Why n-alkanes can be found in organic solvent extract of rubber?
Although natural rubber and synthetic rubber display a poor chemical resistance to alkanes among others, they do not  dissolve in alkanes. Your GC/MS traces (I believe that so because of alkane/alkene identification) have confirmed that since they have shown n-heptane as the main peak (I believe everything else are trace amounts exhibiting below your set-up threshold). 

There is a reason for your finding of straight chain hydrocarbons in synthetic rubber such as SBR in your solvent extracts. For example, SBR is the shorten form for synthetic material, styrene butadiene rubber, which is made by anionic copolymerization of styrene &amp; butadiene. For this anionic copolymerization, frequently used initiator is a solution of lithium alkyl compound in small paraffins, for instance, n-butyllithium in hexanes. The rubber may still have few molecules of these small paraffins, which may diffuse into the n-alkane you use to extract, and hence, would show up in your GC/MS chromatogram.