This invention relates to biodegradable polyesters and to articles prepared from such polymers Biodegradable polyesters of both natural and synthetic origins are well known

See for example, Encyclopedia of Polymer Science and Technology, Second Edition, Volume 2, pp 220-243 However, these known biodegradable polyesters have several limitations which render them unsuitable for many intended applications For example, biodegradable polyesters of natural origin which are isolated as products of fermentation processes require o extensive separation and purification in order to provide a product of purity suitable for typical plastic uses These polymers also have general ly poor physical properties and are difficult to fabricate into useful articles Synthetic biodegradable polyesters are also known Although these synthetic polyesters do not have the separation and purification problems associated with natural materials, they often suffer from the deficiencies in mechanical properties and fabπcabiiity encountered in natural biodegradable polymers

It is known to prepare thermoplastic polyesters by reacting difunctional acids and alcohols It is also known that employing reactants which contain more than two reactive functional groups per molecule will lead first to branching, and then to gelation and crosslmking Thus, preparation of thermoplastics from monomers containing more than two reactive groups is possible only if a suitable protecting group is employed to prevent reaction of the excess reactive group during polymerization Deprotection of these groups can provide a functionahzed polyester However, such a process requires the use of costly reagents and additional undesirable separation and purification processes to provide a product suitable for use as a practical plastic material It would be desirable to provide synthetic biodegradable polymers which can be prepared and fabricated easily and have good mechanical properties

In one aspect, the present invention is a biodegradable hydroxy-functional polyester comprising moieties derived from hydroxy-functional aliphatic diacids and diglycidyl ethers or diglycidyl esters In another aspect, this invention is a process for preparing a biodegradable hydroxy-functional polyester which comprises contacting a hydroxy-functional aliphatic dicarboxylic acid or a mixture of dicarboxylic acids containing hydroxy-functional al iphatic diacids, with a diglycidyl ether or diglycidyl ester in the presence of an onium catalyst in an ether solvent under conditions suitable for forming the polyester In another aspect, this i nvention is a biodegradable article comprising a hydroxy-

-functional polyester

Definitions

The following terms are used in this application and have the meanings and preferred embodiments set out hereinafter unless otherwise specified.

As used herein, the term "aromatic moiety" means any group having one or more aromatic rings and from 5 to 25 carbon atoms. The aromatic rings may have one or more non- carbon atoms in the ring such as, for example, sulfur, nitrogen and oxygen, or one or more substituent groups bonded to the aromatic ring. These substituent groups may be alkyl, cycloalkyl, aryl, alkoxy, aryloxy, amido, halo, nitro, or cyano groups.

The term "hydrocarbylene" means a divalent aliphatic hydrocarbon moiety, such as alkylene, alkenylene or ycloalkylene having from 2 to 20 carbons and optionally containing a heteroatomic group, such as oxygen, sulfur, imino, sulfonyl, carboxyl, carbonyl or sulfoxyl, in the chain or substituent thereto.

As used herein, the term "hydroxy-functional aliphatic diacid" means a compound generally represented by the formula:

fi . II HO-C-R---C-OH

wherein R ¹ is a hydrocarbylene substituted with at least one hydroxyl group.

The term "diglycidyl ether" means a compound generally represented by the formula:

o

CH Λ ₂ ~ CHCH ₂ 0-R ² -OCH ₂ C AH — CH ₂

wherein 2 is an aromatic moiety or a hydrocarbylene. The term "diglycidyl ester" means a compound generally represented by the formula:

C AH ₂ - CHCH ₂ 0- fiC -R -C-OCH ₂ C AH— CH ₂

wherein R2 is as defined before

The term "biodegradable" means that the articles, when exposed to a biologically active environment, suffer from substantial changes in mechanical properties or molecular weight, or provide a source of nutrients that will support the growth of microorganisms Illustrative biologically active environments include soil, aquatic and marine environments, composition systems and activated sewage sludge Hydrolytic and photochemical degradative processes may be operative in such biologically active environments, and other conditions such as temperature, moisture level, pH, ionic strength, aeration, distribution of microbial population and polymer morphology, are well known to affect the rate and the nature of the degradative process The biodegradabihty of plastics can be measured using several different methods

Articles, for example molded films or bars, can be exposed to a biologically active environment, and samples removed periodically for mechanical or molecular weight characterization The plastic material can be placed into a vessel containing a culture of microorganisms, and the products of cellular metabolism, such as carbon dioxide, can be measured to characterize the metabolism of the material Alternatively, a culture of microorganisms, in a medium containing all nutrients needed for cellular growth except for carbon, can be supplied with a plastic material Cell growth indicates that the polymer is being used for the metabolic processes of the cells For purposes of the present invention, a polymer is biodegradable if (1 ) its molecular weight and other physical properties, such as tensile strength, are reduced upon exposure to a biologically active environment or (2) microorganisms grow in a medium containing the polymer as the sole source of carbon

