one or the other.

[0018] The carboxylic acid typically has at least one carboxylic acid group in any useful amount. More typically, there are 1 or 2 to 4 or 6 hydroxyl groups or thiols. The carboxylic acid may be a small molecule or a polymer. For purposes herein, small molecules are less than about 1000 g / mole and polymers are greater than about 1000 g / mole. Even smaller molecules may be desirable for improved processability and diffusibility, such as those having less than about 500 g / mole or 300 g / mole.

[0019] When the stabilizer is a cyclic anhydride, it may be any cyclic anhydride, but typically has a five-membered ring, which may or may not be further substituted. For example, a five-membered cyclic anhydride is

[0020] [ka] [wherein R may be any of the substituents described above, such as alkyl, alkenyl or aryl groups, and two R may form a ring structure which may be aromatic or substituted. The substituent may be an electron-withdrawing group.] The substituents may be any electron-withdrawing group that does not interfere with the function of stabilizing the PHA. The substituents may be halogens, carboxylate groups, carboxylic acids, and the like. Illustrative cyclic anhydride stabilizers include maleic anhydride and itaconic anhydride. Illustrative cyclic anhydrides include phthalates and substituted phthalates. With reference to the disclosed formula, phthalates have R forming an aromatic ring that may be substituted. Exemplary phthalates are:

[0021] [ka] Any useful halocarboxylic acid may be used, and may have aromatic and aliphatic groups. Typically, the halocarboxylic acid is a halo fatty acid (HAA). Typically, the halocarboxylic acid has equal amounts of halo and carboxylic acid groups. For example, the HAA may have one halo and one carboxylic acid group. The HAA may have any structure, such as those described above, but is typically linear. The HAA is preferably a small molecule as defined above. The HAA may comprise any halide, such as Br or Cl. The HAA may be derived from a naturally occurring carboxylic acid.

[0022] PHAs may be formed from the ring opening of lactones or may be biosynthesized. In the biosynthetic process, PHAs are produced by manipulating parameters in fermentation to stimulate the formation of PHA via an enzymatic pathway.

[0023] PHAs are

[0024] [ka] [In the formula, R is the above-mentioned aryl, arylalkenyl, or alkenyl, but typically R is H or C1-C 18 Or C 12 is an unsubstituted alkyl, y can be 1 to 6, typically 1, and z can be any value that results in a polymer having a molecular weight greater than about 10,000 g / mol (e.g., z is at least about 100 to 10,000). The PHA may comprise a blend of biosynthesized PHAs. In certain blends, the blend may have a bimodal distribution, with a peak above 250,000 g / mol and one peak below 250,000 g / mol. Such a blend, a bimodal distribution blend, may be formed by blending a biosynthesized PHA with a PHA prepared by lactone ring-opening polymerization. PHAs typically prepared by ring-opening polymerization have a Mw of less than about 250,000 g / mol to about 10,000 g / mol.

[0025] PHA is available in the following sizes: 10,000 g / mol, 100,000 g / mol to 300,000 g / mol, 500,000 g / mol, 1×10 6 g / mol or 2 x 10 6 The copolymer may have a Mw of 0.01 to 0.01 g / mol. The copolymer may have a low polydispersity of 0.01 to 0.01 g / mol, with a polydispersity index (PDI) of 0.01 to 0.01 g / mol, ...

[0026] PHAs may be copolymers with other monomers, so long as the polymers have similar degradation properties. PHAs may comprise PHAs with varying z and x. PHAs may be block, graft or random polymers. Examples of possible PHAs from ring-opening polymerization of different lactones are described in co-pending U.S. Provisional Patent Application No. 63 / 224,137, filed July 21, 2021, paragraphs 20-25, and in PCT Application No. US 2022 / 037611, filed July 19, 2022, both of which are incorporated herein by reference. Biosynthesized PHAs typically comprise the polymerized units (residues) 3-hydroxypropionate, 3-hydroxybutyrate, 3-hydroxyvalerate, 3-hydroxyhexanoate, 3-hydroxyoctanoate, 3-hexadecanoate. The PHA may comprise 3-hydroxypropionate (3HP), 3-hydroxybutyrate (3HP), or a mixture thereof. The PHA may comprise a homopolymer of the aforementioned polymerized units or a blend of homopolymers or copolymers thereof.

