Patent Application
20220112332
The present disclosure is related generally to frontal polymerization and more particularly to a method of initiating frontal ring-opening metathesis polymerization (FROMP) with light.
Bulk polymerizations convert neat or highly concentrated monomeric mixtures to polymers in an exothermic fashion while providing minimal contamination of products. The resulting polymeric materials typically display both high optical clarity and molecular weights, without requiring significant post-polymerization purification or processing. Controlled bulk polymerizations may thus have reduced economic and environmental impacts on both company and consumer. As a result, bulk polymerization is a ubiquitous technique employed in a variety of industrial settings. Several key drawbacks exist, however, that directly stem from the associated reaction exothermicity and activation energy. Uncontrolled heat dissipation, for example, results in localized hot spots, which may degrade the material or initiate auto-acceleration phenomenon (i.e., gel-effect). Elaborate reaction vessels equipped with mixing apparatuses may circumvent these effects, although with a sizable capital investment cost.
Frontal polymerizations (FPs) are a useful subset of bulk polymerizations which harness the exothermic and auto-accelerating events in a controlled fashion to enable productive and rapid monomer consumption. Highly localized and short lived external stimuli activate a latent initiator or catalyst to induce a reaction zone (i.e., front). The resultant heat of polymerization promotes further reactions to rapidly polymerize available monomer. While thermal initiation is easily implemented with a soldering iron, poor spatial and temporal control limits this process by requiring close contact to the heat source.
A method of light-promoted frontal ring-opening metathesis polymerization includes providing a monomer solution including dicyclopentadiene, a ruthenium-based catalyst and a phosphite inhibitor, and exposing the monomer solution to ultraviolet light having a wavelength in a range from 315 nm to 400 nm, thereby initiating an exothermic polymerization reaction and generating a self-propagating polymerization front. The polymerization front moves through the monomer solution and polymerizes the dicyclopentadiene.
A UV-A initiated strategy for frontal polymerization is described in this disclosure. Light is an attractive stimulus to achieve spatial and temporal control over reaction events in a non-contact manner. Referring to
Surprisingly, the monomer solution 102 does not require a photoinitiator. This is noteworthy because the ruthenium-based catalyst 106 is not known to be photoactive and has not been designed for photoactivity. Nor has a mechanism for the photoactivity of this catalyst been previously postulated. The inventors believe that exposing the monomer solution 102 to ultraviolet light 110 may induce phosphine or phosphite dissociation from the ruthenium-based catalyst 106, as illustrated in
The ultraviolet light 110 may be emitted by a UV source 118 such as a lamp, a light emitting diode (LED), or a laser. The wavelength of the UV-A light is typically in the range from about 365 nm to about 375 nm. The ultraviolet light 110 may be focused on or beneath a surface of the monomer solution. In some cases, the ultraviolet light 110 may be emitted by multiple UV sources. It is contemplated that interference effects may be employed to generate multiple polymerization fronts in the monomer solution 102.
Preferably, the exposure to ultraviolet light 110 takes place shortly after preparation of the monomer solution 102 to prevent premature curing or gelation. For example, the exposure may take place within one or two hours of preparing the monomer solution 102. Preparation of the monomer solution 102 may comprise forming a mixture of the DCPD 104, the ruthenium-based catalyst 106 and the phosphite inhibitor 108. The mixing may entail mechanical stirring and/or sonication. The mixture may optionally include an organic solvent. For example, prior to mixing with the inhibitor 108 and the DCPD 104, the catalyst 106 may be dispersed and/or dissolved in an organic solvent. The preparation may take place at room temperature (e.g., 20-25° C.). In some cases, before or after mixing, the monomer solution may be passed through basic alumina for purification. Due to the liquid state of the monomer solution 102 prior to polymerization, the monomer solution 102 may have a storage modulus of less than 1 Pa. The monomer solution 102 may be contained in a mold having a predetermined shape or another container for frontal polymerization. The poly(DCPD) 114 formed upon polymerization may have a size and shape determined by the mold or container, and optionally may be further processed (e.g., by machining or cutting) to a desired size and shape.
