364 BELL SYSTEM TECHNICAL JOURNAL 



In an analogous way we may consider N2 molecules of butadiene, B, to 

 polymerize into chain molecules of various lengths: 



N2B -> biBi + boB^ + 63^3 + • • • + bnB„, . (5) 



Now if styrene and butadiene molecules react together, as in the produc- 

 tion of GR-S, we can represent their copolymerization as the insertion of 

 the styrene chains (or portions of them) of (4) at random points in the 

 butadiene chains of (5) to form chains SjBk . That is 



NiS + NoB -> S aiSjBk (6) 



where 7 and k take on a variety of integral values and in any particular chain 

 the arrangement of S and B units is probably random. 



In practice, in the reaction represented by (6), iV2/i\^ has the value of 

 approximately 6 since 75 parts by weight of butadiene are employed to 25 

 parts of styrene. Each chain molecule therefore would be expected to 

 contain about 6 butadiene residues to each one of styrene. It is actually 

 found, however, as indicated above that the starting ratio is not adhered to 

 throughout the reaction, the molecules formed early being richer in butadiene 

 and those formed later being poorer in butadiene than the starting ratio of 

 6 to 1 . But , not only is the ratio Bk/Sj a variable from molecule to molecule 

 of the copolymer formed but also their sequence along the chain is variable. 

 Thus, in equation (6), even when equal numbers of styrene and butadiene 

 molecules are present, a strict alternation is apparently not maintained but 

 "strings" of one pure component or the other, form. 



In the GR-S reaction the weight ratio of butadiene to styrene in the first 

 molecules formed may be as high as 4:1 or more from a starting charge of 

 ratio 3:1. Thus, the average weight percentage of styrene in the GR-S 

 copolymer first formed is about 8% below that in the original charge (25%) 

 and increases with conversion so that at the point where the reaction is 

 stopped the copolymer forming contains about 29% styrene. Analogously 

 there is evidence to show that in GR-S no regular sequence of butadiene 

 and styrene along the chain molecules exists but rather a more or less random 

 entrance of the two residues into the molecules with a frequency approximat- 

 ing the 6 to 1 ratio, as the extent of combination (percentage conversion) of 

 the two ingredients approaches completion where obviously the two must 

 become equal. Figure 3 illustrates this behavior for a typical sample pre- 

 pared in the laboratory-. An integral curve showing the cumulative per- 

 centage styrene and a differential curve representing the percentage styrene 

 in the increment of the copolymer are illustrated. 



It must be left to future research to determine how important the mole- 

 cule to molecule variations in styrene content are in terms of useful properties 



