This set of Polymer Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Chain Transfer”.

1. What does the effect of chain transfer on R_{p} depend on?

a) re-initiation rate constant, k_{i}^{’}

b) monomer concentration

c) k_{i}^{’}/k_{p}

d) none of the mentioned

View Answer

Explanation: The effect of chain transfer on R

_{p}depends upon the value of re-initiation constant relative to the value of propagation constant. If k

_{i}

^{’}<< k

_{p}, then chain transfer agent would play the role of an inhibitor or retarder and R

_{p}would be adversely affected.

2. How is the value of average degree of polymerization defined, considering the chain transfer mechanism?

a) ratio of overall rate of polymerization to total rate of production of pairs of chain ends

b) ratio of total rate of production of chain ends to overall rate of polymerization

c) ratio of overall rate of polymerization to rate of formation of pairs of chain ends by chain transfer

d) ratio of rate of formation of pairs of chain ends by chain transfer to overall rate of polymerization

View Answer

Explanation: The average degree of polymerization is redefined from the kinetic view point as the ratio of overall rate of polymerization to total rate of production of pairs of chain ends.

3. What is the value of chain transfer constant for a transfer agent?

a) rate constant for chain transfer reaction, k_{tr}

b) product of k_{tr} and k_{p}

c) ratio of k_{tr} to k_{p}

d) none of the mentioned

View Answer

Explanation: The chain transfer constant C is defined as the ratio of rate constant, k

_{tr}for chain transfer reaction to the propagation rate constant.

4. What is the value of intercept, when (1/X_{n}) is plotted against R_{p}, if only monomer transfer has a significant occurence?

a) 0

b) C_{M}

c) C_{I}

d) [I]

View Answer

Explanation: In the absence of any solvent in polymerization system, the expression for (1/X

_{n}) is arranged as-

1/X

_{n}= [(y+2)/(y+1)](k

_{t}/k

_{p}

^{2})(R

_{p}/[M]

^{2}) + C

_{M}+ C

_{I}[I]/[M] When only monomer transfer takes place, the intercept on 1/X

_{n}axis gives a measure of monomer transfer constant, C

_{M}.

5. What is the value of intercept and slope of the straight line, when the term [1/X_{n}– [(y+2)/(y+1)](k_{t}/k_{p}^{2})(R_{p}/[M]^{2})] is plotted against the ratio [I]/[M], when both monomer and initiator transfer are considered?

a) C_{M}, C_{I}

b) C_{I}, C_{M}

c) 0, C_{I}

d) none of the mentioned

View Answer

Explanation: The plot of [1/X

_{n}– [(y+2)/(y+1)](k

_{t}/k

_{p}

^{2})(R

_{p}/[M]

^{2})] vs [I]/[M] gives a straight line, whose intercept gives a direct measure of CM and slope gives the value of initiator transfer constant, CI.

6. How does the value of solvent transfer constant, C_{S} vary in the solvents with increasing substitution at the α-carbon?

a) increases

b) decreases

c) constant

d) none of the mentioned

View Answer

Explanation: The reactivity increases with the increasing substitution at the α-carbon, which leads to increasing values of C

_{S}in the order benzene < toluene < ethyl benzene < isopropyl benzene.

7. Which of the following solvents is not useful for controlling the degree of polymerization?

a) carbon tetra-bromide

b) pentaphenyl ethane

c) t-butyl mercaptan

d) toluene

View Answer

Explanation: The other three solvents exhibit high chain transfer activity; they surpass styrene in reactivity towards chain radical and have C

_{S}values greater than unity. Toluene, on the other hand, has a very low C

_{S}value to control the degree of polymerization.

8. What is the slope of the linear plot of log [S] v/s log [M] in significant solvent transfer mechanism?

a) C_{M}

b) C_{S}

c) k_{tr},S

d) C_{I}

View Answer

Explanation: The ratio of rate of consumption of the transfer agent and the rate of consumption of monomer can be written as, (d[S]/dt)/(d[M]/dt) = k

_{tr,S}[M]/k

_{p}[S] = C

_{S}[M]/[S] d log[M]/d log [S] = C

_{S}

Hence for this system, the linear plot of log [M] against log [S] renders the slope of solvent transfer constant, C

_{S}.

