Functional materials with reversible crosslinking

Abstract

A method for the reversible crosslinking of, for example, adhesives or coating materials, and a composition stable on storage at room temperature for implementing the crosslinking reaction. The reversible crosslinking method allows very rapid crosslinking even at a low first temperature, and undoing of the crosslinks at higher temperatures, thereby recovering thermoplastic processability and, for example, allowing the originally bonded substrates to be parted from one another again easily. A particular aspect is that a plurality of cycles of crosslinking and undoing of the crosslinks are possible with the present system. A feature of the system used for the reversible crosslinking is that it contains two components, A and B, where component A is a compound having at least two protected dithioester functionalities, preferably cyanodithioester functionalities, and component B is a compound having at least two diene functionalities.

Claims

1. A reversibly crosslinkable formulation crosslinkable via a hetero-Diels-Alder reaction, the formulation comprising a component A which has at least two protected dithioester groups, and a component B which has at least two diene functionalities.

2. The formulation according to claim 1, wherein the protected dithioester is a compound having the structure ##STR00012## where Z is an electron-withdrawing group, R.sup.m is a plurivalent organic group or a polymer, the group R.sup.n is a divalent alkylic, aromatic or oligoethereal group, and n is a number between 2 and 20.

3. The formulation according to claim 2, wherein the group Z is a cyano group.

4. The formulation according to claim 2, wherein the group R.sup.n is a benzyl group and n is a whole number between 2 and 4.

5. The formulation according to claim 1, wherein components A and/or B comprise a polymer.

6. The formulation according to claim 5, wherein the polymers are polyacrylates, polymethacrylates, polystyrenes, copolymers of acrylates, methacrylates and/or styrenes, polyacrylonitrile, polyethers, polyesters, polylactic acids, polyamides, polyesteramides, polyurethanes, polycarbonates, amorphous or semicrystalline poly--olefins, EPDM, EPM, hydrogenated or unhydrogenated polybutadienes, ABS, SBR, polysiloxanes and/or block, comb and/or star copolymers of these polymers.

7. The formulation according to claim 1, wherein at least one of the two components, A or B, has at least three of the respectively stated functionalities.

8. The formulation according to claim 1, wherein component A is a low molecular mass organic compound having 3 to 4 protected dithioester groups, and in that the formulation can be activated at a temperature T.sub.1 between 35 and 70 C. and is thereby crosslinkable and the crosslinking can be reversed to an extent of at least 50% at a temperature T.sub.2 which is at least 5 C. higher than the temperature T.sub.1.

9. A process for reversible crosslinking, comprising: activating a formulation according to claim 1 at a temperature T.sub.1 of at least 35 C.; subsequently crosslinking the formulation via a hetero-Diels-Alder reaction; and at a temperature T.sub.2 which is at least 5 C. higher than the temperature T.sub.1, undoing at least 50% of the crosslinks via a retro-hetero-Diels-Alder reaction.

10. The process according to claim 9, wherein the temperature T.sub.1 is a temperature between 40 and 80 C. and the temperature T.sub.2 is a temperature between 85 and 150 C.

11. The process according to claim 9, wherein at a temperature above 80 C., at least 90% of the formulation is soluble again in a solvent suitable for the formulation prior to the crosslinking.

12. The process according to claim 9, wherein the crosslinking of the formulation comprising components A and B, on heating to the temperature T.sub.1, takes place within 5 minutes.

13. An adhesive, sealant, molding compound, varnish, paint, coating, ink, or composite material, comprising the formulation according to claim 1.

Description

EXAMPLES

(1) The weight-average molecular weights of the polymers are determined by means of GPC (gel permeation chromatography). The measurements were carried out using a PL-GPC 50 Plus from Polymer Laboratories Inc. at 30 C. in tetrahydrofuran (THF) against a series of polystyrene standards (approximately 200 to 1.Math.10.sup.6 g/mol).

(2) The NMR analyses were carried out on a Bruker AM 400 MHz spectrometer.

