FIELD OF THE INVENTION

The invention relates to the recycling of waste materials comprising polyurethane. The invention further relates methods of preparing polycarbamylated compounds.

BACKGROUND OF THE INVENTION

Polyurethane (PU) materials are commonly produced by the reaction of polyol compounds, mainly polyether or polyester polyols, with polyisocyanate compounds, in particular diisocyanates. The properties of the resulting polyurethane materials can easily be tuned by for instance varying the type and relative ratio of the polyol and/or polyisocyanate compounds, resulting in polyurethane materials with significantly different chemical and physical properties which can be applied for a wide range of applications. Common applications of polyurethane materials include flexible or semirigid foams used as matrasses or in furniture, rigid foams used as thermal insulation materials in buildings or refrigerators, and polyurethane elastomers that can be used for instance as adhesives, coatings, and in shoe soles.

Increased environmental awareness of polyurethane producers, stricter regulatory policies of government agencies, and the public demand for sustainability have led to an increased interest in recycling of end of life (EoL) polyurethane waste and polyurethane scrap from the manufacturing process. Common recycling processes include mechanical and chemical recycling methods.

Mechanical recycling generally involves chipping of waste polyurethane foam into small flakes which are glued back together. However, due to a tremendous decrease in foam properties of the rebonded materials, these materials only have a limited applicability and are generally classified as low quality polyurethane products. Hence, they poorly address the issue of the huge waste streams of EoL polyurethane products by the fast growing polyurethane industry, since the market for low quality polyurethane products is small and saturated.

Alternatively, chemical recycling methods have been developed in which the polyurethane materials are depolymerized into their monomers (and oligomers) by using a suitable chemical splitting agent and often a catalyst. After separation and optional purification or chemical conversion steps, the obtained monomers (and oligomers) can be reused as raw materials for the production of new, high quality polyurethane materials.

Common chemical recycling reactions for polyurethane materials include hydrolysis, aminolysis, ammonolysis, and alcoholysis of polyurethane materials, with alcoholysis, and in particular glycolysis, being the most commonly used method. In alcoholysis, a compound containing at least one reactive hydroxyl group is added as a chemical splitting agent to the material comprising polyurethane. In order to reach industrially relevant reaction rates, a catalyst that accelerates the alcoholysis is commonly added. This mixture is then heated and after the desired reaction time, a liquid solution is obtained consisting generally of excess alcohol splitting agent; (oligomeric) compounds containing hydroxyl end groups (the recovered polyol); polycarboxylic acids or the salts thereof; polycarboxylic acid esters; as well as derivatives of the original polyisocyanate compounds, such as polycarbamate compounds, mainly dicarbamate compounds; amine-carbamate compounds; and polyamine compounds, mainly diamine compounds.

It is also long known to those skilled in the art that carbamates, and in particular carbamates formed from an acidic alcohol, are relatively unstable and can be thermally cleaved into an alcohol and an isocyanate [US4349484; EP1870397; W02019/043180]. This thermal splitting process can be seen as an alternative for the more commonly used phosgenation process to obtain isocyanate compounds, with the advantage that the isocyanate compounds are produced without the need for large amounts of extremely toxic phosgene and the expensive process steps required for handling phosgene.

Only in rare cases, acidic monofunctional alcohols, such as methanol, have been applied as splitting agents for the alcoholysis of polyurethane materials:

Asahi et al. (Polym. Degrad. Stab. (2004), 86, 147) disclose a procedure to depolymerize polyurethane in methanol at temperatures up to 300 °C, in the absence of a catalyst and pressures up to 150 bar, with methanol being (partially) in the supercritical state, to reach satisfactory conversions of >90%. This study was complemented by three studies of Liu et al. (Polym. Degrad. Stab. (2013) 98, 2520; Polym. Degrad. Stab. (2017) 140, 17; Polym. Degrad. Stab. (2017) 140, 126) in which the depolymerization of a thermoplastic polyurethane elastomer was investigated in sub- and supercritical methanol. Again, high temperatures ranging from 220 °C to 260 °C and pressures from 30 to 130 bar were needed to reach >90% conversion of the polyurethane elastomer. The main disadvantage of these methanolysis procedures is the high temperature needed to reach satisfactory conversions. At such high temperatures, side reactions occur that hamper product separation and purification after the depolymerization step. Furthermore, the related high pressures require special pressure-resistant reactor equipment, which complicates upscaling.

However, industrially relevant depolymerization rates can be obtained at milder reaction temperatures and pressures by adding a suitable catalyst that accelerates polyurethane methanolysis. EP 3590999 describes the depolymerization of polyurethane in methanol with the addition of an alkali methanolate as catalyst to ensure satisfactory conversions after 5 hours at moderate reaction conditions (temperatures of 158 - 168 °C and pressures of 15 - 19 bar). However, when strong bases, such as alkali methanolates, are used as catalyst at these reaction temperatures, methanolysis of materials comprising polyurethane yields almost exclusively polyamine compounds, mainly diamine compounds, as derivatives of the original polyisocyanate compounds and only traces of polycarbamate compounds are obtained, as was evidenced in the examples of this patent. This is due to the relative instability of polycarbamate compounds formed from acidic alcohols.

Zhao et al. (ACS Omega (2021), 6, 4175) described recently that satisfactory conversions (90%) and adequate selectivities towards polycarbamate compounds, mainly dicarbamate compounds, can be obtained in the methanolysis of a thermoplastic polyurethane elastomer with the aid of strong alkali base catalysts, such as t-BuOK, when very mild reaction conditions (65 °C, atmospheric pressure) are applied. However, the depolymerization rate is very low under these mild reaction conditions, which necessitates large amounts of t-BuOK catalyst (2 - 3 equivalents per urethane group of the polyurethane elastomer) and long reaction times of 20 hours, impeding industrial implementation.

It is clear from the prior art that there is a compelling need for new, industrially relevant methods that use acidic alcohols as splitting agents for the depolymerization of polyurethane materials and operate under moderate reaction conditions with a suitable catalyst to ensure industrially relevant depolymerization rates and acceptable reaction times. It is of particular economic interest that the catalyst also ensures good selectivities towards the unstable polycarbamate compounds, so that after isolation, these polycarbamate compounds can be directly converted into polyisocyanate building blocks, without the need for large amounts of toxic phosgene and the expensive process steps required for the safe handling of phosgene. SUMMARY OF THE INVENTION

The present invention provides novel methods for alcoholysis of polyurethanes with acidic alcohols as chemical splitting agents that fulfils the above-mentioned needs of combining high depolymerization rates with high selectivity for polycarbamate compounds.

The present invention provides methods for preparing dicarbamylated or polycarbamylated compounds, which are soluble in the solution used in the reaction, by chemolytic splitting of a solid material comprising polyurethane in the presence of a monofunctional alcohol as splitting agent and of a catalyst.

