Mixtures of monomers corresponding to general formula I, in which Z is a direct bond and R1 to R4 are hydrogen atoms (the so-called 4,5-dihydrooxazoles) and which contain monomers of which the substituent R7 is derived from one of the fatty acids mentioned above in quantities of 25 to 95% by weight, preferably 50 to 90% by weight and, more preferably, more than 70% by weight and monomers of which