Ring-Expansion Metathesis Polymerization under Confinement

Area of Science:

• Organometallic Chemistry
• Polymer Chemistry
• Materials Science

Background:

• Catalysis using transition metal complexes is crucial for polymer synthesis.
• Confining catalysts within porous materials can alter reactivity and selectivity.
• Ordered mesoporous silica (OMS) offers tunable pore sizes for catalyst immobilization.

Purpose of the Study:

• To immobilize a cationic molybdenum alkylidyne N-heterocyclic carbene (NHC) complex within OMS.
• To investigate the effect of pore confinement on ring-expansion metathesis polymerization (REMP) of cyclic olefins.
• To explore the influence of confinement on polymer molecular weight and stereoselectivity.

Main Methods:

• Synthesis and characterization of the molybdenum NHC complex [Mo(C-p-OMeC6H4)(OCMe(CF3)2)2(IMes)][B(ArF4].
• Immobilization of the complex into OMS with varying pore sizes (66, 56, and 28 Å).
• Ring-expansion metathesis polymerization (REMP) of cyclic olefins including cis-cyclooctene (cCOE), 1,5-cyclooctadiene (COD), (+)-2,3-endo,exo-dicarbomethoxynorborn-5-ene ((+)-DCMNBE), and 2-methyl-2-phenylcycloprop-1-ene (MPCP).
• Analysis of polymer products using matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry.

Main Results:

• Selective immobilization of the molybdenum complex within OMS pores.
• Observation of a strong confinement effect on REMP, leading to low-molecular-weight cyclic polymers even at high monomer concentrations.
• Exclusive formation of cyclic polymers confirmed by MALDI-TOF.
• Confinement-induced enhancement of Z-selectivity and cis-syndiospecificity.

Conclusions:

• Confinement of molybdenum NHC catalysts in OMS is an effective strategy for controlling REMP.
• The pore size of OMS significantly influences the polymerization outcome, enabling the synthesis of cyclic polymers with tailored properties.
• This approach offers a pathway to precisely control polymer architecture and stereochemistry through catalyst confinement.