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.
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