30 KADIAIION lUoLOfJV 



POLYiMKHIZATION AM) 1)1 MKRIZATION 



There are two general types of polymerization: .simj)l(' reactions leading 

 to the formation of definite molecules, such as dimers or t rimers, and 

 chain reactions, whose products are macromoleciilcs of indefinite molecu- 

 lar weight. One of the first polymerizations to be studied with reason- 

 able care (Luther and Weigert, 1905) is the dimerization of anthracene. 

 At moderately elevated temperatures (80°-200°C') the dimer reaches a 

 measurable sleady-state concentration in a dilute solution of anthracene 

 illuminated with ultraviolet light (wave length 3GG0 or 3130 A). Over 

 the range of intensities and concentrations studied, the steady-state con- 

 centration is directly proportional to the intensity of the absorbed radia- 

 tion. The quantum yield of dimerization increases with increasing 

 anthracene concentration, approaching a limiting yield of about 0.5 

 (Weigert, 1927). The available measurements are insufficient to deter- 

 mine the mechanism of the process. It appears very probable that the 

 first five steps which were proposed in the discussion of the oxidation of 

 aromatic hydrocarbons occur in the polymerization reaction. However, 

 any of the three following alternative reactions may be responsible for the 

 formation of the dimer: 



A* + A-> Ao, 



A' + A -^ Ao, 



2A' -^ A,. 



The last of these is consistent with the observed limiting yield of 3-^. 

 This interesting reaction is certainly worthy of further study. 



The gas-phase polymerization of cyanogen (Hogness and Ts'ai, 1932) 

 is at least superficially simple. The absorption of this compound is 

 fairly strong in the wave-length region 2150-2250 A. The absorption 

 bands are diffuse, and the gas is nonfluorescent. Under the conditions of 

 measurement the ciuantum yield is 3.0. The product is a brownish solid. 

 The authors propose the following mechanism: 



C2N2 -\- hv —* 2CN (primary process), 



CN + C2N2^ (CN)3 (secondary process). 



This is probably an oversimplification, since it is difficult to understand 

 why the (CN):i molecules would react with themselves to form para- 

 cyanogen but would not r&act with the remaining cyanogen. 



The formation of large polymer molecules (Mark and Raff, 1941 ; Bawn, 

 1948) ma}^ occur either by successive condensation or by addition of sim- 

 ple molecules. Addition polymerization is essentially a chain reaction 

 and, as such, can be studied effectively by photochemical methods. 

 Determinations of the chain lengths of thermal reactions can be made 

 only indirectly, usually by the use of inhibitors (Alyea and Biickstrom, 

 1929). In a photochemical reaction the ratio of the over-all (juantum 



