The present invention refers to polyetherpolyols which are characterized by a low content of cyclic by-products as well as to a method for producing the polyetherpolyol and an adhesive composition comprising the same.

Polyetherpolyols are well-known compounds suitable for a variety of applications. One common application is the employment of polyetherpolyols in the production of adhesive systems, in particular polyurethane adhesives wherein the polyetherpolyol is reacted with an isocyanate component to obtain the desired polyurethane. In particular with regard to the application of polyetherpolyols in adhesive systems, a high content of linear OH-terminated polymer chains is desirable. However, in most cases, the polyetherpolyols also contain a certain amount of cyclic oligomeric by-products which lead to a decrease in the mechanical performance of the adhesive or thermoplastic polyurethanes. It is therefore desirable to keep the content of cyclic oligomers as low as possible.

A number of attempts have been made to obtain suitable polyetherpolyols, one being the polymerization of cyclic ethers. While three- and four-membered cyclic ethers readily undergo polymerization reaction due to high ring-strain, polymerization of the thermodynamically stable five- and six-membered ring systems remains challenging.

A. Ishigaki describes the copolymerization of 2-substituted tetrahydrofurans with some cyclic ethers using BF3-(C ₂ H5) ₂ 0 as initiator in Die Makromolekulare Chemie 1964, 79, p. 170. 2- Methyltetrahydrofuran (MTHF), which was known to not homopolymerize, was found to copolymerize with such monomers as epichlorohydrin and 3,3-bis(chloromethyl) oxacyclobutane (BCMO). The report in particular highlights the low reactivity ofthe 2-substituted tetrahydrofurans which were found to only undergo polymerization with halogenated co-monomers.

However, it was found that the polyetherpolyols produced according to the known methods still contain high amounts of cyclic by-products which impair the performance of any products obtained from the polyetherpolyols.

In light of the above, it is the object of the present invention to provide a polyetherpolyol which allows for the production products showing good mechanical performances.

It was surprisingly found that polyetherpolyols having a reduced number of cyclic by-products lead to mechanically stable products, such as adhesives and coating systems. It was surprisingly found that the amount of undesired cyclic by-products can be significantly reduced if the respective polyetherpolyols are prepared by polymerization of an epoxy-containing compound and a further compound capable of undergoing ring-opening reaction in the presence of a Bnansted acid with a weakly coordinating and non-nucleophilic anion. A first object of the present invention is therefore a polyetherpolyol obtained by co-polymerization of a compound A containing at least one epoxide moiety and a compound B capable of undergoing a ring-opening reaction wherein compound A and compound B are different from each other and wherein the co-polymerization is carried out in the presence of a Bnzsnsted acid with a weakly coordinating and non-nucleophilic anion.

It was surprisingly found that the inventive polyetherpolyol could be obtained in good yields and with chain lengths suitable for adhesive applications.

The presence of cyclic by-products, in particular cyclic oligomers is preferable to be avoided due to the negative impact on the mechanical performance of the final product like an adhesive, sealing or a coating. It was surprisingly found that formation of cyclic by-products was hardly observed during the preparation of the inventive polyetherpolyol. As a measure for the presence of undesired cyclic by-products, the difference in the number average molecular weight M _n of the inventive polyetherpolyol determined by gel permeation chromatography (GPC) and the number average molecular weight M _n of the inventive polyetherpolyol determined by end group titration was used. A significant difference between the number average molecular weight determined by GPC and the number average molecular weight determined by end group titration was interpreted being due to the presence of cyclic by-products which would not be registered by determination by end group titration, resulting in the mentioned difference in molecular weight. Therefore, an embodiment of the present invention is preferred wherein the difference between the number average molecular weight Mn of the inventive polyetherpolyol as determined by GPC and the number average molecular weight Mn of the inventive polyetherpolyol as determined by end group titration is less than 50%, preferably less than 40%, in particular less than 20%, based on the number average molecular weight M _n of the inventive polyetherpolyol as determined by GPC.