The biodegradable hydroxy-functional polyester of the present invention preferably has repeating units represented by the formula

wherein R ¹ is as defined before and, optionally, combined with a divalent aromatic moiety or another hydrocarbylene, R2 is as defined before, each R3 is independently hydrogen or lower alkyl such as a C1-C. ₄ alkyl, x is from 0 05 to 0 4 and y is from 0 to 0 5

In the more preferred polymers, x is from 0 1 to 0 3, R ¹ is independently (1 ) alkylene, cycloalkylene, alkenylene, alkyleneoxyalkylene, alkylenethioalkylene, or alkylenesulfonylalkylene, substituted with at least one hydroxyl group and, optionally, combined with (2) arylene, alkylenearylene, dialkylenearylene, diaryleneketone, diarylenesulfone, diarylenesulfoxide, alkylidene-diarylene, diarylene oxide or diarylene sulfide,

or another alkylene, cycloalkylene, or al kenylene, and R ² is independently arylene alkylenearylene, dialkylenearylene, diaryleneketone, diarylenesulfone, diarylenesulfoxide, alkylidene-diarylene, diarylene oxide, diarylene sulfide, diarylenecyanomethane, alkylene, cycloal kylene, alkenylene, al kyleneoxyalkylene, alkylenethioal kylene or alkylenesulfonylal kylene

In the most preferred polymers, x is 0 125 and y is 0 5, R ¹ is independently hydroxymethylene, hydroxyethylene, dihydroxyethylene, hydroxypropylene, dihydroxypropylene, tnhydroxypropylene, hydroxymethylethylene, hydroxybutylene, dihydroxybutylene, tπhydroxybutylene, or tetrahydroxybutylene, and R ² is independently m-phenylene, p-phenylene, isopropylidenediphenylene, biphenylene, diphenylene oxide, methylenediphenylene, diphenylene sulfide, naphthylene, diphenylenecyano-methane, 3,3'-dιalkyldιphenyleneιsopropylιdene, or 3,3',4,4'-tetraalkyldιphenyleneιsopropylιdene

Generally, the polymers of the present invention can be prepared by reacting a hydroxy-functional aliphatic diacid, optionally in the presence of another diacid, with a diglycidyl ether or diglycidyl ester at conditions sufficient to cause the acid moieties to react with the epoxy moieties to form a polymer backbone having ester linkages The polymers can be prepared by well known methods such as, for example, those described in U S Patent 5, 171 ,820

Hydroxy-functional aliphatic diacids which can be employed in the practice of the present invention are those aliphatic diacids which can react with a diglycidyl ether or diglycidyl ester to prepare a biodegradable polymer Suitable diacids include, for example, tartaπc, malic, citramalic and hydroxyglutaπc acids Of these diacids, tartaπc acid is most preferred

Diglycidyl ethers which can be employed in the practice of the present invention are those diglycidyl ethers which can react with hydroxy-functional ali phatic diacids to prepare a biodegradable polymer Suitable diglycidyl ethers include, for example, the diglycidyl ethers of 9,9-bιs(4-hydroxyphenyl)fluorene, hydroquinone, resorcinol, 4,4'-sulfonyldιphenol, 4,4'-thιodιphenol, 4,4'-oxydιphenol, 4,4'-dιhydroxy-benzophenone, 4,4'-bιphenol, bιs(4- -hydroxyphenyO-methane, 2,6-dιhydroxynaphthalene and 4,4'-ιsopropyl-ιdene bisphenol (bisphenol A) The more preferred diglycidyl ethers i nclude the diglycidyl ethers of

9,9-bιs(4-hydroxyphenyl)fluorene, hydroquinone, resorcinol, bis(4-hydroxyphenyl)-methane, and bisphenol A The most preferred diglycidyl ethers include the diglycidyl ethers of 9,9-bιs(4- -hydroxyphenyl)fluorene, hydroquinone, resorcinol and bisphenol A

