PROCESS FOR THE PREPARATION OF POLYISOCYANATES

摘要

Mo3360 LeA 26,273 A PROCESS FOR THE PREPARATION OF POLYISOCYANATES The present invention relates to a process for the preparation of a polyisocyanate comprising (a) thermally decomposing a solution of an N-substituted carbamic acid ester corresponding to said polyisocyanate at temperatures above 150.degree.C in a solvent or solvent mixture serving as a decomposition medium and with provision for continually removing by distillation the alcohol produced by the thermal decomposition of said carbamic acid ester, wherein said solvent or solvent mixture (i) is capable of dissolving the carbamic acid ester, (ii) is stable at the decomposition temperature and chemically inert towards the carbamic acid esters and the polyisocyanate formed during the decomposition reaction, and (iii) has at least one miscibility gap with the extracting agent used according to extraction step (b); (b) extracting the polyisocyanate from the decomposition medium with an extracting agent that is at least partly immiscible with the decomposition medium and is a solvent for the polyisocyanate, and optionally distilling the resultant solution of the polyisocyanate in the extracting agent; and (c) recycling the portion of the decomposition medium remaining after the polyisocyanate is extracted. Le A 26 273