Hemisilicone Elastomers Containing Tactical Defects

The mechanical properties of a series of well-defined covalent network elastomers based on a cross-linked linear carbosilane (hemisilicone) prepolymer were studied. Prepolymers with the repeat unit structure -SiMe₂-O-SiMe₂-CH₂CH₂-(M₂E) are prepared by well controlled ring-opening polymerization of the strained 5-membered ring monomer c-M₂E with precise molecular weights and end-group functionality. Cross-linking of monodisperse PM₂E resins renders materials that closely resemble polydimethylsiloxane (PDMS)─based elastomers in many respects but are unexpectedly and vastly superior to them in terms of mechanical performance. We ascribe the enhanced properties to the absence of defects in PM₂E networks that are unavoidable in the PDMS system using comparably monodisperse prepolymers and identical chemistries. The defect-free structurally uniform PM₂E network elastomers serve as benchmarks and points of reference to compare networks that were intentionally prepared to contain defects of known structure and content. The effects of strand length distribution between cross-links, short chain prepolymers, monofunctional prepolymers, and nonfunctional polymers were studied. The precision with which PM₂E-based elastomers can be prepared permits the preparation of structurally well-defined samples that theories can be evaluated with, that structurally unknown networks can be compared against, and moreover, that exhibit targeted properties resulting from deliberately incorporated network defects.