Solid products from the material and unreacted polyurethane can in this way be easily separated from the dicarbamylated or polycarbamylated compounds obtained after the reaction.

The invention is further summarised in the following statements:

1. A method for preparing dicarbamylated and/or polycarbamylated compounds from polyurethane, the method comprising the steps of:

(i) contacting a material comprising polyurethane with a solution comprising a monofunctional alcohol and a catalyst, wherein the catalyst is Lewis acid catalyst and/or a tertiary amine catalyst and

(ii) heating said solution comprising said polyurethane, said monofunctional alcohol and said catalyst, resulting in a chemolytic splitting of said polyurethane in dicarbamylated or polycarbamylated compounds.

2. The method according to statement 1, further comprising the step of isolating said dicarbamylated and/or polycarbamylated compounds from said solution.

3. The method according to statement 1 or 2, wherein said isolation is performed by a method selected from the group consisting of by distillation, extraction, filtration, ion exchange, precipitation.

4. The method according to any one of statements 1 to 3, wherein said isolation is performed by filtration.

5. The method according to any one of statements 1 to 4, further comprising the step of isolating from said solution one or more of polyol or short diol compounds, polycarboxylic acids or the esters thereof, amine-carbamate compounds, and polyamine compounds.

6. The method according to any one of statements 1 to 5, further comprising the step of isolating from said solution Polyol and short diol compounds . 7. The process according to any one of statements 1 to 6, wherein the polyurethane is a toluene diisocyanate (TDI) based polyurethane and the chemolytic splitting results in the formation of 2,4-toluenedicarbamate or 2,6-toluenedicarbamate.

8. The process according to any one of statements 1 to 6, wherein the polyurethane is a methylene diphenyl isocyanates (MDI) based polyurethane and the chemolytic splitting results in the formation of 4,4'-diphenylmethanedicarbamate, 2,4'- diphenylmethanedicarbamate or 2,2'-diphenylmethanedicarbamate.

9. The method according to any one of statements 1 to 6, wherein the polyurethane is polymeric methylene diphenyl isocyanate (MDI) based polyurethane wherein one or more hydrogen atoms on the arene rings are substituted by additional 4- (alkoxycarbonylamino)benzyl substituents and the chemolytic splitting results in the formation of polycarbamylated compounds.

10. The method according to any of statements 1 to 9 , wherein the monofunctional alcohol is an alcohol with a pKa of maximally 15.5.

11. The method according to any one of statements 1 to 10, wherein the monofunctional alcohol is methanol, 1,1,1-trifluoromethanol or phenol, more preferably methanol.

12. The method according to any of statements 1 to 11, wherein the Lewis acid catalyst contains a cation from periodic table groups 3-15.

13. The method according to any of statements 1 to 12, wherein the Lewis acid catalyst contains a cation from periodic table groups 4-8, 14 and 15.

14. The method according to any of statements 1 to 13, wherein the Lewis acid catalyst cation is selected from the group consisting of Fe, Ti, W, Mo, Bi, Mn, and Sn.

15. The method according to any of statements 1 to 14, wherein the Lewis acid catalyst cation is Fe, Ti, or W.

16. The method according to any of statements 1 to 15, wherein the anion of the Lewis acid catalyst is selected from the group consisting of a halide, carboxylate, oxide, alkyl, diketonate, and their derivatives, and combinations thereof.

17. The method according to any of statements 1 to 16, wherein the Lewis acid catalyst is W0CI4, TiO(acac)2, MoO2(tmhd)2, or dibutyl tin diacetate.

18. The method according to any of statements 1 to 17, wherein the tertiary amine catalyst is N,N-diisopropylethylamine or l,4-diazabicyclo[2.2.2]octane.

19. The method according to any of statements 1 to 18, wherein the solution is heated to, and the chemolytic splitting takes place, at a temperature between 100 - 250 °C, 110 - 230 °C, 120 - 220 °C, 150 - 200 °C. 20. The method according to any of statements 1 to 19, wherein per kg of the material comprising polyurethane between 0.1 and 200 kg monofunctional alcohol is used, between 0.25 and 100 kg monofunctional alcohol is used, between 0.5 and 20 kg monofunctional alcohol is used, between 1 and 8 kg monofunctional alcohol is used, or between 2 and 4 kg monofunctional alcohol is used. The method according to any of statements 1 to 20, wherein the mixture in step (ii) is subjected to a pressure of between 1 - 100 bar, between 1.5 - 40 bar, between 5 - 30 bar, or between 15 - 25 bar.

21. The method according to any of statements 1 to 20, wherein step ii is performed for between 1 - 1500 minutes, between 5 - 720 minutes, between 10 - 360 minutes, or between 15 - 180 minutes.

22. The method according to any of statements 1 to 20 wherein the amount of catalyst relative to 1 kg of polyurethane or material comprising polyurethane is between 0.001 and 0.3 kg, between 0.005 and 0.1 kg, or between 0.008 and 0.05 kg.

23. The method according to any of claims 1 to 22, wherein the obtained polycarbamylated compounds are converted into their respective polyisocyanate compounds.

24. The method according to any of claims 1 to 23, wherein the obtained compounds, such as polycarbamylated, polyisocyanate, polyamine, polyol, and short diol compounds are used for production of polyurethanes

DETAILED DESCRIPTION

The prefix "poly" as used herein means "more than one", which when limited to integers is the same as "2 or more" or "at least 2". Hence, the term "polyol" stands for a compound having at least 2 alcohol or hydroxyl (-OH) functional groups, typically between 2 and 6. The term "polyamine" thus stands for a compound having at least 2 primary amine (-NH ₂ ) functional groups. The term "polycarbamate" thus stands for a compound having at least 2 carbamate (-NH(CO)O-) functional groups, typically between 2 and 6. The term "polyisocyanate" thus stands for a compound having at least 2 isocyanate (-NCO) functional groups, typically between 2 and 6. The term "amine-carbamate" stands for a compound having at least 1 amine (-NH ₂ ) functional group, typically between 2 and 6 and at least 1 carbamate (-NH(CO)O-), typically between 2 and 6 functional group. In addition, "polyisocyanate" as used herein refers to compounds having exclusively isocyanate functional groups; "polycarbamate" refers to compounds having exclusively carbamate functional groups; "polyamine" refers to compounds having exclusively amine functional groups; and "amine-carbamate" refers to compounds having at least one amine and at least one carbamate functional group.

The person skilled in the art knows that the polyurethane (PU) is made by reacting at least one polyisocyanate compound with at least one polyol compound, with optionally the addition of a blowing agent and optionally a chain extender or crosslinker, and optionally additives commonly used in the synthesis of polyurethane materials.

A typical length of a polyol is 250 -10 000 g/mol, more particular 500 - 5000 g/mol A typical ratio polyol/polyisocyanate 5 tot 0.25 kg polyol per is tov 1 kg polyisocyanate, 3 tot 0.4 kg polyol is tov 1 kg polyisocyanate.

