why disproportionation of free radicals does not happen in kolbe's electrolysis if we use secondary or tertiary carboxylic salt as substrate? In notes given by chemistry teacher, for secondary carboxylic salts the final products are alkane and ester. 



but i guess, as the reaction intermediates involve secondary/tertiary free radicals, some radicals should also go under disproportionation reaction and produce alkene.



why this doesn't happen?
Because of the free-radical-scavenging activity of the secondary/tertiary  carboxylic salts?

Hence, radicals that could possibly undergo disproportionation reactions resulting in alkene are removed.

As a possible example, see 'Evaluation of the Free-Radical-Scavenging Activity of Diclofenac Acid on the Free-Radical-Induced Haemolysis of Human Erythrocytes'.