In a preferred embodiment, compound A is a low molecular weight epoxy compound, in particular having a molecular weight of 40 to 200 g/mol, preferably 50 to 160 g/mol. In an especially preferred embodiment, compound A is selected from the group consisting of ethylene oxide (EO), propylene oxide (PO) and the compounds shown below.

In an especially preferred embodiment, compound A is propylene oxide (PO).

It was surprisingly found that ring systems which normally do not undergo co-polymerization could be employed as co-monomers in the inventive polyetherpolyol.

In a preferred embodiment, compound B is such selected from five-membered heterocyclic rings and six-membered heterocyclic rings. In an especially preferred embodiment, compound B is selected from the group consisting of five-membered and six-membered cyclic ethers which are optionally substituted. In a further preferred embodiment, compound B is selected from the group consisting of substituted five-membered cyclic ethers and substituted six-membered cyclic ether, the one or more substituents being independently selected from the group consisting methyl, ethyl, propyl, iso-propyl, butyl, iso-butyl and tert-butyl. In an especially preferred embodiment, compound B is 2-methyl tetrahydrofuran.

The Bnansted acid with a weakly coordinating and non-nucleophilic anion employed in the present invention is preferably selected from the group consisting of H3PW12O40* x H2O with 10 < x < 44, preferably 15 < x < 30, H3PM012O40, HBF ₄ , CF3COOH, CF3SO3H, triflimidic acid and HSbFe.

In light of the awareness of limited resources, sustainability and the replacement of commonly used petro-based compounds with compounds obtained from renewable sources, so called bio-based compounds, have become the focus of attention. The inventive polyetherpolyol has the advantage that the monomers employed in the copolymerization may be obtained from renewable sources such as plants thereby contributing to the ever-growing sector of resource-friendly chemical compounds. Therefore, in a preferred embodiment of the invention, compound A and/or compound B are obtained from renewable sources.

It was further surprisingly found that the polymer backbone of the polyetherpolyol can be individually designed by selecting the appropriate ratio of monomers. In this way it is possible to obtain a polyetherpolyol wherein the monomeric ring-opened units in the chain have perfect alternation, wherein the alkene terminated chains are only formed in quantities below the quantification limit. In a preferred embodiment, the initial monomer ratio by weight of compound A to compound B is 1 :20 to 1 : 1 .3, preferably 1 :5 to 1 :1 .5.

The molecular weight of the inventive polyetherpolyol may be adjusted according to the specific application. However, especially in view of applications in adhesives, it is advantageous if the molecularweight does not exceed a certain limit. In a preferred embodiment of the present invention, the polyetherpolyol therefore has an average number molecular weight M _n of 500 to 5000 g/mol, preferably 700 to 3000 g/mol, determined by GPC.

It is also desirable to have a narrow molecular weight distribution. In a preferred embodiment, the inventive polyetherpolyol therefore has a polydispersity index (PDI) of 1 .1 to 5.0, preferably 1 .1 to 2.5, determined according to GPC. Within the present invention, PDI refers to the ratio of the average weight molecular weight and the average number molecular weight of the polyetherpolyol (Mw/Mn). In a preferred embodiment, the inventive polyetherpolyol has a hydroxyl value (OH value) of 10 to 200 mg KOH/g, preferably 50 to 150 mg KOH/g, determined according to DIN 53240.

A further object of the present invention is a method for the production of a polyetherpolyol according to the invention. The method comprises the polymerization of compound A and compound B by providing a reaction vessel charged with compound B and subsequently adding compound A over a period of time t _A in the presence of a Bnansted acid with a weakly coordinating and non-nucleophilic anion. It was surprisingly found that the difference between the molecular weight determined by GPC and the molecular weight determined by end group titration can be further reduced if compound A is added dropwise over a certain period of time. The period of time t _A varies depending on the amount of compound A to be added and may range from minutes to several hours.

It was surprisingly found that the presence of compounds with functional groups that exhibit nucelophilic behavior, such as water, alcohols, amines, thiols, etc., during polymerization also suppressed the formation of cyclic oligomers. These compounds can be referred to as initiators. Therefore, polymerization is preferably carried out in the presence of one or more initiator. However, it was observed that the molecular weight of the polyetherpolyol was reduced if too much initiator was present. In a preferred embodiment, the amount of initiator is therefore 1 to 15 mol-%, preferably 2 to 10 mol-%, with respect to compound A.

