Photodegradable Polymers for Biotechnological Applications

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 plastics: end-of-life design principles

Abstract

Photochemically degradable polymers and plastics are reviewed with an emphasis on the environmental and molecular factors that control the onset of degradation and the rate of degradation. A number of principles are beginning to emerge for the design of viable photochemically degradable plastics. Among the principles discussed are those relating to the effects of chromophores, initiators, antioxidants, temperature, oxygen diffusion into the plastic, polymer crystallinity, tensile and compressive stress, and the absorbed light intensity on the plastic. To obtain a plastic with a controlled lifetime and a specific rate of degradation, many of these parameters can be controlled or adjusted in the design stage of the plastic.     

Aromatic polyamides

Kinetics of the initial degradation of poly(1,3-phenylene isophthalamide) and of poly(chloro-2, 4-phenylene isophthalamide) has been studied by TG in inert as well as oxidative atmospheres. The information derived from the kinetic data is in agreement with our earlier reported studies on the degradation mechanism of these polyamides. The difference-differential method of Freeman-Carroll is shown to have problems when applied to high-char forming polymeric materials. The isoconversion method of Ozawa involving simple computations based on a particular reaction extent, is considered suitable for studying the complex degradation behavior of high-temperature and high-char forming polymer systems. Using this procedure, an activation energy of 215–230 kJ/mole is obtained for the initial degradation of the studied aromatic polyamides in inert and oxidative environments.     

Photodegradable polyurethane self-assembled nanoparticles for photocontrollable release

Light-responsive drug delivery systems are particularly appealing that are capable of releasing active molecules at the appropriate site and rate. We synthesized a series of photodegradable polymers that can form nanoparticles for drug encapsulation. These particles in aqueous solutions are stable in buffers with different pHs or at evaluated temperatures, while light can trigger the crash of particles and the release of encapsulated substances. The release efficiency can reach up to 90% based on Nile red fluorescence intensity upon 15 min light irradiation. Nanoparticle uptake by phagocytic cells and light-triggered release in cells were observed by fluorescence emission of the hydrolyzed fluorescein diacetate upon photoinduced degradation of these nanoparticles. No significant toxicity of these nanoparticles was found at the concentrations up to 1000 μg/mL before or after light irradiation. Further encapsulation and triggered release of a bioactive model drug (Tagalsin G) was evaluated for RAW 264.7 cells. Tagalsin G encapsulated in nanoparticles did not show cytotoxity to cells, while light triggered the release of Tagalsin G increasing cell death dramatically from 9% to 67%. Our model studies show a new promising strategy to trigger drug release in cells.     

Synthesis of Photodegradable Polystyrene with Trithiocarbonate as Linkages

Multiblock polystyrenes (PS) with trithiocarbonate groups as linkages are prepared via reversible addition‐fragmentation chain‐transfer polymerization using polytrithiocarbonate as a chain transfer agent. The photodegradability of the multiblock PS in the solid state is investigated under UV irradiation at room temperature in an air atmosphere. The experimental results demonstrate that the trithiocarbonate linkages in the multiblock PS can be broken under UV light irradiation at room temperature and the multiblock PS is degraded into separate PS blocks. Gel permeation chromatography measurement reveals that the molecular weight of multiblock PS is reduced from 27 900 to 7900 g mol⁻¹ after UV light irradiation for 745 h. Moreover, the thermal stability of the multiblock PS is examined and the results indicate that the incorporation of trithiocarbonate shows little influence on the thermal stability of multiblock PS.

     

N-methyl-substituted aromatic polyamides

A series of N-methyl-substituted aromatic polyamides derived from the secondary aromatic diamines 4,4′-bis(methylamino)diphenylmethane, 3,3′-bis(methylamino)diphenylmethane, 4,4′-bis(methylamino)benzophenone or 3,3′-bis(methylamino)benzophenone and isophthaloyl dichloride, and terephthaloyl dichloride or 3,3′-diphenylmethane dicarboxylic acid dichloride was prepared by high-temperature solution polymerization in s-tetrachloroethane. Compared with analogous unsubstituted and partly N-methylated aromatic polyamides, the full N-methylated polyamides exhibited significantly lower glass transition temperatures (T_g), reduced crystallinity, improved thermal stability, and good solubility in chlorinated solvents.     