[0027] The contacting may include any useful method of mixing the PHA and the stabilizer such that the stabilizer can be incorporated into the PHA and stabilize it. For example, the contacting may include simply physically mixing the powders (particles are typically less than 5 mm, with the majority of particles being less than 1 mm). The physical mixing may be done without the addition of a solvent or liquid. Typical mixers include those known in the chemical arts, such as Muller mixers and V-blenders. The contacting may be done simultaneously with mixing. For example, the PHA and stabilizer may both be dissolved in the solvent, or the PHA may be heated to a molten state and the stabilizer added with the aid of mechanical stirring. The dissolution may be thermally assisted (e.g., heated above ambient temperature (about 20°C) in a closed or pressurized system). Typically, the temperature when forming the composition from the molten PHA is about 100°C to about 200°C. The solvent may be any that dissolves one or both of the PHA and the stabilizer in an amount desired to achieve incorporation of the stabilizer into the PHA without the application of heat. Desirably, the solvent dissolves both the PHA and the stabilizer to achieve mixing of the stabilizer into the PHA. The solvent may be, for example, a chlorinated solvent, a cyclic or acyclic carbonate, an ether, an ester, or a ketone. Typical solvents include tetrahydrofuran, methylene chloride, chloroform, dimethyl carbonate, diethyl carbonate, propylene carbonate, ethylene carbonate, trimethylene carbonate, ethyl acetate, or cyclohexanone. The stabilizer may be used in an amount sufficient to reduce the effect of impurities present in the PHA on the molecular weight of the PHA. The stabilizer is used in an amount of about 0.0001 weight percent or more, about 0.001 weight percent or more, about 0.01 weight percent or more, about 0.1 weight percent or more, about 0.5 weight percent or more, or about 1.0 weight percent or more based on the weight of the PHA and the stabilizer. The stabilizer is used in an amount of about 3 weight percent or less, about 5 weight percent or less, or about 10 weight percent or less based on the weight of the PHA.

[0028] The mixing may be any method such that the stabilizer is mixed at a level that allows it to stabilize the PHA from unwanted thermal degradation. Generally, this means that the mixing is done to distribute the stabilizer uniformly in the PHA. Thermal degradation is a heat-induced decrease in Mw, and too great a decrease in Mw can make the PHA brittle. The stabilizer is not limited in any manner, but may remove metal or quaternary ammonium ions, act as a chain extender, or end-cap the PHA. As an example, the mixing may be done by simultaneously heating (melting) and extruding a dry mixture of the PHA and stabilizer. The PHA may be introduced first into the extruder, and the stabilizer may be further added down the barrel of the extruder. Different temperature profiles and points of introduction of the PHA and stabilizer may be used to generate the desired polymer molecule distribution. The application of heat may also be used to produce a molded article that simultaneously forms the composition. As described, an extruder may be used to form the composition and then pass it through a die to form the molded article. Another example may be a physical mixture of PHAs that is melted by localized heating (e.g., 3D printing) to form the composition and article. The composition may be formed when producing a molded article in an injection mold. EXAMPLES

[0029] [Examples 1 to 18 and Comparative Examples 1 to 3] Polypropiolactone (PPL) homopolymer prepared by ring-opening polymerization of beta-propiolactone (bPL) with tetramethylammonium acetate (TMAA) in methyl tert-butyl ether solvent (MTBE) was used (Mw=198 kDa; D=2.2). Approximately 30 g of PPL was weighed with each of the stabilizers shown in Table 1 at a concentration of 1% by weight of PPL and stabilizer. PPL and stabilizer were mixed in a mortar, ground for 0.5-1 min, and stored in a vial until testing. The amount of initial molar mass remaining after heating to 100-120 °C for 30 min is determined by gel permeation chromatography (CHCl3 against PMMA polymer standards; 1.0 mL / min at 40 °C). The effectiveness is the amount of initial molar mass remaining after heating.

[0030] The above procedure is repeated for malic acid stabilizer, heating to 120° C. and the amount of malic acid is varied from none, 0.1%, 0.5%, 1%, 3%, 5% and 10% and aliquots at different times (initial, 5, 10 and 30 minutes) are obtained and analyzed with Mw as described above and shown in Figure 1. From Figure 1, it is evident that there is an optimum amount of malic acid in the range of about 1% to 3% by weight of malic acid.

