Unraveling Sequence Effect on Glass Transition Temperatures of Discrete Unconjugated Oligomers

Sequence plays a critical role in enabling unique properties and functions of natural biomolecules, which has promoted the rapid advancement of synthetic sequence‐defined polymers in recent decades. Particularly, investigation of short chain sequence‐defined oligomers (also called discrete oligomers) on their properties has become a hot topic. However, most studies have focused on discrete oligomers with conjugated structures. In contrast, unconjugated oligomers remain relatively underexplored. In this study, three pairs of discrete oligomers with the same composition but different sequence for each pair are employed for investigating their glass transition temperatures (Tgs). The resultant Tgs of sequenced oligomers in each pair are found to be significantly different (up to 11.6 °C), attributable to variations in molecular packing as demonstrated by molecular dynamics and density function theory simulations. Intermolecular interaction is demonstrated to have less impact on Tgs than intramolecular interaction. The mechanistic investigation into two model dimers suggests that monomer sequence caused the difference in intramolecular rotational flexibility of the sequenced oligomers. In addition, despite having different monomer sequence and Tgs, the oligomers have very similar solubility parameters, which supports their potential use as effective oligomeric plasticizers to tune the Tgs of bulk polymer materials. A remarkable impact of monomer sequence on glass transition temperatures of discrete unconjugated oligomers is reported. Evidenced by experimental analysis and computational simulations, changing monomer sequence results in altered degree of twisting of the oligomer backbone and distinct rotational flexibility of the sequenced oligomers, which thus caused the variation of glass transition temperatures.