Some phosphorus-containing polyamides

Summary Polyamides derived from p,p'-(phenylphosphinylidene)dibenzoic Acid and some aliphatic and aromatic diamines were prepared and investigated.     

Ferrocene-containing polyesters and polyamides

A series of ferrocene-containing polyesters and polyamides were prepared by refluxing 1,1′-dichlorocarbonylferrocene with various diols and primary diamines in xylene–pyridine solvent. The polyamides were all solids, but some of the polyesters were liquids. Reported are the infrared spectra and solubility characteristics of all the polymers and, where possible, the molecular weight and molecular weight distributions. In general, these polyamides and polyesters were of relatively low molecular weight (below 4000), but the polyesters were readily chain extended and crosslinked by di- and triisocyanates. Elemental analyses are reported for all the polymers prepared.     

Photodegradable, Photoadaptable Hydrogels via Radical-Mediated Disulfide Fragmentation Reaction

Various techniques have been adopted to impart a biological responsiveness to synthetic hydrogels for the delivery of therapeutic agents as well as the study and manipulation of biological processes and tissue development. Such techniques and materials include polyelectrolyte gels that swell and deswell with changes in pH, thermosensitive gels that contract at physiological temperatures, and peptide cross-linked hydrogels that degrade upon peptidolysis by cell-secreted enzymes. Herein we report a unique approach to photochemically deform and degrade disulfide cross-linked hydrogels, mitigating the challenges of light attenuation and low quantum yield, permitting the degradation of hydrogels up to 2 mm thick within 120 s at low light intensities (10 mW/cm(2) at 365 nm). Hydrogels were formed by the oxidation of thiol-functionalized 4-armed poly(ethylene glycol) macromolecules. These disulfide cross-linked hydrogels were then swollen in a lithium acylphosphinate photoinitiator solution. Upon exposure to light, photogenerated radicals initiate multiple fragmentation and disulfide exchange reactions, permitting and promoting photodeformation, photowelding, and photodegradation. This novel, but simple, approach to generate photoadaptable hydrogels portends the study of cellular response to mechanically and topographically dynamic substrates as well as novel encapsulations by the welding of solid substrates. The principles and techniques described herein hold implications for more than hydrogel materials but also for photoadaptable polymers more generally.     

Photodegradable transient bilayered poly(phthalaldehyde) with improved shelf life

Metastable poly(phthalaldehyde) (PPHA) can be triggered to depolymerize under visible light by incorporation of photosensitive compounds, such as a photoacid generator (PAG), which can generate a strong acid in situ. However, photosensitive compounds can be thermally unstable and have limited shelf life, causing inadvertent device triggering. It can also be difficult to fabricate components that are photosensitive because special lighting conditions are needed. In this paper, nonphotosensitive PPHA films were formed and made photosensitive at the point of use. This improved the material shelf life and manufacturability by adding a second, PAG‐containing layer to the original nonphotosensitive layer at an optimal point before use. The catalytic photoacid was generated rapidly by exposure of the PAG‐containing layer to radiation. Depolymerization of PPHA via the acid catalyst was followed by diffusion of the acid into the nonphotosensitive layer causing it to depolymerize. Diffusion of the photoacid into the nonphotosensitive medium was quantified at various temperatures. Photoacid diffusion in a liquid, moving‐front caused depolymerization of the nonphotosensitive PPHA layer. The fabricated bilayer structure allowed for better stability of the structural material using PPHA while still achieving transience.     

