Precursor provides selectivity

Transcription

1 Precursor provides selectivity Novel route to isocyanate-free blocked isocyanates with functional groups. Ton Loontjens, Bart Plum, Tim Kidd, Boudewijn Scholtens, Polyurethane coatings, prepared from polyols and isocyanates, are well known for their excellent properties. However, so far, no functional isocyanates have been used to give special features to coatings, mainly because of the lack of convenient methods to make them. The introduction of functional groups to isocyanates and to the blocked isocyanates (BIs) derived from them is difficult because of the high reactivity of isocyanates. BIs have been prepared from functionalized amines and alcohols in high yields in an alternative approach by using carbonyl biscaprolactam (CBC, "Allinco") as the BI precursor. It is well known that polyurethane coatings possess excellent properties [1]. The properties of these coatings are mainly controlled by the type of polyol and isocyanate (hard-soft balance). Many polyols are commercially available that satisfy the requirements of the market quite well. In contrast to this, only a few blocked isocyanates are offered and consequently improvement of properties by using functional (blocked) isocyanates has hardly been developed[2]. To the authors' knowledge, almost no well-defined commercial diisocyanates with functional groups exist. It is conceivable to produce functional isocyanates from diisocyanates by reacting one of the two isocyanate groups selectively with the -OH or -NH 2 of a functional compound. However, amines react in a non-selective manner with isocyanates and the selectivity of -OH groups with isocyanates is limited. Of the commonly used diisocyanates, only isophorone diisocyanates (IPDI) [3] and toluene diisocyanates (TDI) [4] show a reasonable selectivity with polyols. Commercial blocked isocyanates The preparation of blocked isocyanates (BIs) with functional groups will have the same lack of selectivity, since they are prepared from isocyanates. Commercial BIs are prepared by the trimerization of HMDI or IPDI to reduce the volatility (toxicity) and to increase the functionality. The catalytic trimerization of diisocyanates unavoidably leads to a mixture of compounds [5]. The subsequent blocking reaction of the isocyanate groups proceeds with high conversion, but the reaction does not go to completion and thus there is always some free isocyanate present [3]. Many blocking agents are used, but caprolactam is the favourite due to its low costs and low toxicity. Since no appropriate manner to make functional BIs exists, there is a need for a new methodology. Here we report a new, safe and easy method to make a wide range of (functional) blocked isocyanates by using carbonyl biscaprolactam (CBC, "Allinco"), as a new non-toxic BI precursor compound [6,7]. Investigating carbonyl biscaprolactam (CBC) as a route to BI intermediates A beneficial way to produce BIs would be to do it in a reversed order i.e., by starting from a blocked isocyanate precursor group and attach this onto an amine (Scheme 1). CBC is a new chemical that has been successfully developed to couple during a reactive processing step (T > 250 oc) in an extruder polyester or nylon chains. This implies that two consecutive reactions have to take place. The challenge was now to find conditions to stop the reaction halfway through, thus making (functional) BI intermediates (Scheme 2). It was unexpectedly found that carbonyl biscaprolactam (X = caprolactam), although symmetrical, could fulfil these requirements, since the reactivity of the reactant (BI) differs from that the starting material (CBC). Blocked isocyanates from amines It is known that carbonyl diimidazole (CDI) can convert amino groups of amino acids into imidazole BIs [5]. This procedure works quite well for amino acids, but not with regular primary amines, since the reactivity of CDI is far too high. Carbonyl biscaprolactam (CBC) is less reactive and combines therefore a very high selectivity with still a good reactivity [6]. Moreover, the caprolactam ring is able to open, which enlarges the options to make novel compounds (Scheme 2). Indications from chain extension work with CBC in polyesters showed that the intermediate products were BIs. It is known that BIs are stable below their deblocking temperature (150 oc). Therefore lowering the temperature could give conditions to stop the reaction halfway through. In a typical experiment to prove this concept CBC was mixed with primary amines in toluene for 2 hours at 80 to 100 oc. Surprisingly, the reaction goes completely and selectively to the left (Scheme 2) giving the corresponding BI in nearly quantitative yield. 1H-NMR spectrum The 1 H-NMR spectrum of the crude product from the reaction of stoichiometric amounts of hexyl amine and CBC illustrates the high yields (Figure 1). The characteristic signals of CBC (3.8 ppm) and hexyl amine (2.15 ppm) disappeared completely and, apart from caprolactam, no signals can be seen other than those of the BI. The caprolactam that is formed can be removed by aqueous extraction of the toluene layer. It appears that CBC has exactly the right balance of reactivity. It is not so high that is prevents selectivity but not too low so the products are formed within short reaction times. As a further demonstration of the versatility and generality of this reaction, it was tested with around 20 different primary amines, some of which some are shown (Figure 2). Blocked isocyanates from alcohols In Scheme 2 it was suggested that the reaction of CBC with RXH could also give BIs by ring opening. Some indications from previous work with chain extenders in polyesters suggested that this option may be feasible with alcohols. In some preliminary experiments, alcohols were heated with CBC under the same conditions as for the amines, but a mixture of BI, carbonates and urethanes was obtained. However, when the reactions were carried out in the presence of various types of catalysts a strong improvement in the selectivity was found, whereby BIs were obtained in high yields (> 90 %). Titanium, zirconium and magnesium catalysts appeared to be the most useful for this. The unique preparation of blocked isocyanates from alcohols and CBC is shown (Scheme 3). It is, to the best of the authors' knowledge, the first time that

