Tandem Polymerization of Comb-like <i>i</i>PP-<i>g</i>-PE Copolymers: Synthesis, Characterization, and Compatibilization of Isotactic Polypropylene and Polyethylene Blends

Herein, a tandem catalyst system was successfully developed to prepare comb-like copolymers consisting of an isotactic polypropylene (iPP) backbone and linear polyethylene (PE) branches (iPP-g-PE). This system employs a zirconium-based oligomerization catalyst, bis(β-enaminoketonato)zirconium dibenzyl complex (Cat. 1), and a hafnium-based copolymerization catalyst, pyridylamidohafnium dimethyl complex (Cat. 2), for tandem copolymerization. By simply controlling the reaction durations of oligomerization (Cat. 1/dried methylalumoxane (dMAO)) and copolymerization (Cat. 2/[Ph3C][B(C6F5)4]), the ratio between the iPP backbone and the PE branches can be adjusted. Consequently, a series of iPP-g-PE copolymers with varying incorporations, ranging from 3.1 to 9.0 mol % (equivalent to PE content from 23 to 54 wt %), were synthesized with excellent yields, where both the iPP backbone and the PE branches were crystallizable. Systematic investigation into the tandem copolymerization process, considering the effects of macromonomer concentration and reaction time on the process, was conducted to further elucidate the microstructure and chemical composition of the resultant products. The iPP-g-PE copolymers modified the morphology of PE/iPP blends, inducing phase inversion from droplet-in-matrix to cocontinuous morphology. Notably, the addition of 5 wt % of the iPP46-g-PE54 copolymer improved the strain at break of PE/iPP to over 700% without significantly compromising yield strength. This improvement is attributed to entangled loops trapped by crystallization, which markedly enhance the interfacial strength of PE/iPP blends. This simple and robust tandem copolymerization system is expected to facilitate the thermomechanical recycling of PE/iPP materials.