Mechanistic Studies of Radical-mediated Polyolefin Modifications
radical-mediated , polymer modification
The free radical addition of saturated polymers and small molecules to unsaturated monomers is used to prepare functional derivatives under solvent-free, reactive extrusion conditions. Of particular interest are the dynamics and yields of conventional peroxide-initiated grafting of vinyltrialkoxysilanes to polyethylene, as well as the mechanisms through which bicumene initiates the process at high temperatures. Knowledge of these commercial processes is applied toward the development of new graft modification technology, including radical initiated polymer addition to alkynes, and a new variation of precipitation polymerization chemistry. The thermolysis of bicumene at temperatures ranging from 220°C to 270°C was used to initiate C-H bond addition from alkanes to vinylsilanes in a high-temperature analogue of conventional grafting practice. The initiation mechanism is shown to involve direct hydrogen atom abstraction by intermediate cumyl radicals, as well as autooxidation processes involving cumyl radicals and available oxygen. Conventional peroxide initiated graft modifications of polyethylene with vinylsilanes are examined from the standpoint of reaction dynamics and yields. The influence of peroxide loading and monomer concentration on these reaction variables can be described using a simple quasi-steady state kinetic analysis, while the unusual insensitivity of reaction yields to temperature requires further investigation. A new chemical modification of saturated polymers involving free radical addition to mono-substituted alkynes is presented and examined in terms of reaction yield, graft structure, and changes to molecular weight. Model compounds are used to characterize alkyne grafting products, and to probe the relationship between reagent properties, reaction yields, and product structures. The discovery of cross-linked particles in the products of polypropylene graft modifications with triallyltrimesate has led to a variation of precipitation polymerization wherein C-H bond addition to an allyl monomer contributes to molecular weight growth, thereby incorporating a significant amount of saturated hydrocarbon into the solid phase. The relationships between reaction conditions and solid-phase composition and morphology are discussed.