Rigid aromatic fractal polyamides

The preparation of rigid aromatic, highly branched polyamides is described. Owing to the method of preparation and the chosen ratio of difunctional to trifunctional monomers, these entities are highly porous and not dendrimeric in nature. They better conform with the fractal model and are therefore called fractal polyamides (FPs). The effects of variations in the polymerization procedure, in total monomer concentration, in the ratio of amine to carboxyl groups and in the duration of the polycondensation reaction are investigated. Some characterization was performed and the results are presented and briefly discussed.     

Mechanical relaxations in polyamides

Dynamic mechanical measurements were carried out as a function of temperature (?100 to + 180°C) and frequency (3.5 to 110 cps) for a series of aliphatic terpolyamides, nylon 6 and nylon 12. Effect of crosslinking with toluene diisocyanate, of absorbed water, and of frequency are used to estimate the statistical segment length associated with the α′ relaxation. The effects of variations in the aliphatic chain length in the repeating unit on the temperature of the α′ relaxation are examined by means of copolymer rules with a view to explaining the reported insensitivity of the glass transition temperature of these polymers to changes in (CH₂)/(amide) group ratio. From the estimated length of the segmental motion associated with the α′ relaxation it is inferred that in a series of polyamides of the type nylon X or nylon X,Y (where X or Y = 3, 4, 5, 6, etc.) there should be a relatively small change in the temperature of the α′ transition for those polyamides having X or Y less than about 45. Experiments which are intended to establish the position of the crystalline α–γ transition are discussed.     

Phenoxasilin-containing polyamides and polyesters

Silicon-containing polyamides and polyesters of a new type have been synthesized. They contain phenoxasilin rings with double-stranded structure. The polymers were synthesized by the interfacial polycondensation of 2,8-dichloroformyl-10,10-diphenylphenoxasilin with diamines and bisphenols, and were obtained in nearly quantitative yields. Their reduced viscosities were in the range of 0.53–1.47 dl g^?1 m dimethylformamide (DMF), m-cresol or chloroform. Some of the polyamides were soluble in polar aprotic solvents such as DMF and N-methyl-2-pyrrolidone (NMP) and the polyesters had good solubility in chloroform, phenol-sym tetrachloroethane (60:40 by wt %) and acidic solvents (m-cresol and nitrobenzene). The polymers hardly dissolved in cone. H₂SO₄ and some of them coloured in it. Only the polyester having sulphide bonds was soluble in benzene in addition to the above organic solvents. These polymers hardly degraded below 400° except for the polyamides derived from aliphatic diamines. The polymers from aliphatic diamines melted at 290–325°; the other polyamides and the polyesters decomposed without melting.     

Thermally reactive aromatic polyamides

2,5-Bis(phenylethynyl)terephthaloyl chloride and 4,6-bis-(phenylethynyl)isophthaloyl chloride were synthesized in a multistep reaction scheme from 2,5-dibromoterephthaldehyde and 4,6-dibromoisophthaldehyde, respectively. Low temperature solution polycondensation of these novel monomers and tolane-2,4′-dicarbonyl chloride with aromatic diamines yielded aromatic polyamides containing phenylethynyl moieties. Inherent viscosities of 0.20–0.51 dL/g were recorded. Attempts to carry out the homopolymerization of 2-(3-aminophenylethynyl)benzoyl chloride hydrochloride under similar conditions led to low molecular weight polyamide. Under differential scanning calorimetry and thermal mechanical analysis, the polyamides exhibited strong exotherms with onset occurring in the 185–225°C range. The exotherms were attributable to intramolecular cycloaddition of phenylethynyl moieties with amide groups to give polybenzalphthalimidine structures. Curing of a pressed pellet specimen for 16 h at 250°C under a nitrogen atmosphere resulted in partial conversion to a polybenzalphthalimidine structure with a concomitant increase in the polymer glass transition temperature. Isothermal aging in air of the cured specimen at 316°C (600°F) led to 25% weight loss after 200 h.