Facile Formation of Long Chain Branched Polypropylene via Reactive Processing

Polypropylene (PP) is widely used due to its excellent properties, but its linear structure and corresponding low melt strength greatly limit its application. We herein investigate the preparation of long chain branched polypropylene (LCB-PP) via reactive processing with the employment of a novel monomer, 4-vinylguaiacol (4-VG). The results show that the introduction of 4-VG effectively hinders the degradation of PP during the reactive processing and promotes the formation of long chain branched structures, which was confirmed by high-temperature gel permeation chromatography. The branching degree of prepared LCB-PPs could be easily controlled by adjusting the molar ratio of 4-VG to the dicumyl peroxide (DCP) initiator. It has been demonstrated by the Mark–Houwink plots that LCB-PP₂ exhibits the highest branching degree when the molar ratio of 4-VG to DCP is approximately 2:1. Besides, LCB-PPs with a higher branching degree also possess a higher melt strength and higher expansion ratio after being foamed. Notably, LCB-PP₂ shows remarkably high melt strength and expansion ratio of up to 1.7 × 10⁴ Pa s and 30.7, which are 10 and 28 times those of PP, respectively. Also, pronounced tensile strain hardening behavior is observed for these LCB-PPs. It is believed that the unique combination of conjugated double bonds and the phenolic hydroxyl group in 4-VG plays an important role in hindering degradation and introducing long branches during the reactive processing of PP. This work provides a new perspective for the preparation of high-performance and functionalized PP materials.