Bulk polymerization

Abstract: Radical polymerization with reversible addition−fragmentation chain transfer (RAFT polymerization) can be used to synthesize a wide range of polymers of controlled architecture and narrow molecular weight distribution. The polymerizations use addition−fragmentation chain transfer agents (RAFT agents) that possess high transfer coefficients in free radical polymerization and confer living character on the polymerization. This paper explores the effect of the substituents R of dithiobenzoate RAFT agents [SC(Ph)S−R] on the outcome of polymerizations of styrene, methyl methacrylate (MMA) and butyl (BA) or methyl acrylate (MA). In MMA polymerization at 60 °C, effectiveness depends strongly on R decreasing in the order where R is: −C(Alkyl)2CN ∼ −C(Me)2Ar > −C(Me)2C(O)O(alkyl) > −C(Me)2C(O)NH(alkyl) > −C(Me)2CH2C(Me)3 ≥ −C(Me)HPh > −C(Me)3 ∼ −CH2Ph. Of these, only the compounds with R = −C(Me)2Ph and −C(Me)2CN provided polymers with substantially narrowed polydispersities in batch polymerization and gave molec...

Abstract: A method is developed to enable emulsion polymerization to be performed under RAFT control to give living character without the problems that often affect such systems: formation of an oily layer, loss of colloidal stability, or loss of molecular weight control. Trithiocarbonate RAFT agents are used to form short stabilizing blocks from a water-soluble monomer, from which diblocks can be created by the subsequent polymerization of a hydrophobic monomer. These diblocks are designed to self-assemble to form micelles. Polymerization is initially performed under conditions that avoid the presence of monomer droplets during the particle formation stage and until the hydrophobic ends of the diblocks have become sufficiently long to prevent them from desorbing from the newly formed particles. Polymerization is then continued at any desired feed rate and composition of monomer. The polymer forming in the reaction remains under RAFT control throughout the polymerization; molecular weight polydispersities are generally low. The number of RAFT-ended chains within a particle is much larger than the aggregation number at which the original micelles would have self-assembled, implying that in the early stages of the polymerization, there is aggregation of the micelles and/or migration of the diblocks. The latexes resulting from this approach are stabilized by anchored blocks of the hydrophilic monomer, e.g., acrylic acid, with no labile surfactant present. Sequential polymerization of two hydrophobic monomers gives completely novel core-shell particles where most chains extend from the core of the particles through the shell layer to the surface.

Abstract: Ring-opening polymerization of various lactones and lactides initiated by lanthanum isopropoxide has been investigated. Analysis of molecular weights and molecular-weight distributions of the resulting polymers shows that the ring-opening process is a controlled reaction, which is initiated by a variable number of isopropoxy groups. This number depends on both the monomer and the [monomer]/[initiator] ratio. Kinetic studies indicate that all the polymerizations are equilibrated, and the corresponding thermodynamic parameters and the equilibrium monomer concentrations have been calculated. Comprehensive kinetics carried out for e-caprolactone abd δ-valerolactone polymerizations allows the determination of kinetic order relative to both monomer and initiator concentrations, along with that of the apparent rate constants of polymerization. It is also demonstrated that propagation occurs on aggregated (δ-valerolactone) or unaggregated (e-caprolactone) active polymer chains and that the ring-opening process proceeds via [O-acyl] bond cleavages.