Rapid Controlled Synthesis of Large Polymers by Frontal Ring-Opening Metathesis Polymerization

Abstract

Tunable molecular weight and well-defined polydispersity are the hallmarks of a controlled polymerization. This process relies on vanishingly small termination rates, minimal chain transfer, and initiation rates faster than propagation rates. Ring-opening metathesis polymerization (ROMP) is a well-known controlled polymerization based on the opening of strained cyclic olefins. The exothermic nature of ROMP allows rapid conversion of neat monomers to polymers through frontal ROMP (FROMP). Unlike traditional ROMP, FROMP uses the exothermic heat from the opening of strained cyclic olefins to thermally activate the initiator that sustains the propagation of a cascading reaction front. Although the reaction mechanisms for ROMP and FROMP are the same, the reaction conditions differ greatly, especially in the temperature and monomer concentration. The ability to control the polymerization under FROMP conditions has yet to be investigated, as well as its potential in the synthesis of well-defined polymers without the use of solvents and with minimal energy input. Here, we show that FROMP rapidly transforms monomers into polymers of high-molecular weight (M n ) with good fidelity and low dispersity (Đ). Specifically, the synthesis of polymers with M n up to 700 kg/mol and Đ of 1.5 was achieved with a rapid, solvent-free, and oxygen-tolerant frontal polymerization technique. Further control of the polymerization was possible with the addition of a phosphite ligand that lowered the Đ to 1.2. We anticipate that controlled FROMP will become a valuable macromolecular synthetic tool due to its reliability, speed, scalability, and simplicity.

Frontal Polymerizations: From Chemical Perspectives to Macroscopic Properties and Applications

“…While most studies of FROMP focus on the fabrication of thermoset materials, several examples exist that afford linear polymers in the absence of DCPD . As examples, successful FROMP occurs with COD or partially hydrogenated DCPD bearing a pendent cyclopentane instead of a cyclopentene (i.e., 2,3,3 a ,4,7,7 a -hexahydro-1 H -4,7-methanoindene) . The resultant soluble polymers enable traditional characterization methods to deduce aspects of the polymer’s microstructure and molecular weight.…”

“…These elevated temperatures may result in competitive catalyst decomposition reactions (i.e., nonliving polymerization), though the details of such processes are not yet understood. Recent work from Alzate-Sanchez et al 393 suggests that the metathetic activity of G2 after FROMP is retained; the catalyst survives FROMP and can induce chainextension when fresh monomer is introduced.…”

“…As examples, successful FROMP occurs with COD or partially hydrogenated DCPD bearing a pendent cyclopentane instead of a cyclopentene (i.e., 2,3,3a,4,7,7a-hexahydro-1H-4,7-methanoindene). 393 The resultant soluble polymers enable traditional characterization methods to deduce aspects of the polymer's microstructure and molecular weight. Perhaps unsurprisingly these linear polymers exhibit M n in the range of 10 5 to 10 6 Da; while one may be tempted to invoke a mechanistic argument, we suspect that such observations simply result as a function of the formulation composition.…”

Frontal Polymerization of Stereoisomers: Distinct Microstructures and Properties in Endo- and Exo-Dicyclopentadiene Thermosets

“…Our group and others have thoroughly described the resin formulation space in the frontal ring-opening metathesis polymerization (FROMP) of DCPD catalyzed by Grubbs-type complexes (Scheme ). ,− Mechanical properties and morphologies of materials derived from FROMP depend on the resin formulation. Different catalysts, , inhibitors (to mitigate premature background reactivity), ,, monomers/cross-linkers, ,,,, additives, ,, and triggering stimuli , all influence the polymerization process as well as the thermomechanical and chemical properties of the resultant polymers.…”

“…Nevertheless, some circumstantial conclusions exist based on analogy to known E/Z-selectivities in G2-catalyzed CM reactions 45,46,52−54 as well as prior FROMP reports with partially hydrogenated DCPD (exo-DCPD-H 2 ). 19 Prior work demonstrated that NHC-containing Grubbs catalysts (e.g., G2) favor the production of E-over Z-olefins. 45,46,52−54 Based on this, we expect that the E/Z ratio within the backbone of o(DCPD) is greater than unity.…”