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CHEM 370/770
FINAL EXAMINATION
Monday December 19, 2011, 12:30-15:00 (2 hours), MC-4041.
Answer any fourof the following five questions. Each question has equal value, so be reasonable about
the amount of time you spend on each. Plan your answers carefully and be thorough, since manyquestions cover at least two aspects of the topic. Explain the effects, dont just state them.The use of
appropriate equations and diagrams is often indispensable to justifyyour answers. There are no negative
marks. Accordingly, try to answer the questions even if you have to speculate. Pay attention not to
leave any answers blank, particularly if you answer the questions out of sequence! Please specify the
question you skippedon the cover pageof your exam booklet.
1. Derive the Mayo equation giving the number-average degree of polymerization obtained in a reaction
where chain termination takes place by combination, and for which chain transfer to the initiator, the
monomer, and the solvent are the only significant chain transfer events taking place. Start by
considering the reactions and rate equations for all the chain transfer reactions involved. Note that the
degree of polymerization attained without chain transfer may be simply defined as onX and does not
itself need to be derived.
2. (a) Explain how the relative magnitude of kpvs.ktr, and kpvs.kaaffects Rpand nX when chaintransfer reactions are significant.
(b) Explain what is meant by a steady state in a radical polymerization reaction, and show how the
concentration of radical species can be calculated based on this assumption.
(c) Identify 2 constitutionalisomers and 2 configurationalisomers obtained in the polymerization of
1,3-butadiene.
3. (a) Explain why head-to-tail linkages are most common (>98%) in the polymerization of vinyl
monomers. Give one example for which this proportion is lower, and explain why it is so.
(b) Explain what is meant by an on-off particle growth mechanism in Stage II of emulsion
polymerization.
(c) Show the structure of the three stereoisomersthat can be obtained when propylene is polymerized,
anddiscuss the influence of stereoisomerism on the physical properties of polypropylene.
4. (a) Why are in particular peroxide and azo compounds used as radical initiators rather than other
organic compounds?
(b) Compare the homogeneous nucleation and the micellar nucleation mechanisms of emulsion
polymerization, and explain under what circumstances one mechanism or the other may be more
appropriate to describe a reaction.
(c) Explain why a sample of polystyrene you prepared in the laboratory in may have a polydispersity
index (PDI) of 1.5, while a commercial sample may have a PDI = 10.
5. (a) Explain how and why the initiator efficiencyfis affected by the monomer and initiator
concentrations, and by the viscosity of the polymerization medium.
(b) Discuss how i) the reactivity of the propagating centre and ii) the initiator concentration can affect
the rate of chain transfer reactions (Rtr).
(c) In simple (non-mathematical) terms, define i) the hydrodynamic radius Rh, ii) the radius of gyration
Rg, and iii) the fully extended length of a polymer chain. Drawings should be useful to explain
your answer in each case.
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pt
p
1/2
dt
p
Rk
Mk
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[M]k=
2
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[M]
[XA]C+
[M]
[S]C+
[M]
[I]C+C+
)X(
1=
X
1XASIM
onn
k
[I]kf[M]k=R
t
d
1/2
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FT+
L
U=f
LT
NR
[M]Nk=X
Ai
p
n
2/1
2/1
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m
rm=R
1/22
i
2ii
1/2
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N
n[M]Nk=R
A
p
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