The polyurethane fraction of the of the material comprising polyurethane that is used in the methods of the present invention also envisages polymeric materials that contain both urethane and isocyanurate groups and are often referred to as polyisocyanurates.

Polyisocyanate compounds used for the synthesis of the polyurethane fraction can be selected from aliphatic, or cycloaliphatic, or araliphatic polyisocyanates, such as 1,5- pentane diisocyanate (PDI), 1,6-hexane diisocyanate (HDI), isophorone diisocyanate (IPDI), 1,4-cyclohexane diisocyanate (CHDI), 4,4'-dicyclohexylmethane diisocyanate (H12MDI) and m- and p- tetra methylxylylene diisocyanate (TMXDI), and especially aromatic polyisocyanates with an isocyanate functionality of at least two, such as toluene diisocyanates (TDI), phenylene diisocyanates and methylene diphenyl isocyanates (MDI). The toluene diisocyanates (TDI) may be selected from pure 2,4- TDI and isomeric mixtures in any proportions of 2,4-TDI and 2,6-TDI. The methylene diphenyl isocyanates (MDI) may be selected from pure 4,4'-MDI, isomeric mixtures in any proportions of 4,4'-MDI, 2,4'-MDI and 2,2'-MDI, and/or higher homologs like crude and polymeric MDI (pMDI) having isocyanate functionalities of more than 2. Most preferably, the polyisocyanate compounds to produce such polyurethane fractions are selected from toluene diisocyanates (TDI) and methylene diphenyl isocyanates (MDI) and higher homologs like polymeric MDI (pMDI). In another embodiment, the polyisocyanate compound used for the synthesis of the polyurethane fraction is a modified polyisocyanate compound, which is commonly synthesized by reacting a polyisocyanate compound with a low molecular weight polyol or polyamine. In addition, the modified polyisocyanate compound can be prepared by reacting polyisocyanate compounds with themselves, yielding polyisocyanate compounds that also contain allophanate, uretonimine, carbodiimide, urea, biuret or isocyanurate groups.

If desired, a mixture of two or more polyisocyanate compounds as mentioned above can be used for the synthesis of the polyurethane fraction.

The polyol compounds used for the synthesis of the polyurethane fraction can be selected from polyether, polyester, polyesteramide, polythioether, polycarbonate, polyacetal, polyolefin and polysiloxane polyols, biobased polyols and mixtures of two or more thereof.

The polyether polyols used for preparing such polyurethane fractions may be synthesized by reacting one or more alkylene oxide or substituted alkylene oxide, such as ethylene oxide, propylene oxide, tetrahydrofuran, butylene oxides, styrene oxide, epichlorohydrin, and epibromohydrin, with one or more active hydrogen containing initiators, such as water, ethylene glycol, propylene glycol, 1,4-butanediol,

1.6-hexanediol, glycerol, trimethylolpropane, pentaerythritol, sorbitol, sucrose,

1.2.6-hexanetriol, bisphenols, ethylenediamine, diaminopropanes, ethanolamine, aniline, 2,4-toluenediamine, 2,6-toluenediamine, 2,4'-diaminodiphenylmethane, and 4,4'-diaminodiphenylmethane.

The polyester polyols used for preparing such polyurethane fractions may be synthesized by reaction of one or more hydroxyl-terminated compounds such as ethylene glycol, propylene glycol, diethylene glycol, 1,4-butanediol, 1,6-hexanediol, glycerol, penta erythritol, and polyether polyols with one or more polycarboxylic acid compounds or their ester-forming derivatives, such as succinic, glutaric and adipic acids or their dimethyl esters, sebacic acid, phthalic anhydride, tetrachlorophthalic anhydride, and dimethyl terephthalate. The polyester polyols may also be synthesized by the polymerisation of lactones, such as caprolactone.

The polyurethane fraction of the material comprising polyurethane for use in the methods of the present invention may also contain other ingredients that are commonly used in synthesis formulations to prepare polyurethane materials. Nonlimiting examples of such additional ingredients include chain extending and crosslinking agents, blowing agents, catalysts, surfactants, stabilizers, flame retardants, organic and inorganic fillers, pigments, and anti-oxidants.

The material comprising polyurethane for use in the methods of the present invention can be added in the form in which it is received but preferably it is reduced in size or increased in density in a suitable way known to the person skilled in the art, such as cutting, grinding, milling, pelletizing, pressing, densification and any combinations thereof.

As indicated in the introduction, common applications of polyurethane materials include flexible or semi-rigid foams used as matrasses or in furniture, rigid foams used as thermal insulation materials in buildings or refrigerators, and polyurethane elastomers that can be used for instance as adhesives, coatings, and in shoe soles. Such materials are suitable for use in the claimed methods.

Other non-limiting examples of materials comprising polyurethane for use in the present invention are products that comprise PU and can be used in the present invention are old furniture such as sofas and chairs, mattresses, cushions and pillows, carpets or rugs, insulation materials, or automotive parts, foam packaging.

Preferred materials for use in the present invention are matrasses, soles of shoes and isolation material.

Depending on the type of material and eventual pre-purification the material comprising polyurethane comprises (%wt) more than 10 %, 20 %, 30 %, 40 % , 50 %, 60 %, 70 %, 80 %, 90 %, or 95 % polyurethane.

Typically, the material comprising polyurethane the material comprises (%wt) preferably at least 50 %, more preferably at least 80 %, even more preferably at least 95 %.

In method step (i), a material comprising polyurethane is contacted with a solution comprising a monofunctional alcohol as splitting agent. Typically, the monofunctional alcohol is an acidic monofunctional alcohol. The term "acidic monofunctional alcohol" relates in this invention to chemical compounds having only one free hydroxyl group attached to a carbon atom with a p/< _a of maximally 15.5 as measured in an aqueous solution and described in one of the four International Union of Pure and Applied Chemistry (IUPAC) books of aqueous p/ _a measurements, more preferably a p/ _a in the range of 9 - 15.5, more preferably a volatile acidic alcohol with a boiling point at atmospheric pressure of maximally 70, 80, 90, or 100 °C, more preferably methanol or trifluoroethanol, most preferably methanol. [Perrin DD (1965) Dissociation constants of organic bases in aqueous solution, Ist edn. Butterworths, London; Perrin DD (1972) Dissociation constants of organic bases in aqueous solution, supplement, 1st edn. Butterworths, London; Kortum G, Vogel W, Andrussow K (1961) Dissociation constants of organic acids in aqueous solution, 1st edn. Butterworths, London; Serjeant E, Dempsey B (1979) Ionisation constants of organic acids in aqueous solution, 1st edn. Pergamon Press, Oxford].

Other typical alcohols for use in the present invention are ethanol, isopropanol. Equally suitable as alcohol for use in the present invention is phenol.

Monofunctional alcohols suitable for use in the present invention have a single free hydroxyl group attached to a carbon atom, but may comprise other functional groups such as carbonyl, carboxyl, carboalkoxy, alkoxy, halo, sulfide, thiol, nitrile and/or tertiary amine functional groups.

