Free-Radical Chain-Growth Polymerization - Principles of Polymer Chemistry - page 137

molecular weight [ 327 ]. Also, Hawkett and coworkers reported using surface active RAFT agents to
emulsify the dispersed phase, stabilize the particles and also control the molecular weight. This
yielded polymer latexes that were free from surfactant and costablilizer [ 328 ]. One of these special
RAFT agents was illustrated as follows:

S COOH CH 3

OH

S

C 4 H 9 S

y

x

O

The other two RAFT agents used by them had similar structures. A surface active iniferter was also
reported by Choe and coworkers [ 330 , 331 ]:

S

S COONa

This RAFT agent allowed polymerization of methyl methacrylate initiated by ultraviolet light
irradiation in the absence of added surfactant or initiator.
Rieger and coworkers [ 332 ] reported a surfactant free RAFT emulsion polymerization of butyl
acrylate and styrene using poly(

N N

-dimethylacrylamide) trithiocarbonate macromolecular transfer
agent. They observed that the polymerizations were fast and controlled with molar masses that
matched well the theoretical values and low polydispersity indexes. Monomer conversions close to
100% were reached and the polymerizations behaved as controlled systems, even at 40% solids
contents. The products were poly(

,

N , N -dimethyl acrylamide)-b-poly( n -butyl acrylate) and poly( N , N -

dimethylacrylamide)-b-polystyrene amphiphilic diblock copolymers formed in situ.

Review Questions

Section 3.1

1. What are the three steps in free-radical polymerization? Illustrate each step in free-radical chain
polymerization.
2. What is the rate-determining step in free-radical polymerization?
3. Write the kinetic expressions for initiation, propagation, termination, and transfer.
4. What is the steady state assumption? How is it expressed? Why is it necessary?
5. What is the expression for the rate of propagation? Rate of monomer disappearance? The average
lifetime of a growing radical under steady state conditions?
6. Why is an initiator efficiency factor needed for the rate equation? What is a kinetic chain length?
7. In polymerization of styrene in benzene at 60 C using 0.1 mol benzoyl peroxide initiator and 1 mol
of monomer, 10 ─ 5 , assume steady state and calculate the free-radical concentration k d is 1.38

during the reaction. If k p is 176/mol what is the rate of propagation?What is the lifetime of a growing

radical if k t is 7.2 10 ─ 5 ? What is the rate of propagation if the initiator efficiency is 72%?