Patent Application
20070244274
Styrene polymerization was carried out using phenyl verdazyl (see
These results show that this verdazyl is at least as effective as nitroxides in controlling styrene polymerization. GPC (Gel Permeation Chromatography) traces of MW evolution for a typical polystyrene runs are shown in
General synthetic routes to verdazyl radicals have been established for some time. There are two principal strategies depicted in
Verdazyls of Type 1 containing an sp3 carbon center at C6 can be made using the chemistry shown in
Mono-substituted hydrazines (B) are converted to hydrazones (G) and subsequently formazans (H) via diazonium salts; these work best for aryl groups, though N,N-alkyl substituted formazans are also possible via related chemistry. From here formazan alkylation leads to tetrazines (J), which are aerobically oxidized to give the radicals (I). It also is possible to access verdazyl via cationic heterocycles (K) followed by reduction. Again, these radicals are stable enough to be manipulated without any special precautions.
Typical SFRP processes are run under similar conditions for conventional radical polymerization (monomer, few mol % initiator, heat, no O2) but with the stable radical added as well. There are several variables—reaction temperature, the stable radical, the initiator, relative concentrations—that can be systematically varied to provide living polymerization character (molecular weight control as evidenced by linear growth versus monomer conversion, PDIs well below 1.5, chain extension). In the proposed work, a wide range of verdazyl agents that are moderately unstable will be used in the reaction rather than a stable free radical. In addition, the reactions will be conducted at as low a temperature as possible (ideally less than 100° C.), at a reasonable rate (defined as upwards of 100% conversion in 6 hours or less). These conditions are not currently achievable in the nitroxide-based SFRP.
Two polymer systems will be of primary focus. Nitroxide SFRP is now well-established for styrene-based polymers, and provides a valuable set of standards to which we can compare the verdazyl-based processes. This also permits fundamental studies aimed at getting a clearer picture of the factors that affect the verdazyls' efficacy in SFRP. The other systems are acrylate- and methacrylate-based polymers, represented by poly(n-butyl acrylate) and poly(methyl methacrylate) respectively.
Our results indicating that verdazyl derivative (A) (see
Our results also indicate that imidazole derivatives of verdazyls are effective and may work better than the phenyl derivative in that the reaction is slower but more controlled.
The SFRP behavior of (B) with both styrene as well as n-butyl acrylate and methyl methacrylate will be studied. An important derivative targeted for study will be radicals (C) and (D), which are closely related to both 13 and 14; these four radicals provide all perturbations of whether the N-substituents are alkyl or aryl and whether C6 is a carbonyl or an sp3 center. Comparisons between all three of these species will allow us to elucidate some of the specific structural factors that govern polymerization capabilities.
It is well-established that C3-substituents have very little effect on the electronic structure of verdazyl radicals because of the nature of the singly occupied molecular orbital. However, the nitrogen substituents can have stronger effects on the spin distribution and redox characteristics. To this end, we will prepare and study triarylverdazyl radicals having Ar=p-Me2NC6H4 or Ar=p-O2NC6H4 (see
Steric factors play a significant role in affecting the nitroxide-polymer bond through kinetic and/or thermodynamic stabilization of the stable free radical. Similar effects in analogous verdazyl chemistry are anticipated. To this end we will prepare verdazyls bearing bulkier substituents in the 1,3,5-positions, such as t-butyl or ortho-disubstituted aryl (e.g. mesityl).
As described above, the polymerization runs are typically carried out under established SFRP conditions (i.e., monomer+x mol % radical+y mol % initiator). Complementary to these studies will be the synthesis and study of so-called “unimers”-unimolecular precursors based on the stable radical coupled to one monomer unit that are, upon dissociation, capable of acting as both initiator and SFRP mediator. Nitroxide-based “unimers” have been developed as single-component initiator/SFRP mediators, and they also have been useful as model systems with which to study the nature of the radical-polymer bond through studies of bond dissociation energies, polymerization rates, and byproduct formation. The utility of unimers as model compounds is the primary motivation behind the synthesis and study of verdazyl-based unimers having the structures shown in
We will explore the possibility of improving the SFRP process with verdazyls through the use of chemical additives. It is known that during the course of SFRP reactions, small amounts of termination reactions occur which gradually and irreversibly consume active polymer chains. This leads to a buildup of excess radical (nitroxide) which shifts the equilibria in
The foregoing description of a method of synthesizing polymers using moderately unstable verdazyls describes the preferred methods and is not meant to be limiting. As would be apparent to one skilled in the art, there can be, for example, variation in the range of temperatures, additives, and R groups.
This application claims the benefit of the earlier filing date of U.S. patent application No. 60/793,109, entitled Verdazyl Agents for the Production of Free Standing Polymers, which was filed on Apr. 18, 2006, and is incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 60793109 | Apr 2006 | US |
