Consider the radical copolymerization of a benzene solution that is 1.5 M in sty
ID: 526123 • Letter: C
Question
Consider the radical copolymerization of a benzene solution that is 1.5 M in styrene and 3.0 M in methyl methacrylate a. What is the initial copolymer composition if the polymerization is carried out at 60 degree C using benzoyl peroxide at a concentration of 5.0 times 10^4 M? How is the copolymerization composition affected if 3.0 times 10^3 M benzoyl peroxide is used? b. How will the presence of 5.0 times 10^5 M n-butyl mercaptan affect the initial copolymer composition? c. What would you expect (qualitatively) for the copolymer composition if the reaction were initiated by n-butyl lithium? Or by BF_3 plus water?Explanation / Answer
In chain-growth copolymerization, the composition of the polymer depends on the concentrations and relative reactivities (r) of the monomers.
(a) Both styrene and MMA have similar r- values. so, they would form random copolymer.
fsty (initial mole fraction) = [M1]/{[M1] + [M2]} = 0.3333
fMMA(initial mole fraction) = [M2]/{[M1] + [M2]} = 0.6666
Fsty (after adding initiator) = (rstyfsty2+fstyfMMA)/(rstyfsty2+2fstyfMMA + r2fMMA2)
=[(0.46 x (0.3333)2 + (0.3333 x 0.6666)]/[[(0.46 x (0.3333)2 + 2(0.3333 x 0.6666) + 0.52 x (0.6666)2]
= [0.0511 + 0.2222]/[0.0511 + 2 x 0.2222+0.2311]
= 0.2733/0.7266 = 0.3761
FMMA = 1-Fsty = 1-0.3761 = 0.624
change in initiator concentration would not alter the copolymer composition. It would affect only the overall rate of polymerization and molecular weight.
(b) n-butyl mercaptan is a chain transfer agent. this also would affect only the overall rate of polymerization and molecular weight.
(c) in ionic polymerization reactivities of monomers completely varies. for example. MMA shows negligible reactivity in cationic polymerization whereas styrene has an r value of 2.31. So, definetely there would be a change in copolymer composition.
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