The present invention refers to a process for production of a cyclo aliphatic carbonate from a diol, triol or polyol and a carbon dioxide source using a N-heterocyclic carbene or a N-heterocycIic carbene complex, preferably attached to a lipase biocatalyst, to catalyse the reaction. In a further aspect the present invention refers to the use of said cycloaliphatic carbonate as monomer in production of a polyurethane or a polycarbonate.

Synthesis of carbonate analogues, especially five-membered and six-membered cycloaliphatic carbonates, have lately received a lot of attention due to their potential application as monomers for environmentally benign production of for instance polycarbonates and polyurethanes, currently being produced using for instance toxic phosgene and/or isocyanates.

Aliphatic polycarbonates are tough, dimensionally stable thermoplastics widely used in engineering and optical applications. Aliphatic polycarbonates and their copolymers are biodegradable and recyclable and are expected to find use also in the biomedical field due to their biocompatibility and low toxicity.

Polyurethanes are widely used in a variety of applications, such as seating, seals and high performance adhesives. In recent years, a variety of biomedical polyurethane elastomers exhibiting improved hydrolytic stability have been developed. Due to the toughness, durability, biocompatibility and improved biostability, they have been incorporated in a wide variety of implantable biomedical devices. A demand has now emerged for new types of polyurethanes, providing unaltered properties, produced without the use of toxic starting materials.

The building and construction industry, transportation industry as well as the furniture and bedding industry by far accounts for the greatest usage of polyurethanes, these three application areas together account for more than 70% of the total amount of polyurethanes produced. The polyurethanes produced by the process outlined above can for example be utilised in applications such as food and drink containers, packaging materials, adhesives, paints and varnishes, flexible and rigid foams, toys, flexible foam seating, upholstery, bedding, insulation material, furniture and in household appliances, such as refrigerators, clothing, automotive suspension bushing, electrical potting compounds, (microcellular foam) seals and gaskets, carpet underlay, fireproofing material, inflatable boats, wheels and electronic components.

Polycarbonates have excellent electrical and thermal insulation properties combined with strong material properties. The polycarbonates, such as those produced by the process of the present invention can be utilised in for instance heat resistant and flame retardant applications as well as in electronic components, construction materials, data storage, automotive, aircraft and security components, such as bullet resistant glass, toys, water bottles and food containers.

A typically used way to synthesise polycarbonates is by ring-opening polymerisation of cyclic carbonates in bulk or solution, typically using metallic compounds as catalysts. The use of metal-free or low toxicity catalyst/initiator systems for ring-opening polymerisation of carbonates have increasingly attracted attention. Ring-opening polymerisation initiated by alcohols in the absence of a catalyst has been extensively investigated, since the use of the hydroxyl groups of the alcohols as initiators allows for proper control of the molar mass, improvement in the hydrophilicity, degradability and further functionalisation of the resulting biodegradable polycarbonates.

Lately, a number of reports have appeared on the synthesis of five-membered cyclic carbonates by a phosgene-free route. Transesterification of glycerol, in for instance a solvent comprising media, with dialkyl carbonate using different catalysts, such as K ₂ C0 ₃ , dichloro distannoxanes and immobilised lipase from Candida Antarctica have resulted in moderate to high yields of linear and cyclic glycerol carbonates.

For use in a ring-opening polymerisation process, however, six-membered cyclic carbonates are preferred because they are less thermodynamically stable than their ring-opened polymers and thus retaining C0 ₂ during the polymerisation process. Synthesis of six-membered trimethylene cyclic carbonates is traditionally achieved by reacting for instance 1,3 -propanediol with phosgene or its derivatives. Among other reactions studied, metal catalysed coupling of for instance trimethylene oxide with carbon dioxide has given high yields of trimethylene carbonate. Lipase catalysed transesterification reaction between a dialkyl carbonate and a 1,3-diol in a solvent system, comprising acetonitrile and toluene, for synthesis of cyclic trimethylene carbonate (l,3-dioxane-2-one) monomer with or without a methyl substituent has been reported using very high concentration (900% by weight of the diol) of lipase from Candida Antarctica.

