posted on 2020-04-01, 14:06authored byCuong
Minh Quoc Le, Marc Schmutz, Abraham Chemtob
Very few step polymer latexes have
been produced by aqueous emulsion
polymerization and never with a clear picture of the operating mechanism.
Therefore, finding new means to expand this widely used process industrially
beyond chain radical polymerization is highly desirable. Herein, we
report the successful photo-initiated thiol–ene radical polyaddition
of an emulsion based on two bifunctional monomers diallyl phthalate
and 2,2-(ethylenedioxy)diethanedithiol. After 20 min of irradiation
(λmax = 385 nm, 3.7 mW cm–2), polymerization
resulted in a conversion above 99%, a coagulum-free poly(thioether)
latex with a solid content of 10 wt %, a number-average molecular
weight of 14 kDa, and an average particle diameter of 60 nm. Reaction
kinetics, particle size distribution, and molecular weight progress
have been studied in detail. To clarify the mechanism dictating particle
formation, the number of particles per unit volume of water has been
determined depending on reaction time and surfactant and initiator
concentration. The nucleation mode is found to depart significantly
from conventional chain radical emulsion polymerization. Polymer particle
formation mainly proceeds through the precipitation of oligo-radicals
(homogeneous nucleation) even when the concentration of the surfactant
exceeds its critical micelle concentration. The end of nucleation
(interval I) occurs until a monomer conversion of about 95%. The dispersion’s
solid content can reach as high as 40%, and the molecular weights
remain reasonable (>4 kDa) even if the functional groups are stoichiometrically
imbalanced. In contrast to conventional thermally induced polymerization,
temporal control is demonstrated through sequential “on”/“off”
cycles, and tuning of molecular weights is achieved by precise energetic
dosage of radiation. This new step photopolymerization in emulsion
thus opens an avenue for the synthesis of linear polysulfide latexes
without high energy homogenization and at low irradiance.