Relative sizes of various side groups found in polymers
Since almost all plastics have a carbon-carbon backbone, the strength of these C-C 
bonds are relatively equal, as indicated in the following chart.

Bond           Strength (kcal/mol)


CH3-CH3             88
(CH3)2 CH-CH3       84
(CH3)3 C-CH         80

However, there are huge differences in the strengths of various polymers.  
The differences are caused by both the sizes of atoms and groups in the chain 
and by the presence or absence of secondary bonding forces.

There are three types of secondary bonding forces.

A.  Dipole interactions.

B.  Van der Waals forces.

C.  Hydrogen bonding.