However the monofunctional alcohols typically do not contain primary or secondary amines as functional groups.

Generally the monofunctional alcohol (or a mixture thereof) is a technical grade alcohol, with a content of water < 1 % (v/v).

Optionally, the material comprising polyurethane is dried before it is provided to the method of the invention to fully remove the water or decrease the amount of water that is optionally present in the material comprising polyurethane.

Common drying techniques, such as heat treatment, vacuum drying, freeze drying, and any combinations thereof can be employed.

Typically the methods of the present invention are performed at conditions whereib the water content is less than 5% (v/w) of the total reaction mixture, typically less than 1% (v/w) of the total reaction mixture. Providing dried PU material can result in higher selectivities towards polycarbamylated compounds. Apart from the one or more monofunctional alcohols, the solution may optionally comprise other common (co)-solvents such as ketone, ether, ester, carbonate, hydrocarbon, aromatic, and halogenated solvents.

Typically the amount of such co-solvents in the at the start of the reaction, apart from the monofunctional alcohols is less than 20 vol%, preferably less than 10 vol%, more preferably less than 5 vol%.

In one embodiment, the amount of monofunctional alcohol solvent relative to 1 kg of the material comprising polyurethane is between 0.1 and 200 kg monofunctional alcohol, preferably between 0.25 and 100 kg monofunctional alcohol, more preferably between 0.5 and 20 kg monofunctional alcohol, even more preferably between 1 and 8 kg monofunctional alcohol, and most preferably between 2 and 4 kg monofunctional alcohol.

It is further understood that the chemolytic splitting of the material comprising polyurethane of which the polyurethane fraction is produced from a diisocyanate such as TDI and MDI, results in the formation of dicarbamates, and the chemolytic splitting the material comprising polyurethane of which the polyurethane fraction is produced from a polyisocyanate such as polymeric MDI, results in the formation of dicarbamates and polycarbamates.

In method step (i), the solution also comprises a Lewis acid catalyst and/or tertiary amine catalyst, more preferably a Lewis acid catalyst.

Lewis acid catalysts suitable for use in the methods of the present invention comprise Lewis acids containing a metal atom selected from the periodic table groups 2-15, and combinations thereof, preferably from the periodic table groups 4-8, 14 and 15, more preferably Fe, Ti, W, Mo, Bi, Mn, and Sn, even more preferably Fe, Ti, and W. The anion of the Lewis acid catalyst is selected from the group consisting of halides, carboxylates, oxides, alkyls, diketonates, and their derivatives, and combinations thereof. If desired, the solution in step (i) can comprise a mixture of two or more Lewis acid catalysts.

In another embodiment, the solution in step (i) comprises a tertiary amine catalyst, which includes tertiary amines that are known to the skilled in the art as catalysts for the reaction of isocyanates with hydroxyl-terminated compounds, such as triethylamine, tributylamine, tripropylamine, dimethylbenzylamine, /V-methyl, N- ethyl, or /V-cyclohexylmorpholine, /V,/V-diisopropylethylamine, N,N,N',N'~ tetramethylethylenediamine, /V,/V,/V',/V'-tetramethyl-butanediamine, N,N,N',N'~ tetramethyl-hexanediamine, pentamethyl-diethylenetriamine, tetramethyl- diaminoethylether, dimethylpiperazine, l-azabicyclo[3.3.0]octane, and 1,4- diazabicyclo[2.2.2]octane. If desired, the solution in step (i) can comprise a mixture of two or more tertiary amine catalysts.

The amount of catalyst relative to 1 kg of polyurethane or the material comprising polyurethane is between 0.001 and 0.3 kg, more preferably between 0.005 and 0.1 kg, most preferably between 0.008 and 0.05 kg.

Surprisingly, by careful selection of the type of catalyst, significantly better selectivities towards the polycarbamate compounds, mainly dicarbamate compounds, can be achieved under moderate reaction temperatures and pressures, and within limited reaction times, compared to the methods of the prior art.

Optionally the catalyst can potentially be recovered in a suitable way known to the person skilled in the art, like filtration, and reused again. Alternatively, the catalyst can end up in one of the product fractions or waste fractions.

The mixture obtained in step (i) is preferably placed in a reactor. The reactor preferably comprises a stirrer and the reactor can optionally be sealed gas-tight.

In step (ii), the mixture obtained in step (i) is treated at an elevated temperature and/or elevated pressure, more preferably at an elevated temperature and pressure.

In one embodiment, step (ii) takes place at a temperature of 100 - 250 °C, preferably, 110 - 230 °C, more preferably 120 - 220 °C, and most preferably 150 - 200 °C. The mixture in step (ii) is subjected to a pressure of >1 - 100 bar, preferably 1.5 - 40 bar, more preferably 5 - 30 bar, most preferably 15 - 25 bar. If appropriate, step (ii) can be conducted in the presence of an inert gas known to the person skilled in the art, like nitrogen or noble gasses such as argon, to create an oxygen -reduced or oxygen-free atmosphere.

The reaction time in step (ii) is not particularly limited. Typically, the reaction time is 1 - 1500 minutes, 5 - 720 minutes, 10 - 360 minutes, or 15 - 180 minutes.

In one embodiment, the mixture obtained after step (ii) comprises polycarbamylated compounds as derivatives of the original polyisocyanate compounds like toluene dicarbamate or methylene diphenyl dicarbamate compounds, as well as excess alcohol splitting agent like methanol; Lewis acid or tertiary amine catalysts; (oligomeric) compounds containing hydroxyl end groups like polyether polyols or shorter diols like diethylene glycol; polycarboxylic acids like phthalic acid or the salts thereof; polycarboxylic acid esters like dimethyl phthalate; amine-carbamate compounds like toluene amine-carbamate or methylene diphenyl amine-carbamate compounds; and polyamine compounds like toluenediamine or methylene diphenyl diamine compounds. Optionally, the mixture obtained after step (ii) contains also a chain extender or cross-linker, optionally a blowing agent, and optionally additives conventionally used in preparing polyurethane materials, and combinations thereof.

The polycarbamylated compounds can be isolated from the mixture obtained after step (ii). Conventional separation techniques which are well known to the person skilled in the art, like liquid-liquid extraction, distillation, filtration, precipitation, column chromatography, ion exchange treatments, and a combination thereof, can be used.

In another embodiment, other valuable chemical compounds can optionally be isolated from the mixture obtained after step (ii) via the conventional separation techniques.

A possible use of the polycarbamylated products obtained by this invention is to convert these polycarbamate compounds into their respective polyisocyanate compounds which can be reused for producing new polyurethane materials. The conversion of polycarbamate compounds into polyisocyanate compounds and alcohol compounds is well known in the art using for example thermal splitting [US4349484, EP0396977, EP1870397, W02019/043180]. The polycarbamate compounds can be converted into their respective polyisocyanate compounds directly, or after an additional purification step.

