SYNTUETIC RUBBER RESEARCH 



357 



polymerization type of polymer. Nylon on the other hand is a polyconden- 

 sation polymer. Buna S type synthetic rubber is a polymerization copoly- 

 mer because it is formed by polymerizing together styrene and butadiene 

 monomers. 



One of the important characteristics about reactions leading to the forma- 

 tion of polymers is that they result not in molecules of the same size but in a 

 statistical distribution or mixture of molecules of various sizes. These 

 molecular weight distributions, as they are called, in special cases can be 



y 20 



o 

 o 

 o 



IL 12 



25 50 75 100 125 150 175 200 225 250 



X= NUMBER OF BASE UNITS IN GIVEN CHAIN 



Fig. 1. — Curves showing frequency distribution of molecule species of different chain 

 lengths for linear |)olyesters (Flory — reference 5). 



calculated from the nature of the reaction. In other instances this is not 

 possible, although experim.entally it is often possible to arrive at an approxi- 

 mate curve representing a given polymer distribution. Figure 1 shows a 

 series of curves for a linear polyester in which the reaction conditions are 

 such that the calculated curv^es'^ represent very closely the actual distribution 

 of molecules present. The curves represent the weight fraction of each 

 molecular species present in the mixture at the extent of reaction shown on 

 each curve. 



It is customary to speak of an "average molecular weight" therefore in 

 characterizing these polymer mixtures. Several different types of averages 



