Abstract: A compound or a combination of compounds for crystallization of poly-hydroxy-alkanoate (PHA), poly-lactic acid (PLA) or combinations thereof, wherein each of the compounds has a core motif with two oxalamide motifs, flanked by two arms, wherein the core motif has the formula: R—NH—C(O)—C(O)—NH—(CH2)n—NH—C(O)—C(O)—NH—R?, wherein n ranges from 1 to 10 and the arms R and R? are each independently chosen from: (i) H; (ii) an alkyl group with a total number of carbon atoms between 1 and 20; (iii) an aromatic ring; or (iv) an ester group, for example —X-Ester-Y, or —X-Ester-X-Ester-Y, wherein X is a saturated aliphatic hydrocarbon group having 1 to 20 carbon atoms, Y is chosen from H, an alkyl group with a total number of carbon atoms ranging from 1 and 20 or an aromatic ring. Also described are processes for the crystallization of poly-hydroxy-alkanoate, poly-lactic acid, or combinations thereof.