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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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c+/ae=dxeaxax

pt

p

1/2

dt

p

Rk

Mk

)[I]kfk2(

[M]k=

2

][ 22

[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

pp

T

FT+

L

U=f

LT

NR

[M]Nk=X

Ai

p

n

2/1

2/1

2/1 66

Mk)r(=

m

rm=R

1/22

i

2ii

1/2

g

N

n[M]Nk=R

A

p

p

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