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Preparation of Polymerizable Isocyanate Esters Disclosure Number: IPCOM000028164D
Publication Date: 2004-Apr-29

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Preparation of Polymerizable Isocyanate Esters Isocyanates are known precursors for urethanes, a class of chemicals widely used in paints, foams, adhesives, elastomers, and many other materials. Isocyanates previously have been made from processes that require environmentally and toxicologically unfavorable starting materials, such as phosgene or substituted oxazolines. Isocyanates have also been made from urethanes via thermal elimination processes where elevated temperatures, e.g., above 300°C, are required or from processes that employ toxic materials, such as certain catalysts. More recently, isocyanates have been made from corresponding urethanes or carbamates via less vigorous thermal elimination reactions. For example, U.S. Patent 5,990,345 (Lohmann et al.) describes a process for preparing unsaturated ester isocyanates from the reaction of the corresponding carbamates and trialkyl silyl halide. Subsequent thermal elimination of alkoxy silane leads to the formation of the isocyanate. The silylation step alone takes about 6 hours at 50°C, followed by 8 hours at 100°C for the thermal elimination of alkyloxy (aryloxy) trimethylsilane. This reaction seems to be feasible only when bulky alkyl or aryl carbamates are used. Elimination of alkoxy silane from carbamates with smaller alkyl groups, such as methyl and ethyl, is extremely difficult to carry out. Alper et al. (in U.S. Patent 5,457,229 and in Chem. Communications, [1998], 2575-2576) has described processes for preparing mono-, di-, and tri-isocyanates (both aromatic and aliphatic) from the reaction of alkyl (aryl) urethanes and BCl3, BBr3, BI3 or chlorocatecholborane. Both publications provide details on how to make mono-, di-, and tri- aromatic and alkyl isocyanates, however they are silent regarding the tolerance of their procedures towards the presence of functional groups such as esters, amides, olefins, α,β-unsaturation, etc., that might be present on the starting urethane. We report here two different process routes via which alkyl and aryl urethanes (i.e., carbamates) can be smoothly transformed into isocyanates even if functionalities such as unsaturated esters or acyl groups are present. First, we have shown that isocyanates containing ethylenically unsaturated groups can be made, isolated and purified by extending Alper's method to include α,β-unsaturated esters, such as (meth)acryloxyalkyl isocyanates. Second, we have discovered improved methods for the preparation of isocyanates from urethanes under mild conditions by using di- and tri-chloromethylsilanes (instead of trialkyl silyl halides) as the thermal elimination agents. These methods, unlike the processes disclosed in U.S. Patent 5,990,345, are flexible and work very efficiently with a variety of alkyl and aryl carbamates, including carbamates with ethylenically unsaturated groups, such as (meth)acryloxyalkyl carbamates useful for the production of (meth)acryloxyalkyl isocyanates. Example 1 – Synthesis of 2-Methacryloxyethyl Isocyanate Combined 179 grams (0.956 moles) of 2-methacryloxyethyl carbamic acid methyl ester, 135.48 grams (1.334 moles) of triethylamine, 400 grams (3.099 moles) of dichlorodimethylsilane, and 0.2 grams of BHT with 1023 grams of acetonitrile in a 2-liter flask fitted with a mechanical stirrer and reflux condenser under an atmosphere of nitrogen. The combined mixture was heated to 70°C for 8 hours, followed by the addition of an additional 60 grams (0.465 moles) of dichlorodimethylsilane and with continued heating overnight. The next day GC analysis indicated less than 1.7% unreacted carbamic acid methyl ester remained. The mixture was cooled to room temperature and the resulting solids were removed by filtration and washed with acetonitrile. All of the filtrates were combined and the acetonitrile and most of the excess silane were distilled off at atmospheric pressure. Distillation was continued until the head temperature was 85°C. Distillation under vacuum at 0.05 mmHg afforded product as the material boiling at 53-55°C. The yield was 112 grams or 75.5%. The material was identified by IR (neat), 1H NMR (400 MHz, CDCl3) and 13C NMR as 2-methacryloxyethyl isocyanate.