2 blocked isocyanates have been prepared in a one step reaction from alcohols (and CBC), without using amines or isocyanates. Blocked isocyanates with functional groups To explore this novel chemistry further, functional BIs were prepared starting from alcohols attached to functional groups. In one example, functional blocked isocyanates were prepared from a perfluorine alcohol precursor [5n], as depicted. (Scheme 4). By heating the perfluorine alcohol in bulk with CBC, in the presence of a titanium catalyst, the functional blocked isocyanate is obtained in high yields. Subsequently, several coating formulations were prepared with hydroxy functional polyesters, a common BI crosslinker and various amounts of this novel perfluorine alkyl BI. Comparing surface tensions The coatings had, due to self-stratification, very low surface tensions after curing them at 180 oc (Table 1). It can be seen that the surface energy of this coating, which contains only 4 % fluorine, is very low, even compared with Teflon (which has 76 % fluorine). XPS measurements showed that the surface of the coating was strongly enriched with fluorine, due to the migration of the fluorine-containing compound to the surface. The fluorine compound was fixed at the surface and could not be washed away. Preparation of functional polymers The reaction of CBC with bishexamethylene triamine was chosen to demonstrate the selectivity of CBC with primary and secondary amines (Scheme 5). At 100 oc, CBC reacts quantitatively the primary amine, while no reaction takes place with the secondary amino group. Due to this is a remarkably high double selectivity the product is obtained in high yield (> 95 %) whereby the primary amines are converted into BIs leaving the secondary amine available to introduce other functional groups. Preparation of polyrotaxanes In order to demonstrate the versatility of this new concept several novel polyrotaxanes were prepared [8c] (Scheme 6). Rotaxanes are complex molecules comprised of a linear thread molecule circumscribed by a macrocyclic component. There are no covalent bonds between the macrocycle and the thread so the macrocycle is free to move in a one-dimensional direction along the thread. Bulky stoppers at each end of the thread molecule prevent the macrocycle from falling off. The rotaxane is synthesized by reacting isophthalic acid chloride with xylylene diamine in the presence of a fumaramide thread, which act as a template for the forming tetraamide macrocycle[7a,b]. Different combinations of BI stopper groups attached to the central fumaramide template allowed the construction of a variety of rotaxane monomers, structures of which were confirmed by X-ray crystallographic studies 8c]. When these monomers were heated with diamines, polyureas were obtained in which every monomer unit was fitted with a macrocycle. This novel route to polyrotaxanes affords excellent control over the polymer properties through choice of the rotaxane structure or comonomer (diol, diamine) used. This polymer demonstrates the nearly unlimited number of options that this new methodology offers. Conclusion The preparation of a wide range of blocked isocyanates from the non-toxic carbonyl biscaprolactam ("Allinco") is demonstrated. All amines with one or more primary amine groups can be converted into BIs in a one-pot, one-step synthesis, without using toxic chemicals. The yield of this reaction is nearly quantitative within only a few hours of reaction time even when stoichiometric amounts of the reactants are used. Remarkably, BIs can also be prepared from CBC and alcohols. In this reaction one of the caprolactam rings open whereby the nitrogen atom of the -NCO group comes from caprolactam. The preparation of blocked isocyanates from alcohols has, to the best of the authors' knowledge, never been done before. In one example a BI was prepared starting from a perfluorine alcohol and CBC. A coating formulation of hydroxy polyesters, a blocked isocyanate and this novel perfluorine BI, was cured at 180 oc and showed a very low surface tension (10 mn/m; Teflon has 20 mn/m). The high selectivity of CBC was also found in the reaction with compounds containing primary and secondary amines. It is exclusively the primary amines that react, in a quantitative way, leaving the secondary amines unaffected. The secondary amines were used to introduce a new functionality, next to the blocked isocyanate group. The versatility of this method has been demonstrated by the preparation of various polyrotaxanes. Acknowledgements The authors would like to acknowledge DSM for allowing publication of these results, Prof. D. Leigh for his input on rotaxanes and Dr. M. Ming and L. van Ravenstein for their work on the fluorine blocked isocyanates. References [1] a) T. Misev, Powder Coatings, Chemistry and Technology, Wiley, New York, (1991); b) T. Brock, M. Groteklaes and P. Mischke, European Coatings Handbook, Vincentz Verlag, Hanover, (2000); c) D. A. Bate, The Science of Powder Coatings, vol 1, SITA Technology, London, (1990); d) D. Stoye and W. Freitag, Resins for Coatings, Hanser, New York, (1996) [2] a) I. Marumatsu, Y. Tanimoto, M. Kase and N. Okoshi, Progress in Organic Coatings, 22, (1993), p. 279; b) Z. Wicks, Progress in Organic coatings, 3, (1975), p. 73; c) Z. Wicks, Progress in Organic coatings, 9, (1981), p. 3; d) D. Wicks and Z. Wicks, Progress in Organic coatings, 36, (1999), p. 148; e) D. Wicks and Z. Wicks, Progress in Organic coatings, 41, (2001), p. 1 [3] R. Lomölder, F. Plogmann, P. Speier, J.C.T, 69, (1997), 868, p. 51 [4] P. Manresi, Comprehensive Polymer Sci., 5, (1998), p. 35 [5] R. Gras, Veba-Chemie, DE (1978) [6] a) J.A. Loontjens, B. Plum, WO 98/47940; b) J.A. Loontjens, B. Plum, WO 00/17169; c) J.A. Loontjens, L. Molhoek, B. Spoolder, EP A1; d) J.A. Loontjens, B. Plum, WO 01/66633A1; e) J.A. Loontjens, B. Plum, R. van Benthem, J. Rietberg, WO 01/66609A1; f) J.A. Loontjens, B. Plum, J. Rietberg, EP A1; g) J.A. Loontjens, B. Plum, A. van Geenen, W. Ming, WO 03/ A1; h) J.A. Loontjens, WO 03/ A1 ; i) J.A. Loontjens, B. Scholtens, R. Mülhaupt and S. Maier, Kunststoffe, 92, (2002), 12, p. 83 ; j) J.A. Loontjens and B. Plum, WO 00/17169, (2000) ; k) J.A. Loontjens, NL , (2002) ; l) J.A. Loontjens, J. of Polym. Sci., 41, (2003), p. 3198; m) L. van Ravenstein, W. Ming, R.D. van de Grampel, R. van der Linde, G. de With, J.A. Loontjens, P.C. Thüne, J.W. Niemantsverdriet, Polym. Mater. Sci. Eng., 88, (2003), p.

