Stereochemical Regulation of Poly(methyl(3,3,3-trifluoropropyl)siloxane) via Monomer Ratio: Tuning Crystallization Behavior and Mechanical Properties

The stereochemistry of polymers governs their macroscopic mechanical and thermodynamic properties by regulating chain packing and crystallization behavior. Although the stereochemistry–structure–property relationship has been well-elucidated in symmetrically substituted polysiloxanes, it remains highly complex in asymmetrically substituted counterparts due to the propensity for siloxane backbone rearrangement. Taking asymmetrically substituted poly(methyl(3,3,3-trifluoropropyl)siloxane) (PMTFPS) as a model system, this study quantitatively characterizes its stereochemical configuration using ¹⁹F NMR spectroscopy and systematically elucidates the relationship between the feed ratio of cis-1,3,5-tris(3,3,3-trifluoropropyl)methylcyclotrisiloxane (cis-D₃F) and the stereoregularity of PMTFPS. The results reveal that when the cis-D₃F content exceeds 58%, the resulting PMTFPS exhibits typical features of isotactic polymers and displays semicrystalline behavior. With increasing isotacticity, the d-spacing of the crystalline phase gradually decreases, and the crystal morphology transitions from fibrous and lamellar structures to spherulitic domains under nonisothermal crystallization. Further mechanical testing demonstrates that silica-reinforced PMTFPS elastomers with high stereoregularity exhibit a pronounced self-reinforcement effect, attributed to enhanced strain-induced crystallization (SIC). This work not only uncovers the intrinsic correlations among stereochemical configuration, crystalline structure, crystallization kinetics, and macroscopic properties of PMTFPS but also offers valuable guidance for the stereochemical regulation and molecular design of high-performance asymmetrically substituted polysiloxanes.