Polymer topochemical photopolymerization

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],... 

Asymmetric [2-1-2]-cycloadditions for one-component crystals have been reported. Ethyl ester 30a forms chiral crystals of P-type packing. Irradiation of the single crystals of 30a affords the optically active dimer 31 with an enantiomeric excess (EE) of >90%. In contrast, methyl ester 30b yields the highly crystalline polymer 32 through a typical [ 2-1-2]-topochemical photopolymerization. 

The synthesis of cis-1,4 polymers was also tried by e use of monomers with an s-cis conformation. The solid-state photopolymerization of pyridone derivatives, which is a six-membered cyclic diene amide and is a tautomer of 2-hydroxypyridine, was attempted [100]. Pyridones make hydrogen-bonded cocrystals with a carboxylic acid in the crystalline state. Because the cyclic structure fixes its s-cis conformation, if the polymerization proceeds, a cis-2,5 polymer would be obtained. Actually, however, the photopolymerization did not occur, contrary to our expectation, but [4-1-4] photodimerization proceeded when the carbon-to-carbon distance for the dimerization was small (less than 4 A) [101]. A closer stacking distance of the 2-pyridone moieties might be required for the topochemical polymerization of cychc diene monomers. 

At present, it is common knowledge that not only the photoreactivity, but also the stereochemistry, of the photoproduct is predictable from crystallographic information of starting olefin substrates. This ability of