The other valuable chemical compounds that optionally can be isolated from the mixture obtained after step (ii) may include polyol or short diol compounds, as well as polycarboxylic acids or the esters thereof, which can be used for the synthesis of new plastics, such as polyurethanes or polyesters; amine-carbamate compounds, which can be converted into their respective polyamine compounds via for example hydrolysis or, alternatively, into their respective polycarbamate compounds via for example oxidative carbonylation or alkoxycarbonylation; and polyamine compounds, which can be converted into their respective polyisocyanate compounds via for example phosgenation.

In another embodiment, the mixture obtained after step (ii) is directly used for the synthesis of new plastics, such as polyurethanes or polyesters, without any separation steps.

The polyisocyanate conversion is determined based on data collected by ^X H NMR spectroscopy; the molar amount of isocyanate-derived functional groups (i.e. carbamate and amine functional groups) in the liquid reaction mixture after the reaction is divided by the molar amount of isocyanate-functional groups used to prepare the solid polyurethane material that was added in the reactor (equation 1). polyisocyanate < ^#moles Ar-NH ₂ ) _solllb | _e + (#moles Ar-NH(CO)OR) _soluble conversion (#moles Ar-NCO used in the PU synthesis) _S0 | _id

For TDI-based polyurethane materials, the selectivity towards the polycarbamate products is determined via ^X H NMR by dividing the molar amount of TDI-derived polycarbamate compounds in the liquid reaction mixture after the reaction by the molar amount of TDI-derived polycarbamate, amine-carbamate and polyamine compounds in the liquid reaction mixture after the reaction (equation 2). More information on the identification and quantification of TDI-derived polycarbamate, amine-carbamate and polyamine compounds via ^X H NMR can be found in Vanbergen et al. ChemSusChem. (2020), 13, 3835.

For monomeric and polymeric MDI-based polyurethane materials, the MDI-derived polycarbamate, amine-carbamates and polyamine compounds are first isolated from the liquid reaction mixture after the reaction. The polycarbamate selectivity is then calculated by dividing the molar amount of MDI-derived polycarbamate compounds by the summed molar amounts of MDI-derived polycarbamate, amine-carbamate and polyamine compounds (equation 2).

Eq. 2 polycarbamate selectivity (#moles polycarbamates) _solubte + (#moles amine-carbamates) _S0 | _Ubte + (#moles polyamines) _SO | _Ub!e

For TDI-based polyurethane materials, the selectivity towards the amine-carbamate products is determined, based on data collected by ^X H NMR spectroscopy, by dividing the molar amount of TDI-derived amine-carbamate compounds in the liquid reaction mixture after the reaction by the molar amount of TDI-derived polycarbamate, amine-carbamate and polyamine compounds in the liquid reaction mixture after the reaction (equation 3).

For monomeric and polymeric MDI-based polyurethane materials, the MDI-derived polycarbamate, amine-carbamates and polyamine compounds are first isolated from the liquid reaction mixture after the reaction. The amine-carbamate selectivity is then calculated by dividing the molar amount of MDI-derived amine-carbamate compounds by the molar amount of MDI-derived polycarbamate, amine-carbamate and polyamine compounds (equation 3).

Eq. 3 amine-carbamate (#moles amine-carbamates) _S0luble selectivity (#moles polycarbamates) _SO | _Ub | _e + (#moles amine-carbamates) _S0 | _Ub | _e + (#moles polyamines) _SO | _Ub | _e

For TDI-based polyurethane materials, the selectivity towards the polyamine products is determined, based on data collected by ^X H NMR spectroscopy, by dividing the molar amount of TDI-derived polyamine compounds in the liquid reaction mixture after the reaction by the molar amount of TDI-derived polycarbamate, amine- carbamate and polyamine compounds in the liquid reaction mixture after the reaction (equation 4).

For monomeric and polymeric MDI-based polyurethane materials, the MDI-derived polycarbamate, amine-carbamates and polyamine compounds are first isolated from the liquid reaction mixture after the reaction. The polyamine selectivity is then calculated by dividing the molar amount of MDI-derived polyamine compounds by the molar amount of MDI-derived polycarbamate, amine-carbamate and polyamine compounds (equation 4).

Eq. 4

In case polymeric MDI was used in the preparation of the polyurethane material, polycarbamates refers to compounds having exclusively carbamate functional groups; polyamines to compounds having exclusively amine functional groups; and amine-carbamates to compounds having at least one amine and at least one carbamate functional group.

The polycarbamate yield is calculated via ^X H NMR by dividing the molar amount of carbamate functional groups of the polyisocyanate-derived products in the liquid reaction mixture obtained after the reaction by the molar amount of carbamate functional groups that maximally can be formed from the solid polyurethane material that was added in the reactor (equation 5). The molar amount of carbamate groups that maximally can be formed from the solid polyurethane material that was added in the reactor is calculated by subtracting the molar amount of water added as blowing agent in the polyurethane synthesis recipe from the molar amount of isocyanate functional groups used to prepare the solid polyurethane material, since it can be assumed that all water molecules convert isocyanate functional groups into amine functional groups during the "blowing" reaction.

Eq. 5 EXAMPLES

Example 1: Lewis acid MnBr ₂ as catalyst

700 mg of a standard flexible polyurethane foam, synthesized from TDI 80 as polyisocyanate and VORANOL™ 3322 as polyether polyol via a conventional synthesis procedure as described above, was chipped into small flakes and placed into a pressure-resistant 10 mL reactor that also includes a stirring bar. Next, 7.7 mg Manganese(II) Bromide (MnBr ₂ ) (0.036 mmol) was added as catalyst. After adding 5 mL of methanol, the reactor was closed and sealed gas tight. The reactor was heated to an internal temperature of 185 °C by placing the reactor in a preheated aluminium heating block on a stirring plate and the mixture was stirred inside the reactor. The temperature was kept constant for a reaction time of 2 hours and a pressure of 29 bar was reached.

After the reaction time, the reactor was cooled in an ice bath and opened. 100 pL of dimethylacetamide was added as external standard to the mixture under stirring. An aliquot of 100 pL of the resulting solution was taken and dissolved in 400 pL DMSO- de and analysed via ^X H NMR spectroscopy on a BRUKER AVANCE 300 MHz or BRUKER AVANCE III HD 400 MHz. The polyisocyanate conversion was 96% with a selectivity of 33% towards polycarbamate compounds, 48% towards amine-carbamate compounds, and 19% towards polyamine compounds. The polycarbamate yield was 85%.