3 266; n) L. van Ravenstein, W. Ming, R.D. van de Grampel, R. van der Linde, G. de With, J.A. Loontjens, P.C. Thüne, J.W. Niemantsverdriet, Macromolecules, 37, (2004), p. 408; 0) S. Maier, R. Mülhaupt, J.A. Loontjens, B. Scholtens, Angewandte Chemie, 42 (2003) p. 5094; p) S. Maier, R. Mülhaupt, J.A. Loontjens, B. Scholtens, Macromolecules, 36, (2003), 13, p [7] [8] a) A. Johnston, D. Leigh, R. Protchard and M. Deegan, Angewandte Chemie, 34, no 11, 1209, b) D. Leigh, A. Murphy, J. Smart and A. Slawin, Angewandt Chemie, 36, no 7, 728, 1997; c) T. Kidd, J.A. Loontjens, D. Leigh and J. Wong, Angewandte Chemie, 42, 3379, (2003) Result at a glance - BIs have been prepared from functionalized amines and alcohols in high yields from the retrospective point of view by using carbonyl biscaprolactam (CBC, "Allinco") as the BI precursor. - CBC exhibits very unique selectivity in reactions with amines and alcohols. - All amines with one or more primary amine groups can be converted into BIs in a one-pot, one-step synthesis, without using toxic chemicals. The yield of this reaction is, even when stoichiometric amounts of the reactants are used, nearly quantitative within only a few hours of reaction time. - A diamine containing a primary and a secondary amine reacts exclusively in nearly quantitative yield with CBC via the primary amine. This results in BIs that still have a secondary amino group available for further elaborations. - In amine compounds that contain alcohol groups the reactions occur react solely with the amine in nearly quantitative yield to form caprolactam carbamates (BIs), without affecting the hydroxy group. - It is possible to covalently attach a variety of additional groups to these available sites, thus producing novel functional blocked isocyanates. - The versatility of this method has been demonstrated by the preparation of various polyrotaxanes. The authors: -> Ton Loontjens is principal scientist in macro-organic chemistry at DSM Research of Geleen, the Netherlands. Loontjens is co-author of more than 50 publications and more than 50 patents. -> Boudewijn Scholtens is business development manager for "Allinco" within DSM New Business Development. He is co-author of around 70 publications and 7 patents. -> Tim Kidd now holds a full-time position within DSM Research working on the chemistry and catalysis of UHMwPE. He is co-author of 4 publications. -> Bart Plum works as an associate scientist in the department of Performance Materials Chemistry and Technology (PM-CT) within DSM Research.

4 Figure 1: 1H NMR spectrum of the crude product from hexyl amine and CBC. Figure 2: Some of the BIs prepared in high yields by the CBC method (L = caprolactam). Scheme 1: Preparation of blocked isocyanate from an amine and a precursor blocked isocyanate group. Scheme 2: Possible reactions of carbonyl biscaprolactam (CBC) with amines.

5 Scheme 3: The reaction of an alcohol with CBC in the presence of a catalyst. Scheme 4: The preparation of a perfluorine alkyl blocked isocyanate. Scheme 5: The reaction of bishexamethylene triamine with CBC. Scheme 6: The preparation of a side chain rotaxane monomer provided with two blocked isocyanates as polymerizable groups (L= caprolactam) and a diphenylene unit as stopper group.

6 .