Cross-linking by UV irradiation

Liu and coworkers have investigated PI-PCEMA-PtBA triblock copolymers in which the central PCEMA block was cross-linked by UV irradi-... 

Polyethylene is known to be degraded and cross-linked by UV irradiation, and a tremendous number of investigations have been carried out, mainly from the practical point of view. Several workers have studied... 

Epoxy-functionalized polyisoprene has been cross-linked by UV-irradiation in the presence of a cationic photoinitiator. The reaction kinetics has been monitored by IR spectroscopy and shown to proceed extensively within less than 1 s upon intense illumination. 

Bulk CROP of a soy-based 2-oxazoline (SoyOx) monomer has been performed using a scientific microwave unit, with methyl tosylate (MeOTs) serving as an initiator (Scheme 3.9). Full conversion was achieved within 8 min, leaving the double bonds of the fatty acids side chains unaffected and thus available for further cross-linking by UV irradiation. Statistical copolymers of SoyOx with 2-ethyl-2-oxazoline were also prepared. Rubbery cross-linked materials showing shape-persistent swelling upon absorption of water from the air could be prepared with as little as 13 wt% (5 mol%) of SoyOx. ... 

Fig. 3.8 Variation of the real part G of the shear modulus of the homeotropic NE cross-linked by UV irradiation and oriented with an E-field, as a function of the elongation A, showing that G does not decrease when A approaches Ai. For each G -value, the bars represent the dispersion of the data taken on several samples. The dashed line is a guide for the eyes, and the dotted line shows the behavior calculated with (3.11)-(3.13) and the known values of A and r (see text). The solid and the open squares correspond to a shear applied in a direction respectively parallel (soft-response geometry according to the theory), and perpendicular (non-soft-response geometry according to the theory) to the stretching direction. The stress-strain curve of Fig. 3.6 (panel b) is also reported on the figure to show the position of the thresholds A and A2. Reprinted with kind permission of Springer Science + Business Media (Rogez and Martinoty 2011)...

Hoogenboom and Schubert (2006) observed the microwave-assisted cationic ring-opening polymerization of a soy-based 2-oxazoline monomer (SoyOx). In presence of acetonitrile and without additional solvent, poly(2-oxazoline)s with fatty acid side chains in a living marmer was formed. In this polymerization procedttre, imsaturated side of fatty acids was unaffected. However, rrrtsatrrrated side chains could be cross-linked by UV-irradiation. 

Another industrial application, UV curing, is a technologically advanced process by which monomers/oligomers/polymers, usually in the presence of photoinitiators, undergo polymerization and cross-linking upon UV irradiation. The process is utilized... 

Some newly developed systems are completely solvent free in that the binder, itself, is a liquid and only actually acts as binder on hardening. Examples are mixtures of polythiols and polyenes that can be cross-linked by peroxides, irradiation with electrons, or with uv light (see also Section 27.1). 

Psoralen contains two double bonds that undergo [2+2] photocycloaddition reactions with the 5,6-double bond of thymines (Fig 2). When intercalated at a 5TA3 step in DNA, the two strands of DNA become covalently cross-linked, upon UV irradiation, with psoralen forming cyclobutane rings with two thymines, one on each strand. The structure of this bis-adduct has been determined by NMR studies and shows that the Cg atom of psoralen is located on the major groove side of DNA for the intercalation complex that leads to the cross-... 

Amino acid analysis, by reverse-phase HPLC, of acid-hydrolyzed uncross-linked recombinant resilin and cross-linked recombinant resilin clearly shows the presence of dityrosine in the cross-linked sample (Figure 9.3c). Further evidence of the presence of dityrosine was obtained by UV irradiation (Xmax,ex 315 nm Xmax,em 409 nm). Dityrosine endows natural resilin with pH-dependent blue fluorescence [38] on UV irradiation. The cross-linked recombinant resilin material was similarly fluorescent, strongly suggesting dityrosine cross-links. 

In view of the above, the initiation of polymerization and cross-linking reactions in polymeric systems is different from events induced by photons. Most of the events initiated by UV irradiation occur near the surface, because the absorption is governed by the Beer-Lambert Law, whereas the reactive species produced by electron beam are dispersed randomly throughout the entire thickness of the material (see Figure 5.1). 

For the case of polyethylene oxide-block-isoprene-block-ethylene oxide) (PEEbtPEO) micelles, the pharmacokinetics and disposition of PEO-PI-PEO in mice was studied after intravenous injection (Rolland et al., 1992). In this study, the cores of the PEO-PI-PEO micelles were cross linked by reaction of isoprene double bonds using UV irradiation along with a photoinitiator. 

Chien and Cada [42] have prepared optically active and photoactive SCLC copolymers, 15, with the 4-alkoxyphenyl-4 -alkoxycinnamate chromophore, with the intention of creating LC polysiloxane networks that could be used to prepare macroscopically oriented organic ferroelectric polymers for electro-optical devices. Optical activity was introduced into the polymer by the use of a chiral spacer. Those copolymers which were mesogenic exhibited properties characteristic of a Sc. phase. UV-irradiation of thin films of the polymers in their mesomorphic states at 90°C, led to a loss of the IR absorption at 1635 cm-1 that is due to the cinnamate double bond, and to cross-linking. Long-term irradiation led to... 

The polysilane core part within the shell cross-linked micelles was photodegraded by UV irradiation (>280nm) and dialysis against water produced nanometer-sized... 

Photo-cross-linkable oligophenols were synthesized by the HRP-catalyzed polymerization of cinnamoyl-hydroquinone-ester and cinnamoyl-4-hydroxyanilide in an aqueous 1,4-dioxane.40 UV irradiation of the oligomer films induced the cross-linking by the photochemical [2+2]-cycloaddition of the cinnamoyl function. 

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