Polymers cinnamic acid derivatives

Cinnamic acid derivative 36 crystallizes in the chiral space group P2t and gives the optically pure dimer 37 upon irradiation in the solid state [22], Chiral crystals of 38 gave, upon irradiation, the optically active dimer 39 of 90% ee, whereas the corresponding methyl ester gave a highly crystalline linear polymer through a typical [2 + 2] topochemical photopolymerization [23],... 

The alcohols formed from some cinnamic acid derivatives, namely /7-coumaryl alcohol, coniferyl alcohol (LI), and sinapyl alcohol (L2), commonly known as monolignols, undergo dimerization reactions that yield lignans such as (-l-)-pinoresinol (L3), (-l-)-sesamin (L4), (-)-matairesinol (L5), and podophyllotoxin (L6) (Fig. 13). Several thousand lignans are found to occur in nature. Lignins, the structural components of plant cell walls, are polymers of monolignols and/or lignans. 

Photocyclization is a particularly valuable route to the formation of cyclic compounds. There is a wide variety of photocyclization reactions reported in the literature of organic photochemistry, but relatively few of these have been carried out in solid polymers. The earliest reports concern the photodimerization of cinnamic acid derivatives, leading to crosslinking in solid polymers. These polymers have important applications as commercial photoresists. The chemistry has been reviewed by Delzenne (46) and Williams (47). 

In actual photochemical polymerizations, on the other hand, sterically pure products can occasionally be produced, since here every propagation step must be photochemically controlled. A prerequisite for this, however, is that the density difference between polymer and monomer crystals be very small. An example of this is the polymerization of cinnamic acid derivatives, which, according to the crystal structure of the monomer, lead to a- or j3-truxinic acid products ... 

Photo-cross-linkable polymers described in a Bayer patent (9) were made by grafting cinnamic acid derivatives to lEM homopolymer and lEM/vinyl acetate or lEM/ styrene/male ic anhydr ide copolymers. 

Nonflavonoid compounds comprise simple phenols, phenolic acids, coumarins, xanthones, stilbenes, lignins, and lignans. Phenolic acids are further divided into benzoic acid derivatives, based on a C6-C1 skeleton and cinnamic acid derivatives, which are based on a C6-C3 skeleton [6], The conmarins are phenolic acid derivatives composed of a benzene ring fnsed with an oxygen heterocycle. Xanthones consist of a C6-C1-C6 basic structure, and stilbenes are composed of a C6-C2-C6 skeleton with various hydroxylation patterns [13], Lignins are polymers of C6-C3 units, whereas lignans are made up of two phenylpropane units [13]. The structures of each of these classes are shown in Table 16.1. 

It is important to point out that it is possible to carry out photochemical transformations in mild conditions in order to maintain the activity of the biological molecules or encapsulated cells. For such purposes the photo-transformation of HA is carried out in the presence of several compounds, such as cinnamic acid derivatives, coumarin, thymine, methacrylic anhydride, glycidyl methacrylate and styrene. Figure 5.10 shows how methacrylic esters undergo photopolymerization in the presence of HA to form a grafted polymer [46]. 

Hydroxyl-functionalized cinnamic acid derivatives such as p-coumaric acid (p-hydroxycinnamic acid), ferulic acid, and sinapinic acid are attractive monomers for syntheses of high-performance polyesters. The obtained polyesters are also expected to be biodegradable in the case of copolymerization with aliphatic hydroxy acids such as lactic acid. Tanaka et al reported the thermal polycondensation of p-coumaric acid at 550 °C without any catalyst under high pressure up to 80 kbar (in the solid state) in 1975. They obtained red or brownish-red hard solids insoluble in conventional organic solvents. Higashi and his co-workers synthesized copolyesters of p-coumaric acid and 4-hydroxybenzoic acid or their methoxy substitutions (ferulic acid, vanillic acid, or syringic acid) by polycondensation using hexachlorocyclotri(phosphazene) in pyridine in 1981. The obtained polymers that exhibited UV spectra different... 

Photodimerization of cinnamic acids and its derivatives generally proceeds with high efficiency in the crystal (176), but very inefficiently in fluid phases (177). This low efficiency in the latter phases is apparently due to the rapid deactivation of excited monomers in such phases. However, in systems in which pairs of molecules are constrained so that potentially reactive double bonds are close to one another, the reaction may proceed in reasonable yield even in fluid and disordered states. The major practical application has been for production of photoresists, that is, insoluble photoformed polymers used for image-transfer systems (printed circuits, lithography, etc.) (178). Another application, of more interest here, is the use that has been made of mono- and dicinnamates for asymmetric synthesis (179), in studies of molecular association (180), and in the mapping of the geometry of complex molecules in fluid phases (181). In all of these it is tacitly assumed that there is quasi-topochemical control in other words, that the stereochemistry of the cyclobutane dimer is related to the prereaction geometry of the monomers in the same way as for the solid-state processes. 

The photodimerization of cinnamic acid and similar molecules is observed in crystals, but reactions of the same type occur in some polymers as well. Polymers such as polystyrene are made of long, saturated hydrocarbon chains with pendant groups in close contact dangling from the chain these chromophores can then interact in bimolecular photoaddition reactions. Polyvinyl car bazole and its derivatives are important examples of polymers which lead to such bimolecular interactions (e.g. exciplex formation). 

Photo-crosslinking and the reverse process of photodissociation of pre-existing crosslinks relies on a cycloaddition reaction (and on the reverse dissociation of the cyclic adduct). For example, derivatives of vinyl cinnamic acid can form crosslinks which are dissociated by irradiation with short wavelength light (e.g. 254 nm produced by low-pressure mercury arcs). In this process the polymer chains become separated, and the polymer itself is then soluble in organic solvents. 

Cinnamates occupy an important place in the history of photochemistry. Schmidt and his co-workers [18] used the solid state photochemistry of cinnamic acid and its derivatives to develop the idea of topochemical control of photochemistry in the crystalline state. Minsk [19] developed poly(vinyl cinnamate) as the first polymer for photoimaging. The cinnamate chromophore is still commonly incorporated in photopolymers of all types, including LC polymers, to enable them to be photochemically cross-linked [20], and a number of reports of the photochemistry of such MCLC and SCLC polymers are summarized below. 

Synthesis of New Photocrosslinkable Polymers Derived from Cinnamic Acid... 

Introduction of these photocrosslinkable structures in macro-molecular chains can be performed by esterification of hydroxyla-ted polymers with cinnamoyl chloride. Cellulose Q).condensation products (4, ) and mainly poly(vinyl alcohol) have Been treated( by this method. Other chemical modifications have been studied as ester interchange of poly(vinyl acetate) 7) and Knoevenagel reaction on polyesters (8). Very few results on the synthesis of such photocrosslinkable polymers by polymerization have been reported. Therefore free radical polymerization of cinnamic acid vinyl derivatives did not lead to the expected polymers, but to insolubilization reactions. Howewer cationic procedure can be a good way in some cases since Kato et al. could polymerize by this way with high yields p-vinyl phenylcinnamate (9) and B-vinyloxyethyl cinnamate (10). 

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