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Heat transfer across thermal interface materials is a critical issue for microelectronics thermal management. Polydimethylsiloxane (PDMS), one of the most important components of thermal interface materials presents a large barrier for heat flow due to its low thermal conductivity. In this paper, we use molecular dynamics simulations to identify the upper limit of the PDMS thermal conductivity by studying thermal transport in single PDMS chains with different lengths. We found that even individual molecular chains had low thermal conductivities (κ ∼ 7 W/mK), which is attributed to the chain segment disordering. Studies on double chain and crystalline structures reveal that the structure influences thermal transport due to inter-chain phonon scatterings and suppression of acoustic phonon modes. We also simulated amorphous bulk PDMS to identify the lower bound of PDMS thermal conductivity and found the low thermal conductivity (κ ∼ 0.2 W/mK) is mainly due to the inefficient transport mechanism through extended vibration modes.