152 K. S. PITZER 



CH2- CH2- 



POLYISOBUTYLENE POLYETHYLENE 



Fig. 5 



in the third position we have a CH2 and some more of the chain. At the near 

 end we have two hydrogens and some more chain. 



If you look at it this way yovi will see that the far end of this unit is practi- 

 cally symmetrical because there are two methyl groups, a methylene group that 

 looks like a methyl group so far as the other end of this bond is concerned. 

 Therefore, you can turn this bond by 120 degrees without any appreciable 

 change in the potential energy. If, on the other hand, these methyl groups are 

 replaced by hydrogen in the paratfin, then the situation is like that in n-butane 

 where the potential minima differ in energy about one kilocalorie. 



If you have for each bond in polyisobutylene, not free rotation, but three 

 evenly spaced potential minima, then a long chain has practically as much flexi- 

 bility as if there were free rotation. The chain can get into practically any po- 

 sition. If there is a difference in energy between the three potential minima, 

 then the chain will be in the low energy position for most bonds, and the chain 

 will be much less flexible. Thus in many cases you get the effect of free rotation, 

 not by having the potential barrier zero, but by having several potential minima 

 of equal energy. 



I believe I should say a word about partial double bond systems, although 

 I am sure you are quite familiar with them already. Since these are still mostly 

 single bonds we ought to put it in this frame of reference. Let us take, 1,3- 

 butadiene, and again it does not matter very much if oxygens or other groups 

 are substituted for the terminal CHo groups. There is enough double bonded- 

 ness in the central bond to favor a planar configuration — either cis or trans. 

 The work of Aston, Brickwedde and others on 1 ,3-butadiene indicates that the 

 trans is the low energy conformation. If the central bond is twisted by 90 de- 

 grees there is a potential maximum of about 5 kilocalories. In other words, the 

 barrier is a little higher than in ethane, but not much different. If you let it go 

 back down into the cis potential minimum you get a rotational isomer, or a 

 conformer — which has a higher energy than trans by around 2 to 2^ 9 kilo- 

 calories. Here again is a structural unit which, with other groups than CH2's 

 may be of interest to you. You can take the normal geometry as planar with 