Example 2: tertiary amine DABCO as catalyst

700 mg of a standard flexible polyurethane foam, synthesized from TDI 80 as polyisocyanate and VORANOL™ 3322 as polyether polyol via a conventional synthesis procedure as described above, was chipped into small flakes and placed into a pressure-resistant 10 mL reactor that also includes a stirring bar. Next, 4.0 mg 1,4- diazabicyclo[2.2.2]octane (DABCO) (0.036 mmol) was added as catalyst. After adding 5 mL of methanol, the reactor was closed and sealed gas tight. The reactor was heated to an internal temperature of 185 °C by placing the reactor in a preheated aluminium heating block on a stirring plate and the mixture was stirred inside the reactor. The temperature was kept constant for a reaction time of 2 hours and a pressure of 29 bar was reached.

After the reaction time, the resulting solution was analysed via ^X H NMR spectroscopy as described in example 1. The polyisocyanate conversion was 99% with a selectivity of 32% towards polycarbamate compounds, 47% towards amine-carbamate compounds, and 21% towards polyamine compounds. The polycarbamate yield was 85%.

Example 3: Lewis acid Bi(ND) ₃ as catalyst

700 mg of a standard flexible polyurethane foam, synthesized from TDI 80 as polyisocyanate and VORANOL™ 3322 as polyether polyol via a conventional synthesis procedure as described above, was chipped into small flakes and placed into a pressure-resistant 10 mL reactor that also includes a stirring bar. Next, 26.0 mg bismuth trineodecanoate (Bi(ND) ₃ ) (0.036 mmol) was added as catalyst. After adding 5 mL of methanol, the reactor was closed and sealed gas tight. The reactor was heated to an internal temperature of 185 °C by placing the reactor in a preheated aluminium heating block on a stirring plate and the mixture was stirred inside the reactor. The temperature was kept constant for a reaction time of 2 hours and a pressure of 29 bar was reached.

After the reaction time, the resulting solution was analysed via ^X H NMR spectroscopy as described in example 1. The polyisocyanate conversion was 99% with a selectivity of 30% towards polycarbamate compounds, 53% towards amine-carbamate compounds, and 17% towards polyamine compounds. The polycarbamate yield was 86%.

Example 4: Lewis acid MoO2(tmhd)2 as catalyst

700 mg of a standard flexible polyurethane foam, synthesized from TDI 80 as polyisocyanate and VORANOL™ 3322 as polyether polyol via a conventional synthesis procedure as described above, was chipped into small flakes and placed into a pressure-resistant 10 mL reactor that also includes a stirring bar. Next, 17.8 mg Molybdenum(VI) oxide bis(2,2,6,6-tetramethyl-3,5-heptanedionate) (MoO2(tmhd)2) (0.036 mmol) was added as catalyst. After adding 5 mL of methanol, the reactor was closed and sealed gas tight. The reactor was heated to an internal temperature of 185 °C by placing the reactor in a preheated aluminium heating block on a stirring plate and the mixture was stirred inside the reactor. The temperature was kept constant for a reaction time of 2 hours and a pressure of 29 bar was reached.

After the reaction time, the resulting solution was analysed via ^X H NMR spectroscopy as described in example 1. The polyisocyanate conversion was 97% with a selectivity of 32% towards polycarbamate compounds, 49% towards amine-carbamate compounds, and 19% towards polyamine compounds. The polycarbamate yield was 86%.

Example 5: Lewis acid Fe(acac)s as catalyst

700 mg of a standard flexible polyurethane foam, synthesized from TDI 80 as polyisocyanate and VORANOL™ 3322 as polyether polyol via a conventional synthesis procedure as described above, was chipped into small flakes and placed into a pressure-resistant 10 mL reactor that also includes a stirring bar. Next, 12.7 mg Iron(III) acetylacetonate (Fe(acac)s) (0.036 mmol) was added as catalyst. After adding 5 mL of methanol, the reactor was closed and sealed gas tight. The reactor was heated to an internal temperature of 185 °C by placing the reactor in a preheated aluminium heating block on a stirring plate and the mixture was stirred inside the reactor. The temperature was kept constant for a reaction time of 2 hours and a pressure of 29 bar was reached.

After the reaction time, the resulting solution was analysed via ^X H NMR spectroscopy as described in example 1. The polyisocyanate conversion was 94% with a selectivity of 34% towards polycarbamate compounds, 50% towards amine-carbamate compounds, and 16% towards polyamine compounds. The polycarbamate yield was 86%.

Example 6: Lewis acid TiO(acac)2 as catalyst

700 mg of a standard flexible polyurethane foam, synthesized from TDI 80 as polyisocyanate and VORANOL™ 3322 as polyether polyol via a conventional synthesis procedure as described above, was chipped into small flakes and placed into a pressure-resistant 10 mL reactor that also includes a stirring bar. Next, 9.4 mg Titanium(IV) oxide bis(acetylacetonate) (TiO(acac)2) (0.036 mmol) was added as catalyst. After adding 5 mL of methanol, the reactor was closed and sealed gas tight. The reactor was heated to an internal temperature of 185 °C by placing the reactor in a preheated aluminium heating block on a stirring plate and the mixture was stirred inside the reactor. The temperature was kept constant for a reaction time of 2 hours and a pressure of 29 bar was reached.

After the reaction time, the resulting solution was analysed via ^X H NMR spectroscopy as described in example 1. The polyisocyanate conversion was 94% with a selectivity of 36% towards polycarbamate compounds, 46% towards amine-carbamate compounds, and 17% towards polyamine compounds. The polycarbamate yield was 86%.

Example 7: tertiary amine DIPEA as catalyst

700 mg of a standard flexible polyurethane foam, synthesized from TDI 80 as polyisocyanate and VORANOL™ 3322 as polyether polyol via a conventional synthesis procedure as described above, was chipped into small flakes and placed into a pressure-resistant 10 mL reactor that also includes a stirring bar. Next, 4.7 mg N,N- diisopropylethylamine (DIPEA) (0.036 mmol) was added as catalyst. After adding 5 mL of methanol, the reactor was closed and sealed gas tight. The reactor was heated to an internal temperature of 185 °C by placing the reactor in a preheated aluminium heating block on a stirring plate and the mixture was stirred inside the reactor. The temperature was kept constant for a reaction time of 2 hours and a pressure of 29 bar was reached.

After the reaction time, the resulting solution was analysed via ^X H NMR spectroscopy as described in example 1. The polyisocyanate conversion was 99% with a selectivity of 34% towards polycarbamate compounds, 46% towards amine-carbamate compounds, and 20% towards polyamine compounds. The polycarbamate yield was 88%.

Example 8: Lewis acid WOCI4 as catalyst

700 mg of a standard flexible polyurethane foam, synthesized from TDI 80 as polyisocyanate and VORANOL™ 3322 as polyether polyol via a conventional synthesis procedure as described above, was chipped into small flakes and placed into a pressure-resistant 10 mL reactor that also includes a stirring bar. Next, 12.3 mg Tungsten(VI) Oxychloride (WOCI4) (0.036 mmol) was added as catalyst. After adding 5 mL of methanol, the reactor was closed and sealed gas tight. The reactor was heated to an internal temperature of 185 °C by placing the reactor in a preheated aluminium heating block on a stirring plate and the mixture was stirred inside the reactor. The temperature was kept constant for a reaction time of 2 hours and a pressure of 29 bar was reached.