The syntheses of six-membered cyclic carbonates with functional groups from polyfunctional alcohols, such as trimethylolpropane and pentaerythritol, have so far required more complicated methods with low yields. Polycyclic six-membered carbonates have been prepared by radical polymerisation of acrylic monomers with pendant cyclic carbonate groups. As a different approach, tris or tetrakis(alkoxycarbonyloxy) derivatives, obtained from catalytic transesterification of for instance trimethylolpropane and diethyl carbonate, have at 200-220°C been subjected to thermal disproportionation using Aerosil ^® 200 followed by distillative depolymerisation under reduced pressure, yielding the cyclic product 5-ethyl-5-ethoxyarbonyloxymethyl-l,3-dioxan-2-one at a low yield. It has now quite unexpectedly been found that cycloaliphtic carbonates, especially monocyclic six-membered carbonates, synthesised in a solvent-free medium by a N-heterocyclic carbene catalysed reaction between a polyol and carbon dioxide source can be obtained in high yields. The N-heterocyclic carbene or N-heterocyclic carbene complex catalysed synthesis and derivatisation, according to the present invention, of cycloaliphatic carbonates implies a cleaner and more sustainable route to biodegradable polycarbonates, polyurethanes and copolymers thereof without using toxic organic solvents, isocyanates and/or phosgene. The N-heterocyclic carbene and/or the N-heterocyclic carbene complex is, in the most preferred embodiments of the present invention, attached to a lipase, preferably a lipase B, such as Candida antarctica lipase B. The biocatalytic reaction not requiring any solvent and/or other toxic materials constitutes a green process for synthesis of cycloaliphatic carbonates. It is furthermore possible to convert linear aliphatic carbonates in a yielded product mixture to cycloaliphatic carbonates by thermal treatment under relatively mild conditions as shown in enclosed Fig. 1. The excess of dialkylcarbonate can, subsequent the reaction, be recycled.

The present invention accordingly refers to a process for production of a cycloaliphatic carbonate from a diol, triol or polyol and a carbon dioxide source. The process is performed in a solvent-free medium using a N-heterocyclic carbene or a N-heterocyclic carbene complex, preferably attached to a biocatalyst. The biocatalyst is in preferred embodiments of the present invention an immobilised enzyme, more preferably an esterase, and even more preferably a lipase and most preferably Candida antarctica lipase B. The concentration of lipase is, in preferred embodiments of the present invention, for instance 1 -40% by weight calculated on the diol, triol or polyol.

The process as herein disclosed preferably comprises a transesterification step and a thermal disproportionation step. The temperature interval during said transesterification step is suitably found within the range of 40-80°C, such as 50-70°C, and the temperature during said disproportionation step is preferably in the range of 70-150°C, such as 70-90°C or 75-85°C. The process of the present invention is, furthermore, in preferred embodiments thereof performed under atmospheric pressure.

In a preferred embodiment of the present invention the cycloaliphatic carbonate is a monocyclic carbonate having at a five-membered or six-membered ring and in certain embodiments suitably a hydroxyl and/or alkoxycarbonyloxy functional group.

Said diol, triol or polyol is in preferred embodiments of the present invention a 2,2-dialkyl-l ,3-propanediol, a 2-alkyl-2-hydroxyalkyl-l,3-propanediol or a 2,2 dihydroxyalkyl-l,3-propanediol, which suitably can be exemplified by neopentyl glycol, 2-butyl-2-ethyl-l ,3-propanediol, trimethylolethane, trimethylolpropane, trimethylolbutane and pentaerythritol. Further suitable diols, triols and polyols include glycerol, sorbitol, mannitol and derivatives thereof.

The carbon dioxide source used in the process of the present invention is in preferred embodiments thereof a dialkyl carbonate having for instance at least one C 1-C4 alkyl group, such as a dimethyl carbonate and/or a diethyl carbonate.

The process of the present invention may in addition to said cycloaliphatic carbonate yield at least one linear aliphatic carbonate and/or at least one dimeric or polymeric cyclic carbonate.

In a further aspect, the present invention refers to the use of a cyclic carbonate, produced by the process herein disclosed, as monomer in production of a polyurethane and/or a polycarbonate.

The optimum temperature of lipases (including Candida antarctica lipase B) is in the literature generally reported to be around 60°C. However the optimum can vary depending on substrate properties, solvents and reaction times. The proportion of products yielded by the process of the present invention can accordingly also be changed by varying the ratio of substrates and temperature.

Although some environmentally benign alternatives to the use of organic solvents in organic syntheses of cyclic carbonates are proposed for enzymatic reactions, the most desirable strategy is to carry out the reaction without a solvent. The present invention shows that a carbon dioxide source, such as a dialkyl carbonate, is able to act as solvents in the production of cycloaliphatic carbonates, having hydroxyl and/or alkoxycarbonyloxy functional groups, and being prepared from for instance a diol, triol or polyol and a dialkyl carbonate, in presence of a N-heterocyclic carbene or carbene complex preferably attached to, for instance "His224" in a lipase, such as a said lipase B.

