Chemistry Reference
In-Depth Information
distance of end to end. The entropy change for three-membered rings would have a large negative
value. For larger rings the negative value would be less, because the end to ends would be further
apart. Statistical mechanics treatment has shown that the entropy change of ring closure is [
223
]:
DS
r
ΒΌ R
ln
fPV=
2
xV
s
Ng
where
is the probability of ring closure or the fraction of chain ends that will come together and
close to form ring structures,
P
V
is the total volume of the system,
V
s
is the volume of a constrained
x
N
skeletal atom prior to bond breaking,
is the number of monomer units in the ring, and
is the
Avogadro's number.
Review Questions
Section 5.1
1. Are the mechanisms of ring-opening polymerizations of cyclic monomers chain-growth of step-
growth reactions? Explain
Section 5.2
1. Write the rate expression for propagation in ring-opening polymerizations where there is an
equilibrium between propagation and depropagation.
2. Write the kinetic expression for the total concentration of monomer segments that are incorporated
into the polymer.
Section 5.3
1. Oxiranes can be polymerized by three different mechanisms. What are they? Explain.
2. Write the chemical reactions for the mechanism of polymerization of ethylene oxide with the aid
of stannic chloride. Does a high molecular weight polymer form? If not, explain why.
3. Write the chemical reactions for the mechanism of polymerization of propylene oxide with boron
trifluoride-water.
4. Describe the mechanism and write the chemical reactions of ring-opening polymerizations of
oxiranes with potassium hydroxide. In polymerization of propylene oxide with KOH what type of
tacticity polymer forms. Explain.
5. Describe the mechanism and write the chemical equations for coordinated anionic polymerizations
of propylene oxide by ferric chloride and by diethylzinc-water. Show reaction mechanism.
6. Discuss the general characteristics of steric control in the polymerizations of oxiranes.
7. Explain the mechanism postulated by Tsuruta of steric control in polymerizations of oxiranes with
the aid of organozinc compounds, giving the structure of the catalyst and the mode of monomer
insertion and the mode or ring opening.
