356 BELL SYSTEM TECHNICAL JOURNAL 



chain molecules on polymer behavior. For example, the length of the mole- 

 ♦cules which are present in a polym.er is of critical importance to certain 

 properties such as mechanical strength. These facts, as well as the necessity 

 for order in the arrangement of the molecular units along the chains were not 

 appreciated by the early organic workers. That Carothers realized what 

 many of the older organic chemists did not realize is indicated by his state- 

 ment made in 1934 that the problem of physically characterizing polymers 

 in significant numerical units is of the utmost importance and that it should 

 receive more attention jointly from physicists and chemists. 



Some Physico- Chemical Features of Polymers 



We have seen very briefly how the quest for the origin of properties of 

 rubbers and polymeric substances in general led of necessity to a study of the 

 intimate details of chain molecule structure on the one hand and a study of 

 the general characteristics of large molecules on the other. Before taking 

 up the specific researches on GR-S synthetic rubber, however, it ^\\\\ be 

 helpful to pursue somewhat further the ideas on the formation and constitu- 

 tion of polymers. 



There are two general chemical processes by which polymer molecules are 

 formed, namely pol}Tnerization and polycondensation. Chemists, at times, 

 use the first term to represent all processes leading to the formation of large 

 molecules but it is more convenient to distinguish two processes even though 

 the difference between them is academic in some cases. In poh^merization, 

 chemical molecules called the monomers, become "activated" either by heat 

 energy or by means of special chemical compounds. In this state they 

 spontaneously grow at the expense of their unactivated neighbors until the 

 growth of the chains is abruptly terminated, either by active chains coming 

 together or by a transfer of energ}' to other, often foreign, molecules. The 

 entire growth reaction for any given chain usually takes but a fraction of a 

 second for completion. \\'hen two or more different monomers capable of 

 polymerization enter together into the same chain molecule formation the 

 process is referred to as "copohnnerization". 



In polycondensation, identical or non-identical molecules react to give 

 large molecules just as in polymerizaticni. The difference is that in the 

 former reaction a molecule of water (or other substance) is evolved each time 

 a new molecule is added to the growing chain system. Also the reaction 

 resulting in chain growth is step-wise in the sense that each added molecule 

 follows the same steps in reacting that are followed by any other. No 

 special type of activation on the end of the growing chain is necessar}^ 

 Finally, since there is no activated growth, the phenomena of termination 

 in the sense used above in connection with polymerization do not exist. 



Both kinds of polymers are important technically. Thus polystyrene is a 



