Phosphonate-Functionalized Polypropylenes: Single-Component Flame Retardants

Phosphonate-modified polypropylenes (PPR; R = PPr, P(O)(OiPr)2; PPh, P(O)(OPh)2) have been prepared via a two-step synthesis and display enhanced thermal stability, offering potential as compatible single-component flame-retardant additives for PP systems. PP typically displays high flammability and poor fire resistance, which consequently restricts applications. Solution-phase copolymerization of propylene and 11-bromo-1-undecene using commercial rac-ethylenebis(indenyl)zirconium dichloride afforded poly(propylene)-co-(11-bromo-1-undecene) with tunable comonomer incorporation levels up to 8.1 mol %. Solvent-free, quantitative, postmodification of pendent bromine with phosphite esters yielded alkyl (PPPPr) and aryl (PPPPh) phosphonate-modified PPs. Increased thermal stability (higher T10%, T50%, Tmax, and %residue at 700 °C) in a synthetic air atmosphere was determined by thermogravimetric analysis with respect to PP. Microscale combustion calorimetry of both PPPPr and PPPPh indicates lower values of heat release capacity, peak heat release rate, and total heat release compared to that of PP, indicating enhanced flame retardancy. Both PPPPr and PPPPh can be easily blended with PP at PP:PPR weight ratios of 80:20, 90:10, 95:5, and 99:1, respectively. The polymer blends demonstrated higher thermal stability than the homopolymer, and so such PP-compatible PPR offers an alternate strategy for improving the flame-retardant properties of PP-based materials, which circumvents additive leaching while limiting disruption to the mechanical strength of the base PP material.