Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Radiation sensitive composition or product or process of making
Reexamination Certificate
1999-05-03
2001-01-16
Chapman, Mark (Department: 1753)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Radiation sensitive composition or product or process of making
C430S287100, C522S149000, C522S162000, C522S173000, C522S181000, C522S183000, C526S320000
Reexamination Certificate
active
06174645
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to photoreversible polymeric networks and, particularly, to polymers capable of undergoing photoinduced sol gel transitions.
BACKGROUND OF THE INVENTION
Polymeric networks formed through the action of UV/visible radiation are generally constructed via a free-radical reaction initiated by small-molecule photosensitive activators. These networks can be based on either crosslinkable, preformed polymers or by reactions of monomers with di- and tri-functional crosslinking agents. Crosslinked polymeric networks are found in a wide variety of industrial and medical uses.
Recently, there has been significant interest in synthesizing crosslinked hydrophilic polymers for various biological uses. Hydrogels (that is, crosslinked hydrophilic polymers) generally, because of their characteristic properties such as swellability in water, hydrophilicity, biocompatibility and lack of toxicity, have been utilized in a wide range of biological and medical applications. The anti-thrombogenic and biocompatibility properties of polyethylene glycol) (PEG), for example, have been extensively studied and are well established. In the case of existing PEG-based networks, the crosslinkable groups are traditionally acrylate or methacrylate groups, which readily polymerize in the presence of 365 nm radiation and the appropriate activators. The carbon-carbon bonds formed during the cross-linking reaction are relatively permanent, depolymerizing only at elevated temperatures which would likely destroy the entire material. In a number of applications, however, it would be beneficial to develop reversibly cross-linked hydrogels.
Although conventional chemical crosslinking has been extensively used as a hydrogel preparation method, relatively little work has been reported on the preparation of hydrogels via photopolymerization of water soluble polymers. Current photoinduced systems for hydrogel preparation include: (i) free radical polymerization initiated by long wave ultraviolet light or visible light of acrylate groups attached to water soluble polymers, and (ii) photopolymerization of photosensitive groups such as cinnamate, stilbazolium or coumarin which are pendant to the end of hydrophilic polymers. Although such work has demonstrated photopolymerization, there has been no demonstration of a truly photoreversible system. Indeed, only very limited photoreversibility has been observed.
In that regard, at least: one study has reported a photopolymerized system comprising a water soluble polymer having a cinnamate pendant group, but the extent of photoreversibility of that system, as observed by UV spectroscopy, was less than 25% (as measured by the change of the absorbance at 275 nm). In addition, the time of irradiation that was required to detect such reversible behavior was quite long (that is, on the order of 2 to 3 hours). Likewise, coumarin groups have been reported to produce photocrosslinked polymer systems based on polyoxazolines. A 55% conversion of the photoinduced dimer to the starting material has been reported for such polyoxazoline-based systems. The conversion was calculated from the UV absorbance spectrum after irradiation of the polyoxazoline gel with a low-pressure Hg lamp. The time of irradiation of the polyoxazoline-based systems was quite long, however. Moreover, only the photoreversible behavior of the polyoxazoline-based systems after a single cycle of irradiation was investigated.
It is very desirable to develop efficient photoreversible hydrogel systems. Indeed, it is very desirable to develop efficient photoreversible polymeric crosslinked systems generally (including hydrophobic systems).
SUMMARY OF THE INVENTION
Accordingly, the present invention provides crosslinked polymeric networks that are reversibly crosslinked upon exposure to light of a suitable wavelength. In one embodiment photocrosslinkable “branched,” “starburst” or other “tree-like” polymers containing photochromic groups are synthesized.
Preferably, photochromic groups for use in the present invention have relatively high quantum efficiency for photocrosslinking. Such photochromic groups also preferably exhibit a relatively high degree of photoreversibility. Cinnamylidene groups, for example, have similar chemistry to cinnamate groups, but have much higher quantum efficiency for photocrosslinking. Moreover, the crosslink between cinnamylidene groups of the present polymer systems has been found to be photoreversible in that the crosslink between two such groups forms upon exposure to one wavelength of energy and reverts to the original two groups upon exposure to a different wavelength of energy. To the contrary, cinnamate groups exhibit relatively low degree of photoreversibility. See
Photographic Sci. Eng.,
15:60 (1971).
Cinnamylidene groups and derivatives of cinnamylidene (for example, a cyano derivative) are thus preferably used as the photochromic agents or photocrosslinking agents. In general, any photochromic cinnamylidene derivative or moiety can be used, including, but not limited to the following: cinnamylidene acetyl chloride, &agr;-methylcinnamylidene acetyl chloride, &agr;,&ggr;-dimethylcinnamylidene acetyl chloride, &agr;-phenylcinnamylidene acetyl chloride, &agr;-phenoxycinnamylidene acetyl chloride, and cyanocinnamylidene acetyl chloride.
Generally the present invention provides a photosensitive polymer network synthesized from branched macromeric precursors. To create a crosslinked network, the branched macromeric precursors are preferably functionalized with at least three photochromic moieties. Functionalization, with less than three photochromic moieties results in a termination point. As set forth above, these photochromic moieties are preferably cinnamylidene moieties.
The double bonds of such cinnamylidene moieties undergo intermolecular cross-linking via a 2+2 cycloaddition upon exposure thereof to light of a known range of wavelengths (that is, wavelengths in excess of approximately 300 nm) to from the photosensitive polymer network. The photosensitive polymer networks formed in this manner are capable of undergoing a reversible photoscission upon exposure thereof to light of a second known range of wavelengths. The photoscission reaction occurs upon exposure of the photosensitive polymer network to light having a wavelength less than 300 nm. Such light is preferably in the range of approximately 244 nm to approximately 264 nm.
Preferably, the branched macromeric precursors used in the present invention have a molecular weight of at least approximately 400. Given the ease of functionalization of a specific macromeric precursor with a suitable photochromic crosslinking group, virtually any macromeric precursor can be used in synthesizing the present photoreversible polymeric networks. By way of example only, such macromeric precursors include polytethylene oxides), polyoxylates, polycarbonates, polyurethanes and polyacrylates. The branched macromeric precursors are also preferably functionalized with multiple photochromic moieties (for example, cinnamylidene moieties) such that the resultant functionalized precursor comprises no more than approximately 10 mol % of the photochromic moiety. More preferably, the branched macromeric precursors are functionalized with multiple photochromic moieties such that the resultant functionalized precursor comprises no more than approximately 5 mol % of the photochromic moiety. The content of the photochromic moiety in the functionalized precursor is preferably maintained relatively low such that the bulk properties of the precursor are not significantly affected by the functionalization. It has been discovered that in this manner, photoreversible polymeric networks having a wide range of physiochemical characteristics can be synthesized.
In a preferred embodiment, hydrophilic branched macromeric precursors are used to synthesize a photoreversible hydrogel. Examples of such hydrophilic macromeric precursors include, but are not limited to, one or more of the following: poly(ethylene glycol) (PEG)
Andreopoulos Fotios M.
Beckman Eric J.
Russell Alan J.
Wagner William R.
Bartony & Hare
Chapman Mark
University of Pittsburgh
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