After the reaction time, the resulting solution was analysed via NMR spectroscopy as described in example 1. The polyisocyanate conversion was 96% with a selectivity of 36% towards polycarbamate compounds, 48% towards amine-carbamate compounds, and 16% towards polyamine compounds. The polycarbamate yield was 89%.

Example 9: Lewis acid Fe(tmhd)3 as catalyst

700 mg of a standard flexible polyurethane foam, synthesized from TDI 80 as polyisocyanate and VORANOL™ 3322 as polyether polyol via a conventional synthesis procedure as described above, was chipped into small flakes and placed into a pressure-resistant 10 mL reactor that also includes a stirring bar. Next, 21.8 mg Iron(III) 2,2,6,6-tetramethyl-3,5-heptanedionate (Fe(tmhd)3) (0.036 mmol) was added as catalyst. After adding 5 mL of methanol, the reactor was closed and sealed gas tight. The reactor was heated to an internal temperature of 185 °C by placing the reactor In a preheated aluminium heating block on a stirring plate and the mixture was stirred inside the reactor. The temperature was kept constant for a reaction time of 2 hours and a pressure of 29 bar was reached.

After the reaction time, the resulting solution was analysed via ^X H NMR spectroscopy as described in example 1. The polyisocyanate conversion was 100% with a selectivity of 34% towards polycarbamate compounds, 50% towards aminecarbamate compounds, and 16% towards polyamine compounds. The polycarbamate yield was 92%.

Example 10: Lewis acid FeCL as catalyst

700 mg of a standard flexible polyurethane foam, synthesized from TDI 80 as polyisocyanate and VORANOL™ 3322 as polyether polyol via a conventional synthesis procedure as described above, was chipped into small flakes and placed into a pressure-resistant 10 mL reactor that also includes a stirring bar. Next, 5.8 mg Iron(III) Chloride (FeCh) (0.036 mmol) was added as catalyst. After adding 5 mL of methanol, the reactor was closed and sealed gas tight. The reactor was heated to an internal temperature of 185 °C by placing the reactor in a preheated aluminium heating block on a stirring plate and the mixture was stirred inside the reactor. The temperature was kept constant for a reaction time of 2 hours and a pressure of 29 bar was reached.

After the reaction time, the resulting solution was analysed via ^X H NMR spectroscopy as described in example 1. The polyisocyanate conversion was 100% with a selectivity of 35% towards polycarbamate compounds, 50% towards amine- carbamate compounds, and 15% towards polyamine compounds. The polycarbamate yield was 94%.

Example 11: MDI-based polyurethane elastomer with Lewis acid Fe(tmhd)s as catalyst

700 mg of a standard polyurethane elastomer, synthesized from monomeric MDI as polyisocyanate and a conventional polyether polyol via a standard synthesis procedure as described above, was chipped into small flakes and placed into a pressure-resistant 10 mL reactor that also includes a stirring bar. Next, 21.8 mg Iron(III) 2,2,6,6-tetramethyl-3,5-heptanedionate (Fe(tmhd)3) (0.036 mmol) was added as catalyst. After adding 5 mL of methanol, the reactor was closed and sealed gastight. The reactor was heated to an internal temperature of 185 °C by placing the reactor in a preheated aluminium heating block on a stirring plate and the mixture was stirred inside the reactor. The temperature was kept constant for a reaction time of 2 hours and a pressure of 30 bar was reached.

After the reaction time, the reactor was cooled in an ice bath and opened. The mixture was separated in a polycarbamate fraction; an amine-carbamate fraction; an polyamine fraction; and a polyol fraction, and analysed via ^X H NMR with dimethylacetamide as external standard.

The polyisocyanate conversion was 100% with a selectivity of 95% towards polycarbamate compounds, 5% towards amine-carbamate compounds, and 0% towards polyamine compounds. The polycarbamate yield was 97%.

Example 12: pMDI-based polyurethane rigid foam with tertiary amine N _r N- diisopropylethylamine (DIPEA)

350 mg of a standard rigid polyurethane foam, synthesized from polymeric MDI as polyisocyanate (functionality of 2.7 and NCO value of 31%) and a conventional polyether polyol for rigid polyurethane foams via a standard synthesis procedure as described above, was chipped into small flakes and placed into a pressure-resistant 10 mL reactor that also includes a stirring bar. Next, 4.7 mg N,N- diisopropylethylamine (DIPEA) (0.036 mmol) was added as catalyst. After adding 5 mL of methanol, the reactor was closed and sealed gastight. The reactor was heated to an internal temperature of 185 °C by placing the reactor in a preheated aluminium heating block on a stirring plate and the mixture was stirred inside the reactor. The temperature was kept constant for a reaction time of 3 hours and a pressure of 30 bar was reached.

After the reaction time, the reactor was cooled in an ice bath and opened. The mixture was separated in a polycarbamate fraction; an amine-carbamate fraction; an polyamine fraction; and a polyol fraction, and analysed via ^X H NMR with dimethylacetamide as external standard.

The polyisocyanate conversion was 99% with a selectivity of 51% towards polycarbamate compounds, 47% towards amine-carbamate compounds, and 2% towards polyamine compounds. The polycarbamate yield was 86%.

Example 13: short reaction time with Lewis acid (Fe(tmhd)s) as catalyst 700 mg of a standard flexible polyurethane foam, synthesized from TDI 80 as polyisocyanate and VORANOL™ 3322 as polyether polyol via a conventional synthesis procedure as described above, was chipped into small flakes and placed into a pressure-resistant 10 mL reactor that also includes a stirring bar. Next, 21.8 mg Iron(III) 2,2,6,6-tetramethyl-3,5-heptanedionate (Fe(tmhd)3) (0.036 mmol) was added as catalyst. After adding 5 mL of methanol, the reactor was closed and sealed gas tight. The reactor was heated to an internal temperature of 185 °C by placing the reactor in a preheated aluminium heating block on a stirring plate and the mixture was stirred inside the reactor. The temperature was kept constant for a reaction time of 30 minutes and a pressure of 29 bar was reached.

After the reaction time, the resulting solution was analysed via ^X H NMR spectroscopy as described in example 1. The polyisocyanate conversion was 85% with a selectivity of 38% towards polycarbamate compounds, 46% towards amine-carbamate compounds, and 16% towards polyamine compounds. The polycarbamate yield was 80%.

