SYNTHETIC RUBBER RESEARCH 



371 



are between them. There are at least four important ways in which the 

 chain structure of GR-S can deviate from the simplest structure, namely 



(9) 



which would arise from a regular addition of butadiene and styrene. In the 

 first place, as noted before, the styrene and butadiene units can be badly 

 mixed up in the chain and not arranged in any special order. Secondly, the 

 butadiene or the styrene units may be reversed end for end in the chains. 

 This will make no difference in the case of the butadiene provided the mole- 

 cule has a center of symmetry but this is probably not the case. Again, 

 the butadiene residue may assume either the well-known cis or the trans 

 configurations shown in (10) because of the double bond present in it. 



(10) 



Finally, the butadiene unit may be combined into the chain as a 1,2 or 

 as a 1,4 unit. In the former case a vinyl group is appended to the chain 

 molecule whereas in the latter it is absent : 



(11) 



If these possibilities are considered, one representation of the chain molecule 

 over a distance we should consider within the definition of micro is as 

 follows: 