[0031] [Table 1]

[0032] [Examples 19 to 22] Phthalic anhydride stabilizer Polypropiolactone polymer was mixed with a number of phthalate anhydrides as shown below at 1 weight percent in the manner described in the previous examples. Comparative examples were also run using acyclic anhydrides. Samples are heated at 120°C and 140°C for 30 minutes and the mass of each sample is measured as indicated for 0, about 5, 15, and 30 minutes, respectively. Control samples without stabilizer are also tested as indicated. Data for samples tested at 120°C are shown in Figure 2 and data for samples tested at 140°C are shown in Figure 3. The structures of the stabilizers tested are as follows:

[0033] [ka]

[0034] The data shows that the samples with phthalic anhydride and substituted phthalic anhydrides provide very good thermal stability. The aliphatic anhydrides do not provide good thermal stability.

Claims

1. A composition comprising a polyhydroxyalkanoate and a stabilizer comprising one or more of an aliphatic carboxylic acid, a cyclic anhydride, or a halocarboxylic acid having a hydroxyl group or a thiol group.

2. The composition of claim 1 , wherein the aliphatic carboxylic acid has at least one hydroxyl group or thiol group at the α, β or γ position of the carbonyl of the fatty acid.

3. The composition of claim 1 , wherein the aliphatic carboxylic acid is naturally occurring.

4. The composition of claim 1 , wherein the aliphatic carboxylic acid is derived from a naturally occurring acid.

5. The composition according to any one of claims 1 to 4, wherein at least one of the hydroxyl groups is in the β position.

6. The composition of claim 5 , wherein each of the hydroxyl groups is in the β position.

7. A composition according to any one of claims 1 to 6, wherein only one hydroxyl group is present.

8. The composition of any one of claims 1 to 7, wherein the aliphatic carboxylic acid has from 2 to 4 carboxylic acid groups.

9. 9. The composition of claim 8, wherein the aliphatic carboxylic acid comprises at least one of malic acid, citric acid, and thiol analogs thereof.

10. The composition of claim 1 , wherein the cyclic anhydride is maleic anhydride.

11. The composition of claim 1 , wherein the halocarboxylic acid comprises an aromatic group.

12. The composition of claim 1 , wherein the halocarboxylic acid is aliphatic.

13. The composition of claim 12 , wherein the halocarboxylic acid is derived from a naturally occurring carboxylic acid.

14. The composition of claim 3 , wherein the aliphatic carboxylic acid is naturally occurring in a fruit.

15. 15. The composition of claim 14, wherein the aliphatic carboxylic acid is malic acid or citric acid.

16. The composition of claim 1 , wherein the stabilizer is phthalic anhydride or a substituted phthalic anhydride.

17. (i) contacting a polyhydroxyalkanoate with at least one stabilizer comprising one or more of an aliphatic carboxylic acid having a hydroxyl group or a thiol group, a cyclic anhydride, and a halocarboxylic acid; and (ii) mixing a polyhydroxyalkanoate and a stabilizer.

18. 20. The method of claim 17, wherein the mixing is performed by heating.

19. 20. The method of claim 17, wherein the mixing is assisted by mechanical stirring.

20. 20. The method of claim 19, wherein the mixing is accomplished at least in part by dissolving at least one of the polyhydroxyalkanoate and the stabilizer in a solvent.

21. 20. The method of any one of claims 17 to 19, wherein the polyhydroxyalkanoate comprises a biosynthesized polyhydroxyalkanoate.

22. The method of any one of claims 17 to 20, wherein the polyhydroxyalkanoate comprises a polymer of a ring-opened lactone.

23. 23. The method of any one of claims 17 to 22, wherein the polyhydroxyalkanoate comprises residues of beta-propiolactone or a substituted beta-lactone.

24. 24. The method of claim 23, wherein the residue comprises a beta propiolactone residue.

25. The method of any one of claims 17 to 24, wherein the polyhydroxyalkanoate comprises a biosynthesized polyhydroxyalkanoate.

26. The method of any one of claims 17 to 24, wherein the polyhydroxyalkanoate comprises a ring-opened lactone residue.

27. The method of any one of claims 17 to 26, wherein the polyhydroxyalkanoate is a polyhydroxypropionate, a polyhydroxybutyrate, a copolymer thereof, or a blend thereof.

Images