Preparation and crystalline structure of random copolymers of glycine and β-alanine

Copolymers of glycine and β-alanine within a range of amount-of-substance compositions from 3:1 to 1:9 were prepared by polycondensation of mixtures of the respective pentachlorophenyl ester hydrobromides. Number-average molecular weights between 2 500 and 9 000 were obtained, the lower values corresponding to those copolymers with a higher content in glycine. Sequence distributions were evaluated by means of 50,3 MHz ¹³C NMR spectroscopy and the crystalline structure was examined by wide-angle X-ray diffraction. Random copolymers having similar contents in glycine and β-alanine were found to crystallize in a bidimensional hexagonal lattice (a = 4,79 Å) with chains packed in a similar manner as they do in the crystalline structure of polyglycine II. On the contrary, a heterogeneous product consisting of homopolymer and random copolymer fractions results from mixtures which are enriched in one of the two amino acids. The random copolymer poly(glycine-ran-β-alanine) adopts a packing scheme similar to that found for the helical form of the alternating copolymer nylon 2/3. Chains are hexagonally arranged and interlinked by a three-dimensional network of hydrogen bonds as described for the well known model of polyglycine II, although in the present case no order along the chain axis should be expected.