Polymers with Pendant Cinnamoyl Functional Groups

Minsk et al. [117] may have been the first to synthesize a photocross-linkable polymer, namely poly (vinyl cinnamate). The photochemistry of this compound is similar to the photo-cyclization of cinnamic acid that is discussed bellow in this section. It is interesting that the reaction of cyclization of cinnamic acid can take place even in the solid crystalline stage. This illustrates that the reaction requires very little molecular motion. Similar reactions occur in polymeric materials that are functionalized with cinnamate groups. The photocross-linking of poly (vinyl cinnamate) is illustrated below:

Much earlier, well before Minsk, in 1895, Bertram and Kursten [118] recognized that solid cinnamic acid undergoes a chemical change when exposed to light. Following this, Ruber [119] established that the change is a dimerization of the acid to form a cyclobutane derivative. This dimerization results in formation of truxillic and truxinic acids:

Schmidt and coworkers [120, 121] studied the reaction mechanism and came to the following conclusions:

 1. Photo-dimerization of cinnamic acid and its esters is controlled by the crystal lattice.

2. Dimerizations are possible when olefinic double bonds of the two neighboring molecules in the crystals are 4.1 A ˚ or less apart.

3. Dimerizations are not possible when the double bonds are 4.7 A ˚ or more apart.

 The dimerization reaction takes place upon irradiation with light of a wavelength longer than 300 nm. It was demonstrated subsequently [122], however, on poly (vinyl cinnamate) that the adducts dissociate again upon irradiation with light of 254 nm. Photo-dimerization and formation of cyclobutane groups were demonstrated to be enhanced as a result of sensitized irradiation [123]. When dealing with poly (vinyl cinnamate), it is reasonable to assume that the degree of order in the relationship of one cinnamic group to another is much lower than is found in a crystal lattice of cinnamic acid. On the other hand, it should be higher than in solutions of cinnamic acid, where the groups are far enough apart so that very little photo-dimerization takes place. Photocross-linking of poly (vinyl cinnamate) can include the following reactions [124]:

1. Truxinic acid type dimerization in irradiated poly (vinyl cinnamate) that can occur intramolecularly. It can be shown as follows:

This is accompanied by formation of both folded and parallel chains 2. Truxillic acid type intermolecular dimerization in irradiated poly (vinyl cinnamate), on the other hand, can be illustrated as

dimerization of this type would be accompanied by formation of folded chains. Reactions of formations of folded and parallel chains are similar with the exception that the reacting cinnamic groups are further apart in folded chains and come together only by virtue of chain folding. Chains located parallel to each other but at the right distance can also conceivably yield truxinic acid type dimerization. This would be similar to the arrangements in crystal lattices. Formation of a truxillic dimer, like in reaction 2, shown above, requires favorable folding or two different chains. Also, there is accompanying possibility that the double bonds may simply polymer ize by a chain propagating reaction [125]. This was observed with some cinnamate esters [125]. Attempts were made to determine the reaction products of photocross-linked poly(vinyl cinnamate) by first hydrolyzing it, and then by isolating and identifying the acids. The results showed that a truxillic acid does form. Formation of b-truxillic acid, however, was not demonstrated. In addition, among the reaction products there is also a large quantity of unreacted cinnamic acid. This indicates that only a small portion of the double bonds participate in the reaction. Also, it should not be forgotten that only small changes in unsaturation in polymer molecules, as a result of cross-linking, can have a profound effect on solubility. Studies [126] of electronic structures in photo-isomerization and photo-dimerization of cinnamic acid showed that phosphorescence of cinnamic groups occurs at about 20,000/cm. Also, it was demonstrated when photosensitizers are present, the critical distance between donor, sensitizer, and acceptor molecules (cinnamic acid) is about 10 A ˚ [126]. Although all the details of incipient photocross-linking of poly (vinyl cinnamate) have to date still not been fully worked out, most accept that all three mechanisms take place. These are: dimerizations to truxillic and truxinic acid type structures and polymerizations through the double bonds. The excited states of the molecules can be produced by direct irradiation and also through intersystem crossing from an appropriate photosensitizer [126]. Several criterions were derived from proper selection of sensitizers [127, 128]. These are:

1. The triplet state must be at the energy level close to 50–55 kcal/mole for the cinnamate moiety.

2. The quantum yield of the ratio of phosphorescence to fluorescence should be higher than unity.

3. The mean lifetime of a triplet state must be greater than 0.01 s. The photosensitizing activity (characterized by the triplet state) of derivatives of cinnamic acid is beyond the phosphorescence of the cinnamate group (at about 20,000/cm, as stated above). An energy transfer diagram for poly (vinyl cinnamate) photosensitization with a sensitizer like 2-nitrofluorene was published [128]. The rate of dimerization obeys first-order kinetics. In addition, polymers consisting of flexible segments exhibit higher rates of photo-dimerization than do those composed of rigid segments.

