SYNTHETIC NI'HH/'IR RESEARCH 



.^65 



and to devise ways of eliminatin<;; them if necessary. For the present, we are 

 perhaps justified in assuming that these variations can be neglected. The 

 control considered here therefore relates to the over-all or average composi- 

 tion of the copolymer. 



° 20 40 60 80 100 20 40 60 80 100 



CONVERSION TO POLYMER IN PER CENT 



Fig. 3. — Lejt: Cumulative percentage hy weight of styrene in the copol_\-mer as a 

 function of percentage conversion for an initial 25 percent styrene charge. Right: Per- 

 centage by weight of styrene in the polymer forming at any instant as a function of con- 

 version for an initial 2h percent styrene charge. 



Uetermixatiox of Stykexe ('onte.nt 



Many suggestions involving both chemical and physico-chemical methods 

 for measuring the average styrene content of GR-S copolymers have been 

 proposed. Physico-chemical methods when applicable have an advantage 

 in speed and precision over straight chemical methods and therefore have 

 been more carefully examined. Both ultra-violet absorption^" and refrac- 

 tion^ have been shown to be applicable but since the absorption method is 

 much more sensitive to impurities, the refraction method has proven the 

 most general. It has the advantage also that it can be employed with 

 polymers containing considerable gel fraction. 



The refraction method is based on the fact that the styrene residues in the 

 copolymer provide a greater contribution to the refraction of light passing 

 through the solid or a solution of the solid than do the butadiene residues. 

 Early work at the Bell Laboratories showed that the determination of the 

 refractive inde.x of the solid un purified copolymer led to errors. In addition, 

 the determination of the refractive index even of purified polymers was not 

 precise if much gel was present, as frequently was the case with the early 

 synthetic product. As a consequence a method, based on the use of the 

 interferometer, was developed*' '". The procedure is to disperse 2.4 grams 

 of the pure copolymer in benzene, transfer the contents \o the interferometer 



