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.
Qian Zhou,Ailian Wang,Huayi Li,Zhi Luo,Tao Zheng,Liaoyun Zhang,Youliang Hu
Five salicylates with different sizes of hydrocarbon substituents were firstly synthesized and employed as ecofriendly internal donors of the Ziegler–Natta catalyst for propylene polymerization. The influences of these salicylates and traditional, industrial, internal donor diisobutyl phthalates on the microstructure of polypropylene and active center in a Ziegler–Natta catalyst were studied. It was found that the catalyst activities of the catalysts containing salicylate internal donors with a proper volume were higher than the catalysts containing diisobutyl phthalate internal donors. GPC results showed that the molecular weights of polypropylene prepared by salicylate internal donors were lower than those prepared by diisobutyl phthalate, which indicated that the polypropylene chains produced by salicylate internal donors were easier to transfer than those prepared by diisobutyl phthalate internal donors. Deconvolution of the GPC curves exhibited that as the volume of the salicylate internal donor increased some of the active centers for low molecular weight transferred into the active centers for high molecular weight. The results of 13C-NMR and SSA both suggested that a salicylate internal donor with an appropriate catalyst size volume was beneficial for increasing the isotactic sequence length, isotacticity index and regular triads “mm” of polypropylene. However, further increasing the volume of the salicylate internal donor in a catalyst would lead to the polypropylene chain containing more stereo-defects. Moreover, the active centers with different stereospecificity parameters, piso, in the catalyst could explain the trend of stereo-defects in polypropylene chains when different internal donors were used. In addition, it was found that the isotactic sequence length and isotacticity index of polypropylene prepared by isobutyl 2-benzyloxy-3,5-isopropyl benzoate were close to that produced by a diisobutyl phthalate internal donor. Moreover, the lamella thickness distribution of the polypropylene produced by a salicylate internal donor was broad, which might have potential application for expanded polypropylene materials.
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