Radical Reactions on Polypropylene in the Solid State

Abstract The chemical modification initiated by radicals and the grafting of isotactic polypropylene (i-PP) in the solid state are reviewed. The attack of a radical onto the polypropylene mainchain is led by abstraction of a hydrogen atom to a tert -carbon radical. The tert -carbon radical of the polypropylene mainchain is unstable and overcomes with the so-called β-scission-reaction. During the β-scission-reaction, the mainchain is broken into two parts with a double bound on the one and a primary radical on the other chain end. The resulting degradation of the molecular weight is one limitation of the radical modification of i-PP in the melt and is the main reason for the development of radical reactions in the solid state. It is well known that the β-scission-reaction depends strongly on the temperature. Below 60 °C, the recombination reactions of the i-PP-radicals predominate so that a cross-linking results. Above 60 °C, the β-scission-reaction increases and the molecular weight of the i-PP decreases in a logarithmic scale. The first part of our paper deals with the mechanism of the grafting reactions of different monomers onto i-PP below the melting point of i-PP and the special processing conditions related to it. To start the grafting at low reaction temperatures, γ or electron-ray scattering or special peroxides are used. As monomers different methacrylates, acrylates, acrylonitrile, styrene, divinylbenzene, maleic anhydride, butadiene, dimethylbutadiene, vinyltrimethoxysilane and other silanes to PP-graft copolymers are investigated. The influence of the resonance stability ( Q -value of the Qe-schema) of the monomer radicals in the process will be discussed. From this knowledge, we will further discuss a special process for the long chain branching of the melting temperature of i-PP in an extruder. The properties of the resulting materials are an important part of our review.