Example 14: trifluoroethanol as monofunctional alcohol and tertiary amine /V,/V-diisopropylethylamine (DIPEA) as catalyst

210 mg of a standard flexible polyurethane foam, synthesized from TDI 80 as polyisocyanate and VORANOL™ 3322 as polyether polyol via a conventional synthesis procedure as described above, was chipped into small flakes and placed into a pressure-resistant 10 mL reactor that also includes a stirring bar. Next, 4.7 mg N,N- diisopropylethylamine (DIPEA) (0.036 mmol) was added as catalyst. After adding 5 mL of 2,2,2-trifluoroethanol (TFE), the reactor was closed and sealed gas tight. The reactor was heated to an internal temperature of 170 °C by placing the reactor in a preheated aluminium heating block on a stirring plate and the mixture was stirred inside the reactor. The temperature was kept constant for a reaction time of 4 hours and a pressure of 15 bar was reached.

After the reaction time, the resulting solution was analysed via ^X H NMR spectroscopy as described in example 1. The polyisocyanate conversion was 93% with a selectivity of 35% towards polycarbamate compounds, 51% towards amine-carbamate compounds, and 14% towards polyamine compounds. The polycarbamate yield was 86%.

Comparative Example 1 (Cl): potassium hydroxide as alkaline catalyst

700 mg of a standard flexible polyurethane foam, synthesized from TDI 80 as polyisocyanate and VORANOL™ 3322 as polyether polyol via a conventional synthesis procedure as described above, was chipped into small flakes and placed into a pressure-resistant 10 mL reactor that also includes a stirring bar. Next, 2.0 mg potassium hydroxide (KOH) (0.036 mmol) was added as catalyst. After adding 5 mL of methanol, the reactor was closed and sealed gas tight. The reactor was heated to an internal temperature of 185 °C by placing the reactor in a preheated aluminium heating block on a stirring plate and the mixture was stirred inside the reactor. The temperature was kept constant for a reaction time of 2 hours and a pressure of 29 bar was reached.

After the reaction time, the resulting solution was analysed via ^X H NMR spectroscopy as described in example 1. The polyisocyanate conversion was 100% with a selectivity of 8% towards polycarbamate compounds, 46% towards aminecarbamate compounds, and 46% towards polyamine compounds. The polycarbamate yield was 47%.

Comparative Example 2 (C2): alkali tert-butoxide as catalyst

700 mg of a standard flexible polyurethane foam, synthesized from TDI 80 as polyisocyanate and VORANOL™ 3322 as polyether polyol via a conventional synthesis procedure as described above, was chipped into small flakes and placed into a pressure-resistant 10 mL reactor that also includes a stirring bar. Next, 4.0 mg of Potassium tert-Butoxide (KOtBu) (0.036 mmol) was added as catalyst. After adding 5 mL of methanol, the reactor was closed and sealed gas tight. The reactor was heated to an internal temperature of 185 °C by placing the reactor in a preheated aluminium heating block on a stirring plate and the mixture was stirred inside the reactor. The temperature was kept constant for a reaction time of 2 hours and a pressure of 29 bar was reached.

After the reaction time, the resulting solution was analysed via ^X H NMR spectroscopy as described in example 1. The polyisocyanate conversion was 100% with a selectivity of 12% towards polycarbamate compounds, 55% towards aminecarbamate compounds, and 33% towards polyamine compounds. The polycarbamate yield was 62%.

Comparative Example 3 (C3): Comparative example with alkali methanolate as catalyst

700 mg of a standard flexible polyurethane foam, synthesized from TDI 80 as polyisocyanate and VORANOL™ 3322 as polyether polyol via a conventional synthesis procedure as described above, was chipped into small flakes and placed into a pressure-resistant 10 mL reactor that also includes a stirring bar. Next, 6.5 mg of a Sodium Methoxide (NaOMe) solution (30 wt% in methanol) was added as catalyst. After adding 5 mL of methanol, the reactor was closed and sealed gas tight. The reactor was heated to an internal temperature of 185 °C by placing the reactor in a preheated aluminium heating block on a stirring plate and the mixture was stirred inside the reactor. The temperature was kept constant for a reaction time of 2 hours and a pressure of 29 bar was reached.

After the reaction time, the resulting solution was analysed via ^X H NMR spectroscopy as described in example 1. The polyisocyanate conversion was 100% with a selectivity of 21% towards polycarbamate compounds, 53% towards aminecarbamate compounds, and 26% towards polyamine compounds. The polycarbamate yield was 73%.

Comparative Example 4 (C4): example without catalyst

700 mg of a standard flexible polyurethane foam, synthesized from TDI 80 as polyisocyanate and VORANOL™ 3322 as polyether polyol via a conventional synthesis procedure as described above, was chipped into small flakes and placed into a pressure-resistant 10 mL reactor that also includes a stirring bar. After adding 5 mL of methanol, the reactor was closed and sealed gas tight. The reactor was heated to an internal temperature of 185 °C by placing the reactor in a preheated aluminium heating block on a stirring plate and the mixture was stirred inside the reactor. The temperature was kept constant for a reaction time of 30 minutes and a pressure of 28 bar was reached.

After the reaction time, the resulting solution was analysed via ^X H NMR spectroscopy as described in example 1. The polyisocyanate conversion was only 58% with a selectivity of 38% towards polycarbamate compounds, 45% towards aminecarbamate compounds, and 17% towards polyamine compounds. The polycarbamate yield was 54%.

Comparative Example 5 (C5): p-TSA as organic acid catalyst catalyst

700 mg of a standard flexible polyurethane foam, synthesized from TDI 80 as polyisocyanate and VORANOL™ 3322 as polyether polyol via a conventional synthesis procedure as described above, was chipped into small flakes and placed into a pressure-resistant 10 mL reactor that also includes a stirring bar. Next, 6.7 mg p- Toluenesulfonic acid monohydrate (p-TSA) (0.036 mmol) was added as catalyst. After adding 5 mL of methanol, the reactor was closed and sealed gas tight. The reactor was heated to an internal temperature of 185 °C by placing the reactor in a preheated aluminium heating block on a stirring plate and the mixture was stirred inside the reactor. The temperature was kept constant for a reaction time of 30 minutes and a pressure of 29 bar was reached.

After the reaction time, the resulting solution was analysed via ^X H NMR spectroscopy as described in example 1. The polyisocyanate conversion was only 61% with a selectivity of 35% towards polycarbamate compounds, 47% towards aminecarbamate compounds, and 18% towards polyamine compounds. The polycarbamate yield was 50%.

Table 1. Summary of experimental conditions and results of the above examples

Abbreviations

Bi(ND) ₃ : bismuth trineodecanoate; DABCO : l,4-diazabicyclo[2.2.2]octane; DIPEA : /V,/V-diisopropylethylamine; Fe(acac) ₃ :

5 Iron(III) acetylacetonate ; Fe(tmhd) ₃ : Iron(III) 2,2,6,6-tetramethyl-3,5-heptanedionate; KOtBu : Potassium tert-Butoxide ;

MoC>2(tmhd)2 : Molybdenum(VI) oxide bis(2,2,6,6-tetramethyl-3,5-heptanedionate) ; p-TSA : p-Toluenesulfonic acid monohydrate; TiO(acac)2: Titanium(IV) oxide bis(acetylacetonate.