Effect of the pH on the RAFT Polymerization of Acrylic Acid in Water. Application to the Synthesis of Poly(acrylic acid)-Stabilized Polystyrene Particles by RAFT Emulsion Polymerization

Abstract

International audienceThe reversible addition-fragmentation chain transfer (RAFT) polymerization of acrylic acid (AA) in water was studied in detail at different pHs using 4-cyano-4-thiothiopropylsulfanyl pentanoic add (CTPPA) as a control agent and 4,4'-azobis(4-cyanopentanoic acid) (ACPA) as an initiator. Well-defined hydrophilic macromolecular RAFT agents (PAA-CTPPA) were obtained and further used directly in water for the polymerization of styrene. The corresponding polymerization-induced self-assembly (PISA) process was evaluated at different pHs and it was shown that working in acidic conditions (pH = 2.5) led to well-defined amphiphilic block copolymer particles (D < 1.4) of small size (below 50 nm). When the pH increased, the control over the growth of the polystyrene (PS) block was gradually lost. Chain extension experiments of PAA-CTPPA with N-acryloylmorpholine (NAM), a hydrosoluble and non-pH sensitive monomer, performed at different pHs showed that the very first addition-fragmentation steps that occurred in water were impeded when PAA was ionized leading to partial consumption of PAA-CTPPA and thus to PS molar masses higher than expected. Varying the PAA-CTPPA concentration at pH = 2.5 led in all cases to stable particles composed of well-defined block copolymers with PS segments of different molar masses

Acceleration and Selective Monomer Addition during Aqueous RAFT Copolymerization of Ionic Monomers at 25 °C

“…In contrast, the reactions in 1.0 M NaCl were faster than in water, suggesting that ionic screening [ 30 ] weakened the repulsion and thus promoted reaction. [ 31 ] …”

Polymerization induced self-assembly: tuning of morphology using ionic strength and pH

“…Compared to traditional ways of preparing such nano-objects via a gradual decrease in concentration of a common solvent by addition of a selective solvent, [10][11][12][13][14] PISA presents various advantages such as ease of implementation, not time-consuming, and perhaps most importantly the ability to access high solids contents (up to 50%). PISA has been implemented as both emulsion polymerization [15][16][17][18][19][20][21][22][23] and dispersion polymerization, [24][25][26][27][28][29] most commonly the latter in recent years. Various parameters influence the morphology of the nanoparticles formed during PISA: (i) the relative lengths of the hydrophilic and the hydrophobic blocks; 24,26,[29][30][31] (ii) the interactions between each block and the solvent; 26,32 (iii) the monomer concentration (in dispersion polymerization), 24,26,29,30 and (iv) in the case of aqueous formulations, the pH value and salt concentration.…”

Kinetic study of RAFT homopolymerization and copolymerization in emulsion

“…RAFT agents are typically certain dithio compounds, which have been found to be extremely versatile and work with a wide range of monomers [7][8][9]. Up to now, many published researches devoted to CLRP are carried out in many media such as bulk, solution, emulsion or miniemulsion [10][11][12]. During the past few years, RAFT polymerization in these media has been well documented, and a variety of polymers with precisely controlled molecular weights and narrow molecular weight distributions have been prepared [13][14][15].…”