Photo-initiation of free-radical polymerization by dimethyl-(2,2′-bipyridyl)platinum(II)+ tetrafluoroethylene

Abstract

The system dimethyl-(2,2′-bipyridyl)platinum(II)(DMBP)+ C₂F₄ is a photoinitiator of free-radical polymerization at λ= 365 nm. At high [C₂F₄] each initiating radical incorporates one atom of platinum and one initiating radical is formed for each molecule of DMBP decomposed. Quantum yields for initiation and decomposition are low. In the absence of C₂F₄, initiation also occurs, but at a considerably reduced rate. In the dark at 25°C, little polymerization is observed. Irradiation of DMBP + C₂F₄ in inert solvents leads to the formation of Me(CF₂CF₂Me)(CF₂CF₂)(2,2′-bipy)Pt^IV(I); this product is also formed (more slowly) in the thermal reaction at 25°C.

Irradiation of DMBP + C₂F₄ is considered to yield an adduct which, in the presence of a monomer M gives rise initially to propagating radicals Me₂(2,2′-bipyridyl)Pt^IIICF₂CF₂M; subsequent disproportionation between Pt^III end-groups produces Pt^II and Pt^IV derivatives. When monomer is absent (I) is formed by an intramolecular reaction of the DMBP + C₂F₄ adduct, probably through the radical intermediate Me₂(2,2′-bipyridyl)PtCF₂CF₂. Participation of radical intermediates in the dark reaction between DMBP and C₂F₄ leading to (I) is improbable. The slow initiation observed in the absence of C₂F₄ may arise from scission of methyl from photoexcited species.

Kinetic mechanisms compatible with the experimental observations are proposed.

When polymers obtained by photoinitiation by DMBP + C₂F₄ are irradiated in the presence of a second monomer block copolymers are formed; possible reaction mechanisms are discussed.