Sequential Reactive Processing for the Synthesis of Branched Polypropylene and Propylene-Ethylene Copolymer

Polypropylene (PP) and random propylene-ethylene copolymer (PEC) with long-chain branches (LCB) have been synthesized from linear polymers by sequential melt reactive processing. The process starts with the grafting of low concentrations of maleic anhydride using an organic peroxide as a radical initiator. Then, halfway through the processing time, the chain-linking agent, m-xylylenediamine (XDA), is added to the reactive medium to generate branched molecular structures. The occurrence of grafting and chain-linking reactions was confirmed by infrared spectroscopy, size exclusion chromatography, and dynamic and transient extensional rheology. Branched polymers with up to similar to 1 branch per 1000 monomer units were achieved without largely altering the average molecular weights of the original materials and their tensile mechanical properties. The amount of LCB obtained in PEC practically doubles that in the branched PP. Properties such as maximum tensile stress, elastic modulus, and tensile stress at yield were found to be primarily a function of the molecular weight and crystallinity level of the polymers, and not of their degree of LCB. Altogether, the results demonstrate that grafting low concentrations of maleic anhydride onto the polymer with the addition of the chain-linking agent at half processing time is an efficient and practical method to produce polymers with improved melt strength.