Effects of Hydride Transfer Ring-Opening Reaction on B(C₆F₅)₃ Catalyzed Polymerization of D₄^H Cyclosiloxane and Dialkoxysilanes toward Thermally Stable Silsesquioxane–Siloxane Hybrid Materials

Abstract

The Piers–Rubinsztajn (PR) reaction catalyzed by a metal-free B­(C6F5)3 catalyst was reported as efficient in synthesizing novel polysiloxanes through polycondensation of dialkoxylsilanes and dihydrosiloxane monomers at room temperature. This study is aimed at developing new cyclosiloxane polymers having no hydrocarbon linker in the main chain via an equimolar PR reaction between tetrafunctional eight-membered cyclosiloxanes (TMCS) having the four reactive silyl hydrides (Si–H) at the Si vertices and bifunctional dialkoxylsilanes. The final polymers were targeted with the remaining Si–H group at the Si vertices of the TMCS repeating units for post-functionalization, good solubility in typical organic solvents for post-processing, and a stable ring structure in TMCS for outstanding macromolecular flexibility. However, hydride transfer ring-opening polymerization (HTRP) of TMCS was reported in the B­(C6F5)3-catalyzed reaction system, which invariably causes uncontrollable gelation. Suppression of HTRP succeeded via control of total monomer concentration and reaction time reduction, thereby obtaining post-processable and post-functionalizable liquid cyclosiloxane prepolymers. Thermal curing of the prepolymers through self-cross-linking of the remaining reactive Si–H groups gave rise to free-standing films with excellent thermal stability exceeding 600 °C and a low dielectric constant. The functional versatility of the Si–H groups produces promising prepolymers for creating various functional materials.

Nanosheets Based on Hard 2-Ureido-4[1H]-pyrimidone Units and Soft Cyclosiloxane Units as Membrane Materials

“…Results show that the onset decomposition temperature of HMCS-UPy became 24 °C higher after the attachment of cyclosiloxane. With the hybrid cyclosiloxane unit, the multiple-hydrogen-bonded molecules were demonstrated to have improved thermal properties, probably caused by the unique thermal stability of cyclosiloxane. , …”

“…With the hybrid cyclosiloxane unit, the multiplehydrogen-bonded molecules were demonstrated to have improved thermal properties, probably caused by the unique thermal stability of cyclosiloxane. 43,44 The DSC curves for four heating−cooling cycles of UPy-3 and HMCS-UPy are shown in Figure 2b,c. In the first scanning curves of UPy-3 (black, Figure 2b) and HMCS-UPy (black, Figure 2c), one peak (46 °C for UPy-3 and 39 °C for HMCS-UPy) and one sharp endothermic peak (153.8 °C for UPy-3 and 138.1 °C for HMCS-UPy) can be found, corresponding to the dissociation temperature of urea groups 45 and melting of UPy dimers.…”

Anchoring Effect of Hyperbranched Carborane in Highly Cross-Linked Cyclosiloxane Networks toward High-Performance Polymers

“…Measurements and Analysis. 1 H nuclear magnetic resonance ( 1 H NMR), 11 B nuclear magnetic resonance ( 11 B NMR), and 13 C nuclear magnetic resonance ( 13 C NMR) spectral analyses were performed on a Bruker AVANCE III Ultra-Shield (Bruker, Fallanden, Switzerland) spectrometer. Tetramethylsilane (TMS) was used as the internal reference material in CDCl 3 solutions at 25 °C.…”

“…A colorless transparent liquid was obtained as the product (90% yield) after column chromatography on a silica gel column using hexane. (4,5,7,8,9,10,11,12). This known compound was fully characterized previously.…”

“…Recently, a cyclosiloxane hybrid polymer (CHP) was fabricated from tetra-functional monomers. CHP has gained much attention due to its molecularly smooth highly cross-linked hybrid network, thermal stability, and hard but elastic nature. − Compared to linear polysiloxane, CHP has better thermal stability due to reduction in “back-biting” of linear siloxane chains . Previous studies have shown the nano-size structure of CHP to be layered but isotropic in bulk due to different orientations of nano-sized layers .…”