1. What is the ratio of degree of polymerization for a polymer when terminated by combination to disproportionation mechanism?
a) 2
b) 0.5
c) 1
d) 4
Explanation: For termination by combination,
Xn = 2v
And for termination by disproportionation,
Xn = v
So, the ratio is 2.
2. What is the degree of polymerization for the product having ratio of incidences of termination by combination and by disproportionation, y is 5, when the chain length is 400?
a) 686
b) 343
c) 500
d) 2000
Explanation: The number average degree of polymerization is related to kinetic chain length, when termination occurs simultaneously by combination and disproportionation, by the following expression-
Xn = [2(y+1)/(y+2)]v
= 686.
3. What does the effect of chain transfer on Rp depend on?
a) re-initiation rate constant, ki’
b) monomer concentration
c) ki’/kp
d) none of the mentioned
Explanation: The effect of chain transfer on Rp depends upon the value of re-initiation constant relative to the value of propagation constant. If ki’<< kp, then chain transfer agent would play the role of an inhibitor or retarder and Rp would be adversely affected.
4. 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
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.
5. What is the value of chain transfer constant for a transfer agent?
a) rate constant for chain transfer reaction, ktr
b) product of ktr and kp
c) ratio of ktr to kp
d) none of the mentioned
Explanation: The chain transfer constant C is defined as the ratio of rate constant, ktr for chain transfer reaction to the propagation rate constant.
6. What is the value of intercept, when (1/Xn) is plotted against Rp, if only monomer transfer has a significant occurence?
a) 0
b) CM
c) CI
d) [I]
Explanation: In the absence of any solvent in polymerization system, the expression for (1/Xn) is arranged as-
1/Xn = [(y+2)/(y+1)](kt/kp2)(Rp/[M]2) + CM + CI [I]/[M] When only monomer transfer takes place, the intercept on 1/Xn axis gives a measure of monomer transfer constant, CM.
7. What is the value of intercept and slope of the straight line, when the term [1/Xn– [(y+2)/(y+1)](kt/kp2)(Rp/[M]2)] is plotted against the ratio [I]/[M], when both monomer and initiator transfer are considered?
a) CM, CI
b) CI, CM
c) 0, CI
d) none of the mentioned
Explanation: The plot of [1/Xn – [(y+2)/(y+1)](kt/kp2)(Rp/[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.
8. How does the value of solvent transfer constant, CS vary in the solvents with increasing substitution at the α-carbon?
a) increases
b) decreases
c) constant
d) none of the mentioned
Explanation: The reactivity increases with the increasing substitution at the α-carbon, which leads to increasing values of CS in the order benzene < toluene < ethyl benzene < isopropyl benzene.
9. 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
Explanation: The other three solvents exhibit high chain transfer activity; they surpass styrene in reactivity towards chain radical and have CS values greater than unity. Toluene, on the other hand, has a very low CS value to control the degree of polymerization.
10. What is the slope of the linear plot of log [S] v/s log [M] in significant solvent transfer mechanism?
a) CM
b) CS
c) ktr,S
d) CI
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) = ktr,S[M]/kp[S] = CS [M]/[S] d log[M]/d log [S] = CS
Hence for this system, the linear plot of log [M] against log [S] renders the slope of solvent transfer constant, CS.