Chap 15 Solns
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| Copolymers
14.15 Sketch the repeat structure for each of the following alternating copolymers: (a) poly(butadiene-chloroprene), (b) poly(styrene-methyl methacrylate), and (c) poly(acrylonitrile-vinyl chloride). Solution This problem asks for sketches of the repeat unit structures for several alternating copolymers. (a) For poly(butadiene-chloroprene) (b) For poly(styrene-methyl methacrylate) (c) For poly(acrylonitrile-vinyl chloride) 14.16 The number-average molecular weight of a poly(styrene-butadiene) alternating copolymer is 1,350,000 g/mol; determine the average number of styrene and butadiene repeat units per molecule. Solution Since it is an alternating copolymer, the number of both types of repeat units will be the same. Therefore, consider them as a single repeat unit, and determine the number-average degree of polymerization. For the styrene repeat unit, there are eight carbon atoms and eight hydrogen atoms, while the butadiene repeat consists of four carbon atoms and six hydrogen atoms. Therefore, the styrene-butadiene combined repeat unit weight is just m = 12(AC) + 14(AH) = (12)(12.01 g/mol) + (14)(1.008 g/mol) = 158.23 g/mol From Equation 14.6, the degree of polymerization is just Thus, there is an average of 8530 of both repeat unit types per molecule. 14.17 Calculate the number-average molecular weight of a random nitrile rubber [poly(acrylonitrile-butadiene) copolymer] in which the fraction of butadiene repeat units is 0.30; assume that this concentration corresponds to a degree of polymerization of 2000. Solution This problem asks for us to calculate the number-average molecular weight of a random nitrile rubber copolymer. For the acrylonitrile repeat unit there are three carbon, one nitrogen, and three hydrogen atoms. Thus, its repeat unit molecular weight is mAc = 3(AC) + (AN) + 3(AH) = (3)(12.01 g/mol) + 14.01 g/mol + (3)(1.008 g/mol) = 53.06 g/mol The butadiene repeat unit is composed of four carbon and six hydrogen atoms. Thus, its repeat unit molecular weight is mBu = 4(AC) + 6(AH) = (4)(12.01 g/mol) + (6)(1.008 g/mol) = 54.09 g/mol From Equation 14.7, the average repeat unit molecular weight is just = (0.70)(53.06 g/mol) + (0.30)(54.09 g/mol) = 53.37 g/mol Since DP = 2000 (as stated in the problem), may be computed using Equation 14.6 as 14.18 An alternating copolymer is known to have a number-average molecular weight of 250,000 g/mol and a degree of polymerization of 3420. If one of the repeat units is styrene, which of ethylene, propylene, tetrafluoroethylene, and vinyl chloride is the other repeat unit? Why? Solution For an alternating copolymer which has a number-average molecular weight of 250,000 g/mol and a degree of polymerization of 3420, we are to determine one of the repeat unit types if the other is styrene. It is first necessary to calculate using Equation 14.6 as Since this is an alternating copolymer we know that chain fraction of each repeat unit type is 0.5; that is fs = fx = 0.5, fs and fx being, respectively, the chain fractions of the styrene and unknown repeat units. Also, the repeat unit molecular weight for styrene is Download 0.7 Mb. Share with your friends:
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