SYNTHETIC RUBBER RESEARCH 359 



structure we mean the detailed architecture of the chain molecule over 

 distances of the order of length of the repeating unit. The kind of atoms 

 involved in the unit and their spatial arrangement in regard to atoms in the 

 same chain as well as in the neighboring chains are included in this definition. 

 It is the micro-chain structure which determines entirely the chemical 

 properties of the polymer and to a large extent the physical properties as 

 well. Thus, the influence of solvents, oxidizability, hardness at a given 

 temperature, softening point, ability to crystallize are determined largely 

 by the micro-structure of the polymer. 



Macro-chain structure on the other hand refers to the long range form of 

 the chain molecule. It ignores composition and concerns itself with the 

 nature of the molecule as a whole and with its intercormections to other 

 molecules. 



Certain terminology has grown up in this connection which can be con- 

 veniently defined at this time. We speak of "linear" polymers when 

 primary valence bonds can be traced through the molecules from one end 

 to the other without passing over the same atoms twice. We say "branched" 

 molecules are present when the process of tracing leads us into one or more 

 offshoots from the main chain. When the degree of branching becomes 

 excessive the molecules may become insoluble in good solvents for the linear 

 or slightly branched molecules. When nteworks of molecules are present 

 we say the polymer is "netted" or "cross-Unked". In this instance closed 

 paths may be traced and the smaller the paths, the "tighter" or more 

 "intense" is the netting. Netted chain molecule systems are invariably 

 insoluble. Insoluble polymers whether because of intense branching or 

 netting are called "gel". We speak of micro-gel when the gel particles 

 (molecules) are microscopic or smaller in size (say less than l/j.) and of macro- 

 gel when the particles are large.^ Usually macro-gel as well as the micro-gel 

 is associated with soluble molecule species. These latter are referred to as 

 "sol" and represent the linear or the less branched molecular components 

 of the mixture. The complete description of every molecule present in a 

 polymer mixture is thus a very difl5cult if not impossible task. We are 

 thus forced to employ a statistical treatment. 



In the case of copolymers, still other considerations arise. There is the 

 probability that the reacting components will not react with one another 

 at the same rates they do with themselves. When this occurs the composi- 

 tion of the molecules in the mixture varies, some containing more of one 

 component than others do. Also the order in which the components are 

 arranged along the chains may vary molecule to molecule. Such circum- 

 stances of course give rise to varying properties in the copolymer mixture. 

 We shall have occasion to consider these questions below in connection 

 with the development of GR-S. 



