Zwitterionic polymerization to generate high molecular weight cyclic poly(carbosiloxane)s

Area of Science:

• Polymer Chemistry
• Organosilicon Chemistry
• Catalysis

Background:

• Ring-opening polymerization (ROP) is a key method for synthesizing polymers.
• N-heterocyclic carbenes (NHCs) are potent nucleophiles and catalysts for various organic transformations.
• Cyclic siloxanes are versatile building blocks for advanced materials.

Purpose of the Study:

• To investigate the NHC-mediated zwitterionic ring-opening polymerization of a cyclic siloxane monomer, 2,2,5,5-tetramethyl-2,5-disila-1-oxacyclopentane (TMOSC).
• To explore the effect of NHC nucleophilicity on polymerization kinetics and polymer properties.
• To characterize the resulting poly(carbosiloxane)s and elucidate the polymerization mechanism.

Main Methods:

• Zwitterionic ring-opening polymerization of TMOSC using two different NHC catalysts (IMes and a more nucleophilic analogue).
• Characterization of polymers using Gel Permeation Chromatography (GPC) and Matrix-Assisted Laser Desorption/Ionization-Time of Flight Mass Spectrometry (MALDI-TOF MS).
• Dilute solution viscosity studies and Density Functional Theory (DFT) calculations to determine polymer structure and reaction mechanism.

Main Results:

• NHC-mediated ROP of TMOSC achieved high molecular weight poly(carbosiloxane)s (Mn up to 940,000 Da) rapidly at room temperature.
• Polymerization rates and molecular weights were significantly influenced by the nucleophilicity of the NHC catalyst.
• The resulting polymers were predominantly cyclic, confirmed by viscosity and mass spectrometry data.
• DFT calculations supported both zwitterionic and neutral cyclic intermediates in the polymerization pathway.

Conclusions:

• NHC catalysis provides an efficient route for the controlled synthesis of high molecular weight cyclic poly(carbosiloxane)s from TMOSC.
• The choice of NHC catalyst is crucial for controlling polymerization kinetics and achieving high molecular weights.
• The predominantly cyclic nature of the polymers opens avenues for their use in advanced material applications.