9. What is the effect of polymer transfer on the polymer product formed by radical polymerization?

a) becomes less crystallizable

b) becomes mechanically weak

c) becomes less resistant to foreign agents

d) all of the mentioned

View Answer

Explanation: The polymer transfer leads to the chain branching which makes it less crystalizable, weak in mechanical properties and less resistant to heat, solvents and chemicals.

10. For polymerization of vinyl acetate with hydrogen peroxide at 60ᵒC, the reciprocal of number average degree of polymerization is given by,

1/X_{n} = 1.75*10^{-4} + 14.0R_{p} + 5.4*10^{4}R_{p}^{2}

What is the value of monomer transfer constant?

a) 1.75*10^{-4}

b) 0.875*10^{-4}

c) 5.4*10^{4}

d) 14

View Answer

Explanation: According to the expression for reciprocal of average degree of polymerization, we have

1/X

_{n}= C

_{M}+ (k

_{t}/k

_{p}

^{2})(R

_{p}/[M]

^{2}) + C

_{I}(k

_{t}/k

_{p}

^{2}fk

_{d})(R

_{p}

^{2}/[M]

^{3})

Comparing the given equation with this one, we get CM as 1.75* 10

^{-4}.

11. Consider the expression for reciprocal of average degree of polymerization, when styrene is polymerized with t-butyl hydroperoxide at 60ᵒC-

1/X_{n} = 0.6*10^{-4}+10.0R_{p}+1.73*10^{4} R_{p}^{2}

Calculate the value of initiator transfer constant, if the monomer concentration is 8.35 mol/l and the value of the term k_{p}^{2} fk_{d}/k_{t}= 3.29*10^{-9}.

a) 0.035

b) 0.07

c) 0.063

d) 0.0175

View Answer

Explanation: According to the expression for reciprocal of average degree of polymerization, we have

1/X

_{n}= C

_{M}+ (k

_{t}/k

_{p}

^{2})(R

_{p}/[M]

^{2}) + C

_{I}(k

_{t}/k

_{p}

^{2}fk

_{d})(R

_{p}

^{2}/[M]

^{3})

Comparing the given equation,

CI(k

_{t}/k

_{p}

^{2}fk

_{d})(R

_{p}

^{2}/[M]

^{3}) = 1.73*10

^{4}R

_{p}

^{2}

CI = 1.73*10

^{4}* 3.29*10

^{-9}*(8.35)

^{3}

CI = 0.035.

12. The transfer constant to the solvent for polymerization of of styrene in toluene at 60ᵒC is 0.125*10^{-5}. How much dilution is required to decrease the molecular weight by 4 times when 1/X_{n0} is 3.5*10^{-4}?

a) 75

b) 42

c) 84

d) 7

View Answer

Explanation: To decrease molecular weight by 4 times, 1/X

_{n}becomes 4/X

_{n0},

4/X

_{n0}= 1/X

_{n0}+ C

_{I}[S]/[M] Substituting the values, [S]/[M] = 84. Thus, dilution factor is 84.

13. The molecular weight of the polymer, when styrene is polymerized at 60ᵒC, is halved by diluting it with ethyl benzene. What is the value of transfer constant to the solvent, if the dilution factor is 7 and the term 1/X_{n0} is 4.8*10^{-4}?

a) 0.335

b) 0.68

c) 1.5

d) 0.82

View Answer

Explanation: We have the expression for solvent transfer,

1/X

_{n}= 1/X

_{n0}+ C

_{I}[S]/[M] When molecular weight is halved, 1/X

_{n}becomes 2/X

_{n0}, then substituting the values and [S]/[M]= 7, we get C

_{I}as 0.68.

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