Example 1

Synthesis of a Cyclopentadiene-Blocked Dicyanodithioester (IPDI-DTE-CP)

Stage 1a: Sodium Carbonocyanidodithioate (1a)

(3) ##STR00007##

(4) A suspension of 5.46 g (0.111 mol, 1.1 eq) of sodium cyanide in 20 ml of DMF is cooled to 0 C. in an ice bath. With vigorous stirring, 6.20 ml (7.75 g, 0.102 mol, 1 eq) of carbon disulphide, diluted with 13 ml of DMF, are added over a period of 10 minutes. Following complete addition of CS.sub.2, the ice bath is removed and the solution is stirred until solidification is complete (precipitation of brown needles over a period of approximately 60 minutes). Then 150 ml of isobutyl alcohol are added and heating takes place to dissolve the precipitated needles. The hot solution is filtered in order to remove unconsumed sodium cyanide. The filtrate is cooled with liquid nitrogen and the reformed precipitate is isolated by filtration and washed with diethylether. The brownish powder is recrystallized once from a 1:1 mixture of isobutyl alcohol and diethyl ether. This gives a yellow solid, with a yield of 90% (30.0 g, 0.09 mol).

Stage 1b: Tetraethylammonium Carbonocyanidodithioate (1b)

(5) ##STR00008##

(6) The following conversion to the ammonium salt is necessary in order to increase the stability of the salt in storage. The corresponding sodium salt from stage 1, however, could also be used.

(7) The entire yellow solid obtained is subsequently boiled at reflux in 110 ml of ethanol, while at the same time 18.9 g (0.09 mol, 1 eq) of ethylammonium bromide are boiled in 50 ml of ethanol. The two boiling solutions are combined and kept at boiling with reflux for a further 10 minutes. On cooling, a brown solid crystallizes out, and is filtered off and recrystallized from ethanol. The end product is a brown, lustrous solid. Yield: 48% (10.0 g, 0.043 mol)

Stage 2: 4-(Bromomethyl)benzyl alcohol (2)

(8) ##STR00009##

(9) Under nitrogen, 50 ml of a 25% strength solution of diisobutyl aluminium hydride in hexane (8.17 g, 57.4 mmol, 2 eq) are cooled to 0 C. in an ice bath, and then 6.57 g (28.7 mmol, 1 eq) of methyl 4-(bromomethyl)benzoate, in solution in 30 ml of dichloromethane, are added via a dropping funnel over the course of 30 minutes. After a reaction time of 4 hours, water is slowly added in order to quench remaining diisobutylaluminium hydride. To dissolve the white precipitate which forms in the course of quenching, 30 ml of concentrated HCl solution and 30 ml of dichloromethane are added. The two phases are separated and the aqueous layer is extracted four times with dichloromethane. The organic phases are combined and dried using Na.sub.2SO.sub.4. Under reduced pressure, the solvent is removed; a white solid is left, in a yield of 77% (4.40 g, 22.0 mmol). The product was identified by .sup.1H-NMR spectroscopy as the target product.

Stage 3: Cyclopentadiene-Blocked Cyanodithioester (3)

(10) ##STR00010##

(11) In a round-bottomed flask, 4.40 g (22.0 mmol, 1 eq) of 4-(bromomethyl)benzyl alcohol from stage 2 are dissolved in 15 ml of acetonitrile and stirred at room temperature. Added to the solution are 5.22 g (22.5 mmol, 1.02 eq) of tetraethylammonium carbonocyanidodithioate from stage 1b, in solution in 2 ml of acetonitrile. After a reaction time of one minute, 5.22 ml (4.02 g, 60.9 mmol, 2.7 eq) of cyclopentadiene are injected and the mixture is stirred for 3 hours. The solvent is removed under reduced pressure and the yellow oil that remains is purified by means of flash chromatography (silica gel/1:1-1:2 hexane:ethyl acetate). The product was identified by means of .sup.1H and .sup.13C NMR spectroscopy as the target product. Yield: 32% (2.03 g, 7.04 mmol)

Stage 4: Cyclopentadiene-Blocked Cyanodithioester Di-Linker (IPDI-DTE-CP) (3a)

(12) 1.00 g (3.40 mmol, 2.3 eq) of cyclopentadiene-blocked cyanodithioester (3) from stage 3, 0.33 g (1.50 mmol, 1 eq) of isophorone diisocyanate (IPDI) and 1.00 g (0.0015 mmol, 0.01 eq) of dibutyltin dilaurate are introduced under a nitrogen atmosphere into a 25 ml two-necked flask, and subsequently admixed with 4 ml of dried THF. This mixture is heated to 55 C. with stirring and at that temperature 0.5 ml (0.362 g, 3.50 mmol, 2.3 eq) of triethylamine is added. The mixture is subsequently stirred at 55 C. overnight and then cooled to room temperature, before the THF is removed under reduced pressure.

