Photodegradable polymers constitute an emerging class of materials that finds numerous applications in biotechnology, biomedicine, and nanoscience. This article highlights some of the emerging applications of photodegradable polymers in the form of homopolymers, particles and self-assembled constructs in solution, hydrogels for tissue engineering, and photolabile polymers for biopatterning applications. Novel photochemistries have been combined with controlled polymerization methods, which result in well-defined photodegradable materials that exhibit light mediated and often controlled fragmentation processes.

Photodegradable polymers for biotechnological applications.

Polymers containing allylic and benzylic carbonate repeating units have been prepared by phase-transfer catalyzed polycondensation of activated bis-carbonates or carbamates and diols. The polymers are highly susceptible to thermal depolymerization and revert to small molecules when heated to temperatures which vary from 140 to 230°C depending on structure. The thermolysis temperatures are reduced to well below 100°C if catalytic amounts of acid are added to the polycarbonates. The thermolysis or acidolysis of bis allylic or benzylic carbonates provides a convenient route to aromatic compounds as demonstrated with both models and polymers. The polycarbonates can be used to formulate highly sensitive resist materials with potential for self-development of positive images. Some benzylic polycarbonates which produce polymerizable divinyl monomers upon thermolysis can be used to create negative images in a process which includes both depolymerization and photocrosslinking.

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Thermally depolymerizable polycarbonates. V: Acid catalyzed thermolysis of allylic and benzylic polycarbonates: a new route to resist imaging

Poly(4-hydroxybutyl vinyl ether) [poly(HOBVE)] and the related copolymers, which showed thermally induced phase separation in water, are synthesized via living cationic copolymerization. When the aqueous solution of copoly(HOBVE) is heated, coacervate droplets are observed in micrograph. This is the thermoresponsive copolymer exhibiting coacervation accompanied by the liquid−liquid-phase separations though unlike precipitation. To obtain hydrogel microspheres from the droplets, highly random copolymers of HOBVE and 2-(vinyloxy)ethyl methacrylate (VEM), that is poly(HOBVE1-x-r-VEMx), are synthesized by sequential living cationic copolymerization, which possess evenly photofunctional units. The initially formed coacervate droplets slightly above phase separation temperature can be cross-linked by UV (254 nm) irradiating in water, to form fine hydrogel microspheres. The obtained hydrogel microspheres show the thermoresponsiveness and adsorption/desorption effect, whose size and form are confirmed by the meas...

Synthesis of Fine Hydrogel Microspheres and Capsules from Thermoresponsive Coacervate

Vinyl polymers containing pendant acetal groups were synthesized using (2,2-dimethyl-l,3-dioxolan-4-yl)methyl acrylate (DMA) and (2,2-dimethyl-l,3-dioxo-lan-4-yl)methyl methacrylate (DMM), and were evaluated as negative electron beam (EB) resists. It was found that the EB sensitivity of polymers containing acetal groups in the side chain was higher than that of polymers containing acetal groups in the main chain. A high sensitivity of 3.6 × 10−8 C/cm2 was observed. Copolymers of DMA or DMM with styrene were also synthesized in order to improve the durability for dry etching process. It was found that the copolymers had an excellent dry etching durability and were adaptable to EB lithography.

Relationship between electron sensitivity and chemical structures of polymers as electron beam resist. VII: Electron sensitivity of vinyl polymers containing pendant 1,3-dioxolan groups

Vinyl ethers having oxyethylene units [CH2=CHO-(CH2CH2O)-nC2H5, n=1–4] were cleanly polymerized by the HI/I2 initiator in a nonpolar solvent (toluene) at low temperatures (−15 and −40°C) to yield living-like polymers with a very narrow molecular weight distribution (Mw/Mn ≤1.2); the oxyethylene units in the monomers hardly disturbed the polymerization. The number-average molecular weight of the polymers increased proportionally to monomer conversion and the monomer-to-HI feed ratio. The polymerization rates of the (poly)ether-containing vinyl ethers were much greater than those of alkyl vinyl ethers under similar conditions, and it is presumed that the pendant ether oxygens intramolecularly activate the growing end. The polymers were soluble in methanol (with n≥1) and in water (with n≥2), depending on the number of oxyethylene units in the pendant.

Living cationic polymerization of vinyl ethers with a functional group

Poly (Fluoro Methacrylate) as Highly Sensitive, High Contrast Positive Resist

High sensitivity and high contrast electron negative resist, chloromethylated polystyrene (CMS) was developed for direct writing electron beam lithography with 1-μm resolution. The resist shows excellent lithographic performances such as high plasma-etching durability and negligible “post polymerization effect”. A series of CMS covering a wide range of Mw, 6,800–560,000, were synthesized by the chloromethylation of nearly monodisperse polystyrenes. The effects of molecular parameters on sensitivity and resolution were investigated. The chloromethylation remarkably improved the reactivity of polystyrene, but which was saturated above 40% of chloromethylation ratio. About 100 times higher sensitivity could be achieved as compared with the starting material. As the increase of chloromethylation ratio (CR) gradually broadened the molecular weight distribution (MWD), the optimum CR was evaluated to be about 40%. In the above range of Mw, the sensitivity varies from 39 to 0.4 μC/cm, whereas the γ-value varies from 3.0 to 1.4. A sharp edge profile was obtained in developed pattern of CMS resist because of its relatively high glass-transition temperature (68–115°C) compared with commercial resists and the suitable selection of a developer. The resolution of CMS was compared with the structually related polymers synthesized from polystyrene with a broader MWD or vinylbenzylchloride and poly(chloroethylvinylether) (CEVE). These polymers show significantly lower resolution than CMS, which indicates the importance of MWD and Tg in electron negative resist.

High performance electron negative resist, chloromethylated polystyrene. A study on molecular parameters

The deep uv and X-ray lithographic characteristics of high-performance negative electron resist chloromethylated polystyrene (CMS) were investigated. The sensitivity and resolution of a series of CMS's synthesized from nearly monodispersed polystyrene were evaluated as functions of molecular parameters. In CMS with a molecular weight of 1.8×105 showing a resolution better than 1.5 µm, a high sensitivity of 29 mJ/cm2 to X-rays and a sensitivity to deep uv about 40 times higher than that of PMMA was achieved. This high sensitivity is attributed to the high reactivity of the chloromethyl group, as verified by the high G value of CMS (5.4), 50 times larger than that of polystyrene. In contrast to most negative resists, where oxygen in air depresses the sensitivity, the sensitivity of CMS is hardly affected by the atmosphere. The sensitivity and resolution in the deep uv system are particularly affected by light absorption, because CMS exhibits strong absorption between 200 to 300 nm.

Shungo Sugawara

Papers

Poly (Fluoro Methacrylate) as Highly Sensitive, High Contrast Positive Resist

High performance electron negative resist, chloromethylated polystyrene. A study on molecular parameters

Chloromethylated Polystyrene as Deep UV and X-Ray Resist

Poly(vinylethers) as X‐Ray Resists

Crosslinking Reactions in Negative Electron Resists Composed of Halogenated Aromatic Polymers