The ruthenium-based catalyst 106 may comprise Grubbs' second-generation catalyst (GC2) or another Grubbs' ruthenium catalyst, examples of which are shown in
Varying GC2 and phosphite inhibitor loadings from 350 ppm to 12.5 ppm are found to result in large differences in the ability to initiate frontal polymerization. Table 1 summarizes results for frontal polymerization performed under the standard conditions shown in
The choice of phosphite inhibitor appears to have little influence on frontal polymerization initiation via heat or light. Initiation times do vary slightly between P(NEt2)3, which has the fastest initiation, and P(OiPr)3, which shows the slowest initiation time. Light initiation is not observed using P(OiPr)3 as the inhibitor, even after two hours of exposure, but instead leads to significant bulk curing. The absence of a phosphorous inhibitor altogether results in immediate reaction and gelation before all components are homogeneously mixed. Typically, the phosphite inhibitor has a loading level in the monomer solution equal to that of the ruthenium-based catalyst.
The energy of the incident photo stimulus affects the efficacy of FROMP. Changes to the wavelength and power density may cause a significant variation in the initiation time. Experiments reveal that higher power densities are associated with faster initiation times. A 375 nm LED at 318 mW/cm2 may initiate FROMP within about 6 minutes, as shown in
A series of control experiments are carried out to delineate the cause of initiation as a photo, photo-thermal, or thermal effect, as shown by data of
The polymerization front moves through the monomer solution and polymerizes the DCPD rapidly after initiation of the polymerization reaction. The speed of the polymerization front may be at least about 0.9 mm/s, at least about 1.3 mm/s, or at least about 1.7 mm/s, and/or as high as about 2.1 mm/s, as shown by the data of
Nearly all polymerization characteristics and polymer thermomechanical properties of the light-activated pDCPD are comparable to those of pDCPD produced by thermally initiated FROMP. Front speeds increase from 0.8 mm/sec to 1.75 mm/sec with increasing catalyst and inhibitor concentration, consistent with what is observed in the thermal case. The glass transition temperatures (Tg) of the various GC2 loadings are comparable for both initiation methods and are higher than previous reports (Tg about 140° C.) since a solvent-free system is used in this investigation. Regardless of the stimulus used for initiation of frontal polymerization, the final polymer product properties may not be significantly affected.
To determine if the heat generated during light-promoted FROMP is sufficient to initiate FROMP on its own, differential scanning calorimetry (DSC) is used by running isothermal holds at a range of temperatures for 20 minutes. The minimum temperature for thermal FP initiation is 50° C. (DSC), while the temperature observed for initiation using a 375 nm LED at 318 mW/cm2 is 39° C. (thermocouple). In contrast to thermally-induced frontal polymerization, a temperature of the monomer solution remains less than 50° C., or less than 40° C., prior to initiation of the exothermic polymerization reaction. Since the irradiated sample never reaches 50° C. before frontal polymerization occurs, initiation promoted by photoexcitation of GC2 is highly likely. This finding is consistent with light-promoted phosphine or phosphite dissociation.
Although the present invention has been described in considerable detail with reference to certain embodiments thereof, other embodiments are possible without departing from the present invention. The spirit and scope of the appended claims should not be limited, therefore, to the description of the preferred embodiments contained herein. All embodiments that come within the meaning of the claims, either literally or by equivalence, are intended to be embraced therein.
Furthermore, the advantages described above are not necessarily the only advantages of the invention, and it is not necessarily expected that all of the described advantages will be achieved with every embodiment of the invention.
The present patent document claims the benefit of priority under 35 U.S.C. § 119(e) to U.S. Provisional Patent Application Ser. No. 63/091,403, which was filed on Oct. 14, 2020, and is hereby incorporated by reference in its entirety.
This invention was made with government support under FA9550-16-1-0017 awarded by the Air Force Office of Scientific Research (AFOSR). The government has certain rights in the invention.
| Number | Date | Country | |
|---|---|---|---|
| 63091403 | Oct 2020 | US |