The Bnansted acid with a weakly coordinating and non-nucleophilic anion is preferably employed in the reaction in an amount of 0.005 to 10 mol-%, preferably 0.01 to 1 mol-%. The Bnansted acid with a weakly coordinating and non-nucleophilic anion is preferably selected from the group consisting of H3PW12O40* x H2O with 10 < x < 44, preferably 15 < x < 30, H3PM012O40, HBF ₄ , CF3COOH, CF3SO3H, triflimidic acid and HSbF6.

The inventive polyetherpolyol can be further functionalized with a number of different reaction partners resulting in acrylate, isocyanate, epoxide, vinyl ether, thiol, amine termination and the like. It is thus especially suitable for applications in the fields of coatings, adhesives, sealings and elastomers (CASE), in particular adhesives. A further object of the present invention is therefore an adhesive composition comprising the inventive polyetherpolyol. In a preferred embodiment, the adhesive composition is a polyurethane adhesive composition or a silane-based adhesive.

Also an object of the present invention is the use of the inventive polyetherpolyol for the production of adhesives, preferably polyurethane adhesives and silane-based systems.

The present invention will be explained in more detail with reference to the following examples which by no means are to be understood as limiting the scope and spirit of the invention.

Examples:

Example A (Reference) A reaction vessel was charged with 2-methyl THF and BF3*2H ₂ 0 was added. Propylene oxide was introduced dropwise into the reaction mixture and the resulting contents were left to stir. Then, an aqueous solution of sodium hydroxide was added. The resulting aqueous phase was extracted with an organic solvent. The obtained organic phase was washed and dried. After removal of the organic solvent, the polyetherpolyol was obtained.

Example B (according to the invention)

A reaction vessel was charged with 2-methyl THF and phosphortungstic acid H3PW12O40* 24 H2O) was added, followed by the addition of an alcohol. Propylene oxide was introduced dropwise into the reaction mixture and the resulting contents were left to stir. Then, an aqueous solution of sodium carbonate was added. Any excess of 2-methyl THF was evaporated under reduced pressure. The obtained polyetherpolyol was dissolved in an organic solvent and filtered through a pad of silica. After removal of the volatiles in the filtrate, the inventive polyether polyol was obtained.

The molecular weight of the obtained polyetherpolyols was determined via GPC-APC and by way of end-group titration. The results are summarized in Table 1 .

Table 1

GPC analysis of the samples was carried out on an Acquity Advanced Polymer Chromatography System available from Waters Corporation (USA) with an Rl and PDA detector. THF with a flow rate of 0.5 ml/minute was used as the eluent and the molecular weights were determined against a polystyrene standard (PSS) of 266 to 1210000 Dalton. Evaluation of the average molecular weight values was carried out using a calibration curve of the 5 ^th order (“Streifenmethode”).

End group titration was carried out according to standard procedure as described in DIN 53240 by acetylation of the free hydroxyl groups of the sample with acetic anhydride in pyridine solvent. After completion of the reaction, water was added, and the remaining unreacted acetic anhydride converted to acetic acid and measured by titration with potassium hydroxide.

The inventive polyetherpolyol was reacted with a diisocyanate (4,4’-methylene diphenyl diisocyanate) at a ratio of 2.2 (NCO/OH) to obtain a polyurethane adhesive. Films were casted (13 cm x 5 cm, thickness of 2 mm) and stored for 7 days at 23 °C and 50% relative humidity. The same procedure was applied using PPG 2000 and pTHF as comparable polyetherpolyols, respectively. The obtained films were then characterized by stress-strain tests. The results are summarized in Table 2. It was surprisingly found that the adhesive based on the inventive polyetherpolyol reached a higher tensile strength than the commonly employed PPG (2.18 n/mm ² compared to 1.54 n/mm ² ) at 50% elongation.

Table 2

The stress-strain test was conducted using a Z010 test system from Zwick-Roell equipped with a 500 N probe head. Speed of sample elongation was 50 mm/min according to DIN 535049.