(13) The residue is dissolved in 40 ml of dichloromethane and the organic phase is washed in succession with 30 ml of 1-molar aqueous NaOH solution, 30 ml of 1-molar aqueous HCl solution, and with NaCl solution. The organic phase is dried over magnesium sulphate and then the solvent is removed under reduced pressure. This gives 1.40 g of a dark solid, which in turn is purified by chromatography over silica gel, using a 2:1 mixture of ethyl acetate and hexane as eluent. This gives a yield of 53% (0.70 g, 0.8 mmol). The product was identified as the target product by means of .sup.1H NMR spectroscopy.

Example 2

Isophorone-Disorbyl (IPDI-SA) (4)

(14) This compound, used in accordance with the invention as a diene, is synthesized from isophorone diisocyanate (IPDI) and sorbyl alcohol in acetone.

(15) ##STR00011##

(16) For the synthesis, 1.092 mol (267.77 g) of IPDI are weighed out into a 2000 ml three-necked flask and then dissolved in 300 g of acetone. Following addition of 0.01% by weight of DBTL, the solution is heated to 60 C., and 2.377 mol (233.26 g) of furfuryl alcohol are added dropwise over the course of 60 minutes. The NCO content of the reaction solution at the beginning of the synthesis is 12.45%. After a reaction time of 4.5 hours, the NCO content is ascertained, in order to determine the progress of the reaction. It is 0.506%. A further hour later, the reaction is ended at an NCO content of 0.20%. The solvent is removed on a rotary evaporator at 100 C. and 5*10.sup.1 mbar, leaving behind a brownish oil which is of very high viscosity at room temperature. Unambiguous characterization of the product took place by means of infrared spectroscopy and by means of .sup.1H NMR and .sup.13C NMR spectroscopy.

Example 3

Reaction of the IPDI-SA with the CP-Blocked Dienophile from Example 1

(17) Equimolar amounts of the IPDI-SA diene from Example 2 and the CP-blocked dienophile from Example 1 (stage 4) are dissolved in dichloromethane and the two solutions are mixed with one another. The dichloromethane is subsequently removed in vacuo. The residue which remains is heated to 75 C. After a reaction time of 20-60 minutes, the heating source was removed and the resultant, cooled polymer was analyzed by means of GPC.

Example 4

Synthesis of Furfuryl-Functionalized Polymethacrylate (PMMA-FU) by Means of Free-Radical Solution Polymerization

(18) For the synthesis of the copolymer, a mixture of 60 parts by weight of n-butyl methacrylate, 20 parts of methyl methacrylate and 10 parts by weight of furfuryl methacrylate are dissolved in 35 parts by weight of xylene in a glass vessel, 4 parts by weight of mercaptoethanol are added, and degassing takes place by passing nitrogen through the vessel. In a further vessel, a 10% strength by weight solution of ,-azobis(2-hydroxyethylisobutyramide) (3 parts by weight) is prepared. The two initial charges are metered in a constant ratio over a period of five hours into a jacketed glass reactor with thermostat, temperature-conditioned to 110 C., under nitrogen, and are allowed to polymerize. After the end of the metering, heating is continued for an hour (110 C.) and the resultant polymer solution is cooled and discharged. A viscous, clear polymer solution is obtained, whose composition is ascertained by means of 1H NMR spectroscopy.

(19) In this way, a diene component having more than two diene functionalities is obtained. This compound acts, accordingly, as a crosslinker.

Example 5

Reaction of the PMMA-FU with the CP-Blocked Dienophile from Example 1

(20) Relative to the dienophile groups and to the furfuryl groups used in the polymerization of Example 4, equimolar amounts of the PMMA-FU from Example 4 and of the CP-blocked dienophile from Example 1 (stage 4) are dissolved in dichloromethane and the two solutions are mixed with one another. The dichloromethane is subsequently removed in vacuo. The residue which remains is heated to 75 C. After a reaction time of 20-60 minutes, the heating source was removed. An extraction test on the resultant solid using dichloromethane under reflux revealed that only 5% by weight of the material is still soluble. In contrast, an extraction with toluene under reflux and with subsequent filtration at approximately 95 C. revealed that about 85% by weight of the material was in solution again at 111 C. This experiment shows that there is no decrosslinking at a temperature of around 40 C., whereas at a temperature of 111 C. a large proportion of the crosslinking sites are undone again.

