The Reinvestigation of Vinyl Acetate Emulsion Polymerization (I) -The Rate of Polymerization

  • K. H. S. Chang
  • M. H. Litt
  • M. Nomura

Summary

The kinetics of the seeded emulsion polymerization of vinyl acetate were investigated thoroughly. The variation of the polymerization rate with changes in particle concentration, type of emulsifier, emulsifier concentration, persulfate concentration, ionic strength, and monomer volume were determined. The rate of polymerization is dependent on the initiator concentration to the 0.6 power, the particle concentration to the 0.12 power, and the vinyl acetate volume to the 0.39 power; however, it is independent of the type of emulsifier, emulsifier concentration, and ionic strength in the emulsion. In all cases, the rate of the polymerization is almost independent of monomer concentration in the particles until 85 or 90% conversion.

The results were rationalized by the following mechanism.
  1. (a)

    The persulfate ion radical enters the particle where it either initiates or terminates the polymerization depending on whether the particle contains a radical or not. As only 1 to 2% of particles contain radicals, it usually initiates polymerization.

     
  2. (b)

    The radical in a particle can chain transfer to monomer generating a monomer radical. It is believed that the kinetically important chain transfer is on vinyl hydrogen. This radical reinitiates relatively slowly and thus the radical can escape from the particle to become an aqueous radical.

     
  3. (c)

    The aqueous monomer radical can do several things, (i) It can be swept up by a dead particle, (ii) it can react with initiator to generate a sulfate ion radical which will then enter a particle and initiate or terminate the polymerization, (in) it can be swept up into a particle containing a radical. Since the time the monomer radical stays in a particle is relatively short, the radical may diffuse out of such a particle without terminating, though it usually terminates under our conditions of polymerization.

     
  4. (d)

    The escape from particles and diffusion back in continues for the monomer radical until it either reinitiates or terminates.

     

The kinetic steps were justified by data from the literature and kinetic equations were derived and integrated. The equations were compared with the experimental data and shown to match very well over the whole range of experimental parameters. The integrated equation followed the course of polymerization exactly over the whole parameter range. The rate constant for reinitiation was calculated to be 29 liter/mole s and for termination to be 1.1×109 liter/mole s at 60°:C.

Keywords

Surfactant Benzene Polystyrene Styrene Acetyl 

Nomenclature

a

partition coefficient of monomer and monomer radicals between the particles and the aqueous phase

c

fractional conversion of polymer

d

particle diameter

D

diffusion constant of monomer radical in particles

Da

diffusion constant of monomer radical in aqueous phase

D0

diffusion constant of monomer radical in particles before disappearance of separate monomer phase

ƒ

the efficiency of unimolecular persulfate decomposition

(I)

initiator concentration

(I0)

initiator concentration at beginning of reaction

k1

unimolecular decomposition constant of potassium persulfate

k2

propagation rate constant of vinyl acetate

k3

chain transfer constant of growing radical to monomer

k4

reinitiation rate constant, chain transferred monomer radical adding to monomer

k7

induced decomposition rate constant, attack by monomer radical on potassium persulfate in the aqueous phase

k80

termination rate constant for monomer radical in a particle at or before 30% conversion

k2

\( = 2 \cdot 75x{{10}^{{ - 9}}}{{k}_{7}}{{\left( {f{{k}_{1}}{{k}_{3}}/{{k}_{4}}} \right)}^{{1/2}}} \)

k3

rate of polymerization at 30% conversion

(M)

monomer concentration in particles

(M)

monomer concentration per liter H2O during polymerization

(M1·)

concentration of particles containing initially chain transferred monomer radical

(M·a)

monomer radical concentration in aqueous phase

(M0)

initial monomer concentration/liter H2O

(\( \left( {{{M}_{{{{a}_{0}}}}}} \right) \))

monomer concentration in aqueous phase before the disappearance of separate monomer phase

(Ma)

monomer concentration in aqueous phase

(M·p)

concentration of particles into which a monomer radical has diffused

(M·pP·)

concentration of particles containing one growing radical and one monomer radical

N

Avogadro’s number, 6.02 × 1023

\( {\bar{n}} \)

fraction of particles with growing radicals

(P)

concentration of particles/liter H2O

(P·)

= (\( \left( {P\bar{n}} \right) \)) = concentration of particles with growing radicals/liter H2O

Pe

probability of escape of monomer radical from particle (= 1/(1 + k 4 Mad 2/4D))

Pt

probability of termination of monomer radical with growing radical in a particle (= 1/(1 + 2NπdD 0/3ak 80))

Rp

rate of polymerization

Vorg

volume of organic phase contained in the particles

Vaq

volume of aqueous phase

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Copyright information

© Applied Science Publishers Ltd 1981

Authors and Affiliations

  • K. H. S. Chang
    • 1
  • M. H. Litt
    • 1
  • M. Nomura
    • 1
  1. 1.Department of Macromolecular ScienceCase Western Reserve UniversityOhioUSA

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