Excellent performance of aromatic polyguanamines induced by multiple hydrogen bondable tetraazacalix[2]arene[2]-triazine ring in their main chain

Abstract

A series of poly(guanamine) (c-PG)s containing tetraazacalix[2]arene[2]-triazine (mPDA₂CyC₂) were successfully prepared by solution polycondensation of mPDA₂CyC₂ with various aromatic diamines in an aprotic organic solvent with a lithium chloride additive (5 wt%) at 150 °C for 6 hours. The number-average molecular weights (M _n)s of these c-PG polymers reached up to 31 500, with a relatively broad molecular weight distribution (M _w/M _n) of 5.3. They showed good solubility in aprotic organic solvents, such as N-methylpyrrolidone and N,N-dimethylacetamide at a concentration of 2 mg mL^-1. The glass transition temperatures (T _g) of the c-PG polymers were in the range 359 °C-392 °C, approximately 160 °C higher than those of counterpart polymers (i.e., with no aza-calixarene-based PG (l-PG)). The coefficients of thermal expansion (CTEs) of the c-PG polymers were 29.7-48.1 ppm K^-1 (at 100 °C-150 °C), much lower than those of l-PG samples, i.e., 59.1-85.1 ppm K^-1. Transparent and almost colorless c-PG films were successfully prepared by a solution casting method, showing maximum tensile strength (σ _S), modulus (E _γ), and elongation at break (E _b) values of 151 MPa, 6.3 GPa, and 4.4%, respectively, for the c-PG polymer from mPDA₂CyC₂ and 4,4'-oxydianiline monomers. The corresponding l-PG film exhibited σ _S, E _γ, and E _b values of just 76 MPa, 5.4 GPa, and 1.6%, respectively. These outstanding thermal and mechanical properties of the c-PG polymers can be attributed to their multiple hydrogen bonding interaction between mPDA₂CyC₂ residues in the polymer backbone. This interaction was identified by infrared spectroscopy measurements at the broad absorption band around 3000-3400 cm^-1.