To systematically address the intriguing question of how siloxane termini of the side chains relative to alkyl terminal groups affect the various inherent properties of conjugated polymers including optical, electrical, and morphological characteristics we have synthesized model polymers (PDPPTT-R-TG and PDPPTT-Si-TG, together with an accompanying reference PDPPTT-ref) containing an identical backbone yet possessing different terminal groups. In order to fairly compare the end functionalities by eliminating molecular weight (M-n) and polydispersity index (PDI) variations that may act as complicating factors, the polymers used in this study have been controlled to have similar M-n and PDI by carefully optimizing the catalyst system and reaction conditions. Although the molecular packing and orientation behaviors of PDPPTT-R-TG and PDPPTT-Si-TG are very different from each other, both polymers exhibit very high mobility exceeding 4.5 cm(2) V-1 s(-1) More meaningfully, organic field-effect transistors (OFETs) based on PDPPTT-Si-TG are highly stable over extended periods in humid environments. Our findings provide new insights into the molecular design strategy aimed at the simultaneous enhancement of charge-carrier mobility and ambient stability, which is of great importance for practical OFET applications.