Microemulsion Polymerization of Butyl Acrylate. Effect of Radical Scavenger and Formation of Primary Radicals

Abstract

:Microemulsion polymerizations of butyl acrylate initiated by dibenzoyl peroxide in the presence of a radical scavenger (RS)(stable radicals: an oil soluble RS: 4-stearoyloxy-2,2,6,6-tetramethyl piperidine-1-oxyl (STPO) and water soluble RS: 4-hydroxy-2,2,6,6-tetramethyl piperidine-1-oxyl (HTPO)}were investigated. STPO and HTPO decreased the rate of polymerization and the decrease was more pronounced with STPO. Inorganic salts and organic additives decreased the rate of polymerization. The shift of the maximum rate of polymerization to lower conversion by increasing temperature and addition of several additives was attributed to increase of particle number and decrease of monomer concentration at the reaction loci. The decrease of the particle size in the presence of RS is attributed to retardation of particle growth. A close packed structure at the droplet surface promoted by current additives acts as a barrier for entering radicals. The larger overall activation energy in runs with STPO is discussed in terms of a depressed entry ofradicals into microdroplets.KEY WORDS Microemulsion / Butyl Acrylate/ Polymerization/ Tetramethyl Piperidine-1-oxyl Radical Scavenger/ Inorganic Salts/ Rate of Polymerization/ Polymer Particles/ In the case of microemulsion polymerization initiated by the oil-soluble initiator, there are two main approaches for the production of primary radicals : 1 -3 1) in the monomer swollen micelles or polymer particles radicals desorb to the aqueous phase and 2) in the aqueous phase radicals are generated from the fraction of the oilsoluble initiator dissolved in water. -6 A partly watersoluble initiator such as a,a'-azobis(isobutyronitrile) (AIBN) forms primary radicals in both the aqueous phase and in the monomer-swollen micelles. In the case of highly hydrophobic initiator the formation of radicals from the fraction of initiator dissolved in the aqueous phase is negligible, as well as exit of hydrophobic primary radicals into the aqueous phase. Microemulsion and miniemulsion polymerizations initiated by hydrophobic initiators such as dibenzoyl peroxide (DBP) or lauroyl peroxide (LPO) are reported to be relatively fast. 1 · 2 • 7 The very low water solubility of hydrophobic initiator (DBP, LPO, etc.) and radical fragments propose a very low rate of polymerization (ca. by two orders in magnitude lower than that for peroxodisulfate initiated polymerization). The strong cage effect within the microdroplets or latex particles depresses the formation of radicals in the monomer phase. However, the rate of DBP-initiated polymerization was ca. 5 times smaller than the water-soluble peroxodisulfate initiated polymerization (polymerizations carried out under the same reaction conditions). Thus, the polymerization behavior is not properly described by two reaction approaches : 1) water-phase initiation and 2) desorption of primary (monomeric) radicals formed in the microdroplets in the aqueous phase.The non-linearity between the molecular weights of the polystyrene (much below 10 5 ) and laser r...

Nonconventional Emulsion Polymerization of Methyl Methacrylate. Effect of Cu(II)/Histidine Complex Catalyst and Different Peroxo-Salts

“…This small particle size of PMMA may be attributed to the retardation in particle growth. Similar results of smaller particle size and retardation in particle growth were reported by Capek et al 25 in the microemulsion polymerization of butyl acrylate in presence of radical scavenger. Thus, complex catalyzed emulsifier-free emulsion polymerization can hopefully be utilized to prepare nanoscale latex particles.…”

Cobalt(III)‐mediated microemulsion polymerization of acrylonitrile: Kinetics and particle morphology

“…From the Arrhenius plot of log R P versus 1/ T (Fig. 4), the overall activation energy, E a , was found to be 39.26 kJ/mol and, taking E P = 16.2 kJ/mol, E t = 15.5 kJ/mol reported for AN,28 the activation energy for initiator decomposition was found to be 61.62 kJ/mol by using the formula and the number of micelles, 0.122 × 10 18 against the theoretical values27 of 10 18 , characteristics of emulsifier‐free emulsion polymerization. Figure 5 shows the relationship between R p with the reciprocal of DP of PAN at different initiator concentrations at 50°C.…”

“…From the experimental evidence, it is observed that particle size increases with increase in concentration of complex at decreasing temperature (Table II). Hence it may be suggested that the polymerization was stabilized by the presence of the complex with reduction of the particle size into the nano range 27…”