Process for producing 2-(C.sub.1 to C.sub.3 alkyl)-.DELTA..sup.2 -oxazoline copolymers

In a process for preparing a linear or slightly branched 2-(C.sub.1 to C.sub.3 alkyl)-.DELTA..sup.2 -oxazoline copolymer from a mixture of monomers more than 50 mole % of which is 2-(isopropyl- and/or n-propyl)-.DELTA..sup.2 -oxazoline and less than 50 mole % of which is 2-(methyl- and/or ethyl)-.DELTA..sup.2 -oxazoline, the copolymer having a viscosity number of 35 to 70 ml/g which is easily reproducible from batch to batch,

• comprising polymerizing the monomers in the presence of a cation-active catalyst whose anion is slightly nucleophilic,
• the improvement wherein,
• (A) the monomers used as reagents contain less than 200 mg of water per liter of reagent and less than 0.003% by weight of basic nitrogen, the monomers being obtained by the following purification steps:
• (i) fractional distillation thereof, optionally carried out after preaddition to the monomer reagent of an inert, organic solvent which acts as an entraining agent for impurities and which has a boiling point at least 15.degree. C. lower at normal pressure than the monomer reagent, and optionally carried out at reduced pressure, which method can be applied to all monomers, or
• addition thereto of an inert, organic solvent which acts as an entraining agent for impurities and which has a boiling point at least 15.degree. C. lower at normal pressure than the monomer reagent;
• followed by distillation to extensively remove this solvent;
• subsequently followed by addition to the resultant sump product of from 0.01 to 2 parts by weight, referred to 100 parts by volume of the monomer reagent, of an aromatic isocyanate boiling at above 100.degree. C./7 mbar;
• and ensuing fractional distillation at reduced pressure, which method can only be applied to 2-isopropyl-.DELTA..sup.2 -oxazoline monomer reagent;
• (ii) subsequent contact with silica gel or neutral aluminum oxide, and
• (iii) optional drying above a molecular sieve having a pore width of 0.3 to 0.4 nm;
• (B) the polymerization is discontinuous and is carried out in a stirring vessel provided with a reflux condenser adequately dimensioned to remove the heat of polymerization by evaporative cooling;
• (C) the polymerization temperature is from 120 to 175.degree. C., and the temperature following the conversion of 90 mole-% of the monomers does not exceed 165.degree. C., and
• (D) the polymerization is stopped after a conversion of 96 to 99.5 mole-% of monomers has been reached.