It was also demonstrated that greater photosensitivity can be obtained by separating the cinnamic group from the polymer backbone by introducing–CH₂–CH₂–O– spacers as follows [129, 130]:

In addition, Tsuda and Oikawa carried out molecular orbital calculations of the electronic structures in the excited states of poly(vinyl cinnamate) [131, 132]. They based their calculations on the reaction of intermolecular concerted cycloaddition that take place according to the Woodward–Hoffmann’s rule. This means that the cyclobutane ring formation takes place if a nodal plane exists at the central double bond in the lowest unoccupied MO(LLUMO) and not in the highest occupied MO (HOMO) of the ground state cinnamoyloxy group. This is within the picture of Huckel MO or Extended Huckel MO theory. The conclusion is that the concerted cycloadditions occur favorably in the lowest triplet state T1 and in the second excited singlet state S2 [132]. The effectiveness of photosensitizers in accelerating the cross-linking reaction of poly (vinyl cinnamate) is illustrated in Table 10.2. Some 4 decades after the original development of poly (vinyl cinnamate) into a useful photocross linkable polymer, a novel optical property of the polymer was observed. When the material is irradiated with linearly polarized light, it exhibits polarization holography [133, 134]. The exposure of thin films of poly (vinyl cinnamate) to linearly polarized ultraviolet light causes uniaxial reorientation into liquid crystal layers [135–142]. Poly (vinyl cinnamate) and its derivatives have the ability to align in thin films the liquid crystal moieties in the direction that is perpendicular to the polarization axis of the linearly polarized ultraviolet light [143–145]. Schadt et al. [136] suggested that the surface settled homogeneous alignment of nematic liquid crystals results from photo-dimerizations of the cinnamatemoietiesandformationofcyclobutanerings(asshownearlier)withanazimuthallyoriented order. This, he feels, determines the direction of the liquid crystal alignment [140]. Ichimura et al. [141] suggested a different photo-alignment process. They claimed that the photo-induced homogeneous liquid crystalline alignment is caused by polarization of photo-chromophores at the uppermost surfaces of the substrates due to repeated A/Z photo-isomerizations, similarly to azobenzenes [140–149]. This was also shown to take place with stilbenes [140]. In addition, it was demonstrated by them[149]thatbothphoto-isomerization andphoto-dimerization contribute to liquid crystalline alignment. Photo-regulation in a polymethacrylate with o-cinnamate side chains displays preferential formation of Z-isomer. Dimerization, on the other hand, takes place more favorably in other polymers, including poly (vinyl cinnamate) [150]. The liquid crystals alignment in films prepared from materials with cinnamate group after irradiating the films with linearly polarized UV light is quite uniform. All the aggregate structures, lamellar crystals, produced by the photocross-linking reaction were found to be square in shape [151]. This has application in flat panel liquid crystal displays. It led others to synthesize polymeric materials that could be useful in photo-alignment. Lee et al. [152] synthesized a soluble photo-reactive polyimide with cinnamate chromophore side groups. The polymer, poly(3,30-bis(cinnamoyloxy) 4,40-biphenylene hexafluoroisopropylidene diphthalimide), has a reasonably high molecular weight and forms good quality films through conventional solution spin-casting and drying.

Thepolymeristhermallystableupto340Candpositivelybirefringent.Thephotochemicalreactions of the polymer in solution and in films, as well as its molecular orientations, are induced by exposure to linearly polarized ultraviolet light. As one might expect, the cinnamate chromophores undergo both photo-isomerization and photo-dimerization. Also, exposure to UV light induces anisotropic orientations of the polymer main chains and of the cinnamate side groups in the films. The irradiated films homogeneouslyalign nematic liquid crystal molecules along a direction at an angle of 107with respect to the polarization of the linearly polarized ultraviolet light. This coincides with the orientation direction of the polyimide chains. Thus, the liquid crystal alignment process is principally governed in irradiated polyimide films by the polymer main chains and the unreacted cinnamate side groups [151]. Nagata and Hizakae [153] reported preparation of a series of photocross-linkable biodegradable polymers by condensation of dichlorides of 4,40-(adipoyldioxy) dicinnamic acid and alkane diols of various methylene lengths. They also used various poly (ethylene glycols) with molecular weights ranging from 200 to 8,300. Among other interesting polymers with cinnamate functional groups are high polymeric phosphazenes that bear cinnamate groups [154]. A typical polymer synthesis is a follows:

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مواضيع ذات صلة

Polymers with Functional Chalcone Groups

Polymers That Photocross-link by Formation of Cyclobutane Rings

Photocross-linkable Polymers

Photosensitizers

The Charge Transfer Processes in Polymers

The Electron Transfer Process

Energy Transfer Process

Interaction of Light with Organic Molecules

The Nature of Light

Electricity-Conducting Polymers

Immobilized Reagents

Immobilized Nonenzymatic Catalysts