Diglycidyl esters which can be employed in the practice of the present invention are those diglycidyl esters which can react with hydroxy-functional al i phatic diacids to prepare a biodegradable polymer Suitable diglycidyl esters include, for example, the diglycidyl esters of terephthal ic acid, isophthalic acid, 2,6-naphthalenedιcarboxyiιc acid, 3,4'-bι phenyl- dicarboxylic acid, 4,4'-bιphenyldιcarboxylιc acid, malonic acid, succinic acid, gl utaπc acid, adipic

acid, pime c acid, suberic acid, azaleic acid, sebacic acid, 1 ,4-cyclohexanedicarboxylιc acid, 1 ,10-decane-dιcarboxylιc acιd, tartaπc acid, malic acid, citramalic acid, and hydroxyglutaπc acid The more preferred diglycidyl esters which can be employed in the practice of the present invention for preparing the biodegradable hydroxy-functional polyester include the diglycidyl esters of terephthalic acid, isophthalic acid, adipi acid, 1 ,4-cyclohexanedιcarboxylιc acid and 1 , 10-decanedιcarboxylιc acid

In general, the reaction of the diacid and ether or ester requires a catalyst or any material capable of catalyzing the reaction The preferred catalytic materials are the onium catalysts Preferred onium catalysts include tetrahydrocarbyl quaternary ammonium halides and tetrahydrocarbyl phosphonium halides, wherein hydrocarbyl is a monovalent hydrocarbon radical such as alkyl, aryl, cycloalkyl, aralkyl and alkaryl, preferably having from 1 to 16 carbon atoms More preferred onium catalysts include ethyltπphenyl-phosphonium iodide, tetraphenylphosphomum bromide, and tetrakιs(n-butyl) ammonium bromide and its corresponding chloride, iodide and fluoride, with tetrakιs(n-butyl) ammonium bromide being the most preferred

The polymers of this invention can be homopolymers containing moieties derived from the hydroxy-functional aliphatic diacids described above, or can be copolymers having one or more other diacid copolymeπzable with the hydroxy-functional aliphatic diacids Illustrative of such other diacids include terephthalic acid, isophthalic acid, 2,6-naphthalene- dicarboxylic acid, 3,4'-bιphenyldιcarboxylιc acid, 4,4'-bιphenyldιcarboxylιc acid, malonic acid, succinic acid, glutaπc acid, adipic acid, pimelic acid, suberic acid, azaleic acid, sebasic acid, 1 ,4-cyclohexanedι-carboxylιc acιd and 1, 10-decanedιcarboxylιc acιd More preferred diacids are terephthalic acid, isophthalic acid, adipic acid, 1,4-cyclohexanedicarboxylιc acid and 1 , 10-decanedιcarboxylιc acid The conditions at which the reaction is most advantageously conducted are dependent on a variety of factors, including the specific reactants, solvent, and catalyst employed but, in general, the reaction is conducted under a non-oxidizing atmosphere such as a blanket of nitrogen, preferably at a temperature from 100°C to 190°C The reaction can be conducted neat (without solvent or other diluents) However, in order to ensure homogeneous reaction mixtures at such temperatures, it is often desirable to use inert organic solvents for the reactants Examples of suitable solvents include ethers such as giyme, diglyme, tπglyme, dioxane or tetrahydrofuran

In some cases, it may be desirable to end-cap residual epoxy groups or control molecular weight with monofunctional reactants (compounds having one reactive group) such as carboxy c acids, thiols, monofunctional sulfonamides, secondary amines and monohydπc phenols Preferred monofunctional reactants include acetic acid, benzoic acid, thiophenol, N-methylbenzenesulfonamide, phenol and tert-butylphenol

The hydroxy ester polymers are recovered from the reaction mixture by conventional methods For example, the reaction mixture containing the polymer can be diluted with a suitable solvent such as the reaction solvent or dimethylformamide, cooled to room temperature, and the polymer isolated by precipitation from a non-solvent such as water 5 The precipitated polymer can then be purified by washing with additional non-solvent The polymer is collected by filtration, washed with a suitable non-solvent, such as water and then dried

The polymers of the present invention are processable as thermoplastics The article can be reheated and reformed any desired number of times, and the polymer does not o undergo crosslinking or network formation during this handling A test for crosslinking can be made by contacting the material with a solvent, preferably after exposure to the temperatures encountered during thermoplastic processing Crosslinked materials will not be dissolved, - while uncrosslinked materials will be dissolved by the solvent Thermoplastics can be extruded at temperatures above their glass transition temperatures or they can be compression molded 5 into films or plaques, and they remain both soluble and processable after such thermal treatment

The following working examples are given to illustrate the invention and should not be construed as limiting its scope Unless otherwise indicated, all parts and percentages are by weight 0 Example 1

A 100 mL mini reactor, equipped with a mechanical stirrer, nitrogen inlet, and condenser, was charged with bisphenol A diglycidyl ether (10 453 g, 30 6 mmol, 172 38 g/equιv E E W ), L-tartaric acid (4 590 g, 30 6 mmol), and tetra-n-butylammonium bromide (4 00 g, 12 5 mmol) Dioxane (35 mL) was added under a stream of nitrogen and the mixture heated to 5 reflux for 4 5 hours Glacial acetic acid (4 mL) was added and heating at reflux continued for 1 hour The mixture was diluted with dimethylformamide (DMF, 30 mL), and the product isolated by precipitation into water (500 mL) in a Waring blendor The product was collected by suction filtration and dried in a vacuum oven at 80 ^C C overnight The product obtained (14 2 g) had an inherent viscosity of 0 32 dL g, measured in DMF at 25°C, at a concentration of 0 5 g/dL, and a glass transition temperature of 71°C Example 2

A series of copolymers derived from varying ratios of tartaπc acid and isophthalic acid and bisphenol A diglycidyl ether was prepared in the manner described in the above example The inherent viscosities and thermal properties are listed in Table I

Table I

Inherent mol X Isophthalic mol X L-Tartaric Viscosity! τ _e 2 ro

(dL/e)

100 0 0.50 97

90 10 0.40 73

75 25 0.40 91

50 50 0.26 87

0 100 0.32 71 llnherent Viscosity in DMF at 0.5 g/dL and 25°C

^Glass transition temperature determined using a DuPont

Model 2100 differential scanning calorimeter (DSC) operation in a heating mode at 20°C/minute

Biodeqradability Tests

The biodegradability of the polymers prepared in Examples 1 and 2 was evaluated by monitoring the growth of a consortium of soil microorganisms in a liquid culture medium over a six-week time period. Under aerobic conditions, the microorganisms were provided samples of the above polymer compositions as a sole source of carbon in addition to all inorganic nutrients required for their growth. The culture medium contained, per liter: potassium phosphate, dibasic (1.55 g), sodium phosphate, monobasic monohydrate (0.85 g), ammonium sulfate (2.0 g), sodium nitrate (2.0 g), magnesium chloride hexahydrate (0.1 g), disodium EDTA (0.5 mg), ferrous sulfate heptahydrate (0.2 mg), zinc sulfate heptahydrate (0.01 mg), manganese chloride tetrahydrate (0.03 mg), boric acid (0.03 mg), cobalt chloride hexahydrate (0.02 mg), calcium chloride dihydrate (0.001 mg), nickel chloride hexahydrate (0.002 mg), and sodium molybdate dihydrate (0.003 mg) Also added was a 100: 1 dilution of a vitamin mixture containing per liter: biotin (0.2 mg), folic acid (0.2 mg), pyridoxin (1.0 mg), thiamine hydrochloπde (0.5 mg), πboflavin (0.5 mg), nicotinic actd (0.5 mg), D,L-calcιum pantothenate (0.5 mg), cyanocobaltamm (B 12, 0 01 mg), para-ammo hydroxybeπzoic acid (0.5 mg) and lipoic acid (0 5 mg)

The pH of the medium was adjusted to 7 0 prior to sterilization Weekly aliquots of the culture broth were serially diluted and plated onto a solid nutrient medium to assess the number of colony forming units per unit volume Control cultures without polymer, and with polymer compositions known to be poorly biodegradable (poly(ethylene terephthalate), PET) were ran to illustrate the biodegradability of the compositions of the present invention Table II shows the peak bacterial populations, expressed as colony-forming units per mil liter of culture broth, for the tested thermoplastic materials

Tabl e I I

Peak Bacteria/mL culture broth

Sample

Replicate 1 Replicate 2

Blank2 0.8-1.5 x 107 0.8-1.2 x 107

Control ³ 1.2-2.2 x 107 0.5-1.2 x 107

Example 1 6.0-7.0 x 107 6.0-8.0 x 107

Example 2 3.4-3.7 x 108 0.8-1.1 x 108

'Listed as peak value measured over six weeks incubation, as the range found in triplicate measurements, for each two separate cultures for each material tested. 2No carbon source in culture medium. 3poly(ethylene terephthalate).

The data shown in Table II illustrate that cultures containing no polymer (blank) or cultures containing polymers known to be poorly biodegradable (PET), show little or no growth. The larger bacterial population found in the cultures containing the polymers of the present invention indicates that the bacteria can utilize the polymer as a source of carbon for cellular processes. The blank (which is not zero) represents a background level of bacteria. Values above this level indicate growth on the polymer.