New applications of catalytic chain transfer polymerization to waterborne binders for automotive paint systems

Abstract

Catalytic chain transfer polymerization (CCTP) is a conventional free radical polymerization technique that allows the preparation of macromonomers in a one step process. Acid functional macromonomers can be copolymerized with acrylate backbone monomers to form graft copolymers that, after neutralization with a base, are water dispersible. Low molecular weight oligomers from CCTP act as chain transfer agents themselves for methacrylate monomers via addition–fragmentation mechanism and lead to (semi) AB-block copolymers. Group transfer polymerization (GTP) is another polymerization technique to make AB-block copolymers but economically less attractive for functional comonomers since they interfere with the initiation mechanism. Graft and AB-block copolymer dispersions offer advantages in waterborne coatings compared to linear polymers of the same overall composition and molecular weight. Examples discussed in this paper are pigment dispersants and dispersion resins in which the backbone or A-segment of the copolymer has specific groups to anchor to the pigment surface and the side chains or B-segment give both charge and steric stabilization. AB-block copolymers with one block water soluble or dispersible also function as copolymerizable surfactants in an emulsion polymerization process and allow the synthesis of surfactant-free emulsions with low amounts of hydrophilic groups. The catalytic chain transfer agents (CCTAs) used in CCTP do have a high chain transfer activity for methacrylate monomers at very low concentrations so that low molecular weight oligomers (e.g. dimers) can be made. This chain transfer activity is lost in an emulsion polymerization process if the CCTA is (partly) water soluble. The paper will further demonstrate the use of dimers in the control of molecular weight in emulsion copolymerization.