Biobased Polyamides with Polyethylene-like Properties through Catalyst-free Polycondensation

The growing demand for sustainable alternatives to petroleum-based plastics has driven the development of biobased long-chain polycondensates with polyethylene-like properties. In this study, we synthesized a series of fully biobased polyamides PA 36,X (X = 6, 8, 10, or 14) via catalyst-free melt polycondensation using plant oil-derived dimer fatty diamine (Priamine 1074) and renewable dicarboxylic acids. The unique structure of Priamine 1074─featuring a flexible long methylene backbone and two alkyl branches─imparts enhanced ductility and toughness to the resulting polyamides, which exhibit mechanical properties spanning commercial low-density polyethylene and linear low-density polyethylene, including high elongation at break (>645%) and tensile strength (23–26 MPa). Systematic investigation revealed that increasing the dicarboxylic acid methylene length reduces melting and crystallization temperatures due to weakened hydrogen bonding, while simultaneously enhancing glass-transition temperature and crystallinity via improved chain mobility and conformational entropy. The polyamides also demonstrate excellent thermal stability (T_d,5% > 448 °C), low water absorption (<1%), and high hydrophobicity (water contact angle > 90°), ensuring dimensional stability and moisture resistance. Moreover, their tunable thermal and mechanical properties, combined with excellent melt processability, enable customizable performance for diverse applications.