Environmentally-Friendly Acrylates-Based Polymer Latices - Advanced Materials for Agriculture, Food and Environmental Safety - page 158

water-soluble thermal initiators are sodium, potassium and ammonium
salt of persulphates, which are generally employed between 50-90

C. h e
decomposition of persulphate is accelerated at acidic pH. h e initiator
ei ciency is reduced at pH < 3. h e use of a redox initiator (combination
of oxidizing and reducing agents), e.g., tert-butyl hydroperoxide-sodium
formaldehyde sulfoxylate; persulphate-bisulphite is also useful at low
temperature for preparing high molar mass polymer with a low level of
branching [82]. Monomer conversion during the polymerization pro-
vides knowledge about the polymerization rate and kinetics of the polym-
erization process, which can be measured gravimetrically and by gas
chromatography.
Lee et al. [83, 84] studied the kinetics of emulsion polymerization of
BA and MMA. h e results showed an increase in polymerization rate and
number of polymer particles with an increase in initiator concentration,
but a decrease in molecular weight and particle size. Schneider et al. [85]
studied the ef ect of initiators in high solid content BA/MMA/AA emul-
sions. h ey found an increase in the reaction rate with the oil-soluble ini-
tiator and a better stability of nucleated particles with the water-soluble
initiator. Capek et al. [86] also studied the ef ect of both water-soluble and
oil-soluble initiators on the emulsion polymerization of BA and found that
the polymerization rate was faster with the ammonium persulphate (APS)
initiator.
h e emulsion copolymerization of EHMA and MMA was carried out
with the use of a bifunctional initiator 1,4-butylene glycol di(2-bromoiso-
butyrate) by Eslami and Zhu [87]. h e ef ects of initiator concentration and
temperature proi le on the polymerization kinetics and latex stability were
examined. Both EHMA homopolymerization and successive copolymeriza-
tion with MMA proceeded in a living manner and gave good control over
the polymer molecular weights. h e polymer molecular weights increased
linearly with the monomer conversion with polydispersities lower than 1.2.
A low-temperature prepolymerization step was found to be helpful in sta-
bilizing the latex systems, whereas further polymerization at an elevated
temperature ensured high conversion rates. h e EHMA polymers were
ef ective as macroinitiators for initiating the block polymerization of MMA.
Triblock poly(methyl methacrylate-2-ethylhexyl methacrylate-methyl
methacrylate) samples with various block lengths were synthesized. h e
MMA and EHMA reactivity ratios determined by a nonlinear least-square
method were ~0.903 and 0.930, respectively, at 70

°

C.
Yo u et al. [88] studied the kinetics of emulsion copolymerization
of EMA/LMA. h ey found that the overall rate of polymerization was
only slightly dependent on monomer and stabilizer concentrations and

°