The method of claim 6, wherein the polyglutamic and polyaspartic acid esters or amides have side functions having formulae (CH2)n COO--CHR1 COOR (I), or (CH2)n COOCR1 R2 --O--COR (II) or (CH2)n CO(NH--CHX--CO)m NHCH(COOH)--(CH2)p COOH (III), wherein R is an alkyl or aryl substituent; R1 and R2 are H or lower alkyls, or R and R1 are connected together by a substit