Thermomechanical properties of conjugated polymers (CPs) are greatly influenced by both their microstructures and backbone dynamics. In the present work, to investigate the effect of the side-chain branch position on the backbone's mobility and molecular packing structure, four poly (3-alkylthiophene-2,5-diyl) derivatives (P3ATs) with different side chains, either branched or linear, were synthesized by a quasi-living Kumada catalyst transfer polymerization (KCTP) method. The side-chain branch position greatly influences the glass transition temperature (T_g) of the backbone of P3ATs as well as the melting temperature (T_m), as measured by dynamic mechanical analysis (DMA) and differential scanning calorimetry (DSC) respectively. Placing the branching point closer to the conjugated backbone leads to increased backbone T_g and T_m. Also, according to grazing incidence wide-angle X-ray scattering (GIWAXS) results, branching closer to the backbone causes tighter packing in the side-chain direction. The tighter packing along the side-chain direction coresponds with the higher T_m, which decreases the free volume of the polymer system and subsequently increases the T_g at the same time. This work provides the first in-depth understanding of branch point influences on the thermal properties of poly(3-alkylthiophenes). It would provide guidance to the future development of side-chain engineering on next-generation CPs with desirable thermomechnical properties for stretchable and wearable electronics.