Example 6

Synthesis of CP-telechelic poly-n-butyl acrylate (PBA-CP)

(21) This telechelic used as diene is a poly-n-butyl acrylate having two terminal cyclopentadiene groups in each case. The preparation process is a two-stage process.

Stage 1: Synthesis of Br-telechelic poly-n-butyl acrylate by means of ATRP

(22) 62 equivalents of n-butyl acrylate (nBA), 1 equivalent of 1,4-bis(bromoisobutyryl-oxy)butane, 0.35 equivalent of copper(I) oxide, 0.0125 equivalent of copper(II) bromide and 0.7 equivalent of pentamethyldiethylenetriamine (PMDETA) are introduced into a 1 l three-necked flask with magnetic stirrer, nitrogen inlet and reflux condenser. Acetone is added to the mixture in an amount sufficient to give 200 ml of a 50% strength (by volume) solution. Oxygen present is removed by the passage of nitrogen for 40 minutes. The mixture is thereafter heated to 60 C. under nitrogen in an oil bath. After polymerization for 1.5 hours, it is terminated by cooling to room temperature and admitting atmospheric oxygen. The copper catalyst is removed by electrochemical deposition on zinc dust in accordance with the method described in WO 2012/007213. The bromine-terminated poly(n-butyl acrylate) is recovered by evaporation of the solvent.

Stage 2: Synthesis of Cp-Telechelic Poly(nBA) (PnBA-CP)

(23) 1 equivalent of polymer from preceding stage 1, 6 equivalents of sodium iodide, 2 equivalents of triphenylphosphine and 2 equivalents of nickelocene are introduced into a 50 ml three-necked flask with magnetic stirrer, reflux condenser and dropping funnel, under nitrogen and in acetone, to form 25 ml of a solution having a molarity of 0.1 based on the polymer. The solution is stirred at room temperature for 12 hours, and the reaction solution is subsequently purified by column chromatography on a short column packed with basic aluminium oxide. The cyclopentadienyl-terminated polymer is precipitated twice from cold ethanol by addition of water. The conversion of the Br end groups to Cb end groups can be demonstrated by nuclear magnetic resonance spectroscopy. The molecular weight is determined by means of GPC with calibration against PMMA standards in THF.

Example 7

Reaction of the PnBA-CP with the CP-Blocked Dienophile from Example 1

(24) Equimolar amounts of the PnBA-CP diene from Example 6 and of the CP-blocked dienophile from Example 1 (stage 4) are dissolved in dichloromethane and the two solutions are mixed with one another. The dichloromethane is subsequently removed in vacuo. The residue which remains is heated to 75 C. After a reaction time of 20-60 minutes, the heating source was removed and the resultant, cooled polymer was analysed by GPC (Example 7a) and compared with the molecular weight of the polymer from Example 6 (see Table 1). Given therein are the values for the number average (M.sub.n) and for the peak maximum (M.sub.p) of the molecular weight, and the molecular weight distribution (PDI).

(25) The product is subsequently taken up in a solution of 10 equivalents of 2,3-dimethylbutadiene in toluene and is heated at 120 C. for 30 minutes. A GPC measurement is then performed on this product as well (see Example 7b in Table 1). The 2,3-dimethylbutadiene is needed here as a diene, in order to scavenge the free cyanodithiocarbonate groups of the decrosslinked dienophile. If this were not done, the cyanodithiocarbonates would react again on cooling with the diene groups of the compound from Example 6 (in accordance with the invention).

(26) In a parallel experiment without 2,3-dimethylbutadiene and toluene, the mixture is heated to 120 C. Here it is found that a fluid composition is obtained from a pastelike material at this high temperature. This decrease in viscosity cannot be attributed solely to the lower viscosity of the polymer at higher temperatures, but instead more particularly to the retro-hetero-Diels-Alder reaction that occurs in accordance with the invention.

(27) TABLE-US-00001 TABLE 1 M.sub.n M.sub.p Example g mol.sup.1 g mol.sup.1 PDI Example 6 12 300 11 100 1.4 (PnBA-CP) Example 1 920 870 1.0 (IPDI-DTE-CP) Example 7a 23 100 37 000 1.9 Example 7b 5 900 11 100 3.1