C-C bond dissociation energy
The terminal bonds are stronger than the internal bonds.

In simple hydrocarbons where strain or electronegative substituents are absent, the bond strengths will correlate with the stability of the resultant radicals that would be formed when the bond breaks.  

Because radicals are electron deficient species (they are one electron short of an octet) they are stabilized by electron donating substituents (alkyl groups for example).  Hence, a tertiary radical is more stable than a secondary radical which is more stable than a primary radical which is more stable than a methyl radical.

Using this information we can see that 
$$\ce{R-CH2. + .CH2-R}$$
is more stable than
$$\ce{CH3. + .CH2-R}$$
Hence the internal bond will have a lower bond dissociation energy and break more readily.