The following preferred specific embodiments are to be construed as merely illustrative and not limitative of the scope of the present invention in any way whatsoever. In the following, Example 1 refers to the structure elucidation of products obtained by solvent-free lipase catalysed reaction between trimethylolpropane (TMP) and dimethyl carbonate (DMC) and diethyl carbonate (DEC), resp. Example 2 refers to the effect of the lipase concentration on reactions between TMP and DMC. Example 3 refers to the effect of temperature on reactions between TMP and DMC. Example 4 refers to the effect of biocatalyst concentration on the reaction between TMP and DEC. Example 5 refers to the effect of thermal treatment in the disproportionation of a reaction product obtained in Example 2. Results from Examples 1-5 are given in Tables 1-5. Figure 1 illustrates the reaction scheme during a carbonisation process according to the present invention. The reaction scheme is illustrated by reaction between a triol and a dialkyl carbonate. Products illustrated in said scheme are PI = triol, P2 = dialkyl carbonate, P3 = monocarbonated triol, P4 and P6 = cyclic carbonates of triol, P5 = dicarbonated triol, P7 = tricarbonated triol and P8 = dimeric cyclic carbonate of triol.

Example 1

Trimethylopropane was at 50-80°C reacted with an excess of dimethyl carbonate or diethyl carbonate using lipase B (Novozym ^® 435) as catalyst. Formed products were identified by GC-MS and Ή-NMR. Quantitative analyses of reaction components were performed using gas chromatography equipped with a capillary column, VF-lms and a flame ionisation detector. The initial column oven temperature was increased from 50°C to 250°C at a rate of 20°C/min. The samples, diluted in acetonitrile to a concentration of 0.1-0.5 mg/ml, were injected in split injection mode of 10% at 275°C. The conversion of trimethylolpropane and the ratio of products were calculated by comparison of peak areas on the gas chromatograms. The molecular masses of products were measured by GC-MS equipped with a capillary column. The initial column oven temperature was increased from 50°C to 275°C at a rate of 15°C/min. The samples were diluted as above and injected at 275°C. The products were isolated from the reaction mixture by flash chromatography. The reaction mixture was loaded on a silica column (25 id x 250 h, mm), which was equilibrated with a solvent system of dichloromethane/ethyl acetate (2: 1). The elution was performed using 3: 1 dichloromethane/ethyl acetate. The structures of the purified samples were elucidated by Ή-NMR and ¹³ C-NMR using 400 MHz NMR. The GC-MS and Ή-NMR data for the structure elucidation of products are given in Table 1. The chemical shifts of the products elucidated by 1H-NMR agree with the products shown in Figure 1.

Example 2

50 part by weight of trimethylolpropane and 1.5 part by weight of dimethyl carbonate were charged in a reaction vessel, equipped with agitation (700 rpm), and heated to a reaction temperature of 60°C. The reaction was initiated by addition of 150 parts by weight of molecular sieves and a lipase B catalyst (Novozym ^® 435) at concentrations of 2.5, 5, 10, 20 and 40% by weight calculated on charged trimethylolpropane. The amounts of cyclic and linear components in the reaction mixture were determined after 48 and 120 hours and the result is given in Table 2.

The result given in Table 2 shows that a lipase concentration of 5-10% calculated on trimethylolpropane, are sufficient to obtain complete trimethylolpropane conversion. At 10% lipase, the trimethylolpropane conversion was over 98% and P3, P4, P5 and P6 were formed in sufficient amounts.

Example 3

50 part by weight of trimethylolpropane and 1.5 part by weight of dimethyl carbonate were charged in a reaction vessel, equipped with agitation (700 rpm), and heated to a reaction temperature of 50, 60 and 70°C. The reaction was initiated by addition of 150 part by weight of molecular sieves and a lipase B catalyst (Novozym ^® 435) at concentrations of 10% by weight calculated on charged trimethylolpropane. A maximum conversion of 98% was obtained. The amounts of cyclic and linear components in the reaction mixture were determined after 48 and 96 hours and the result is given in Table 3.

Example 4

Example 2 was repeated with the differences that dimethyl carbonate was substituted for diethyl carbonate and that a reaction temperature of 70°C was used in addition to 60°C. The amounts of cyclic and linear components in the reaction mixture were determined and the result is given in Table 4.

Example 5

The reaction product obtained in Example 2 using 10% of lipase B at the reaction temperature 60°C was filtered to remove solids including the lipase and molecular sieves, and heated to 60-80°C under atmospheric pressure to allow evaporation of formed methanol. The result of disproportionation of carbonates, obtained by the process of the present invention, by thermal treatment is given in Table 5.