Design and Synthesis of Acrylic Dispersants for Non-aqueous Dispersion Polymerization
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Non-aqueous poly(acrylic) dispersions (NADs) used in automotive coating formulations are heterogeneous high-solids suspension of polymeric nano-size particles (< 200 nm) produced by radical polymerization in organic hydrocarbon medium. An important component of the system is the low molecular weight (MW) reactive polymeric dispersant (5000-6000 Da), which stabilizes the particles formed. A vinyl-terminated butyl methacrylate (BMA) macromonomer dispersant (type A) has been shown to be more effective at stabilizing the nanoparticles formed during the NAD synthesis than a BMA based grafted dispersant with vinyl groups attached at random positions along the backbone. However, incorporating 10 mol% 2-hydroxyethyl methacrylate (HEMA) as a functional comonomer in the macromonomer dispersant, a necessary addition for end-use properties, caused the final dispersion to lose stability. A series of experiments was conducted to show that the particle size distribution, viscosity and stability of the final dispersion can be correlated to the thermodynamic solubility distances among the dispersant, solvent and polymer particle compositions. The investigation led to the use of ethylhexyl methacrylate (EHMA) as the main component of the macromonomer, with a stable NAD system achieved even with 28 mol% HEMA added to the macromonomer composition. The macromonomer, although having controlled double-bond placement through catalytic chain transfer chemistry, still has a molecular weight dispersity of close to two. Thus, a low dispersity P(BMA) macromonomer was synthesized by sequential atom transfer radical polymerization (ATRP) and catalytic chain transfer polymerization (CCTP) to achieve a similar number-average MW and terminal double bond functionality. Using both methyl acrylate homopolymerization and methyl methacrylate/methyl acrylate (70/30 w/w) copolymerization to produce NADs, the lower dispersity P(BMA) macromonomer provides better stabilization per dispersant chain, as characterized by smaller average particle size and higher weight fraction of incorporated dispersant. A model was developed in the Predici® software package (Version 11) to improve the understanding of the sequential ATRP-CCTP process.