This set of Polymer Engineering Multiple Choice Questions & Answers (MCQs) focusses on “Kinetics of Step-Growth Polymerization”.
1. What is the order of a self-catalyzed polyesterification reaction?
a) 2
b) 3
c) 1
d) 4
View Answer
Explanation: For each step of reaction between a –COOH group and –OH group, a second functional group –COOH acts as a catalyst in the reaction. Therefore, the reaction follows the third order kinetics.
2. How is the reactivity of a specific functional group, in general, dependent on the size of the molecule to which it is attached, in a polycondensation reaction?
a) increases with increase in molecular size
b) decreases with decrease in molecular size
c) independent of molecular size
d) cannot be determined
View Answer
Explanation: The reactivity of functional is independent of the size of the molecule to which it is attached.
3. Which of the following is true about polymer systems in polycondensation?
a) have higher viscosity than small non-polymeric molecules
b) insoluble beyond a critical molecular weight
c) all of the mentioned
d) none of the mentioned
View Answer
Explanation: The medium viscosity in polymer systems is much higher than the systems of small non-polymeric molecules. Besides, there tend to become insoluble after a certain range of chain length or molecular weight.
4. Which order kinetics does a strong acid catalyzed polyesterification reaction follow?
a) 1
b) 2
c) 3
d) 4
View Answer
Explanation: This reaction is achieved by addition of a strong acid catalyst, which is not utilized in the reaction as in self catalyzed reaction. So, the reaction clearly, follows second order kinetics.
5. What is the slope of the line plotted between square of the average degree of polymerization and time for a self catalyzed polyesterification?
a) 2c02k
b) 2c0k
c) c0k/2
d) c02k
View Answer
Explanation: The rate of disappearance of carboxyl groups is given by, -dc/dt = kc3 (due to self catalyzation, second COOH group acts as catalyst, and reaction follows third order kinetics)
On integration we get,
2kt = 1/c2 – constant
On substituting c=c0(1-p), we get
2c02kt = 1/(1-p)2 – constant
This equation gives the slope equal to 2c02k.
6. What factors may be responsible for deviation in the high conversion region of self-catalyzed polycondensation?
a) loss of reactants
b) cyclization
c) other side reactions
d) all of the mentioned
View Answer
Explanation: The deviation from linearity in the high conversion region may have arisen due to the factors including loss of reactants, cyclization or other side reactions.
7. What is true about an end functional group, having a terminal reactive centre, of a polymer molecule?
a) has a greater mobility than the polymer molecule as a whole
b) has a lower mobility than the polymer molecule as a whole
c) same as polymer molecule
d) cannot be determined
View Answer
Explanation: The functional group at a free end has a good degree of mobility to collide and faster reaction to take place.
8. How does the reaction rate vary with increasing extent of reaction beyond 94% conversion in self catalyzed reaction?
a) decreases
b) increases
c) remains same
d) none of the mentioned
View Answer
Explanation: The reaction rate decreases with increasing extent of reaction beyond 94%. The factors responsible are loss of reactants, difficulty in removal of byproduct molecule due to high viscosity and other side reactions.
9. What is the average degree of polymerization for a complete polycondensation reaction in a bifunctional system?
a) infinity
b) 0
c) 1
d) none of the mentioned
View Answer
Explanation: For a bifunctional system, average functionality, f=2 and for a complete reaction, extent of reaction, p=1. So the expression for extent of reaction reduces to,
p = [1-(1/Xn)] where, Xn is the average degree of polymerization. Therefore, Xn=∞. Although it is possible only in hypothetical situation.
10. How is the extent of reaction, p, of a polycondensation reaction related to the average functionality, f, of the system?
a) proportional to f
b) proportional to 1/f
c) proportional to f2
d) proportional to 1/f2
View Answer
Explanation: The term for extent of reaction is given by,
p = (2/f)[1-(1/Xn)] Therefore, it is inversely proportional to f.
11. The average functionality of a polyfunctional system is 2.4. What will be the average degree of polymerization for 80% completion of reaction?
a) 20.55
b) 27.67
c) 50.3
d) 2.97
View Answer
Explanation: Using the formula,
p = (2/f)[1-(1/Xn)] and putting the values of f=2.4 and p=0.8, we get the value of Xn as 27.67.
Sanfoundry Global Education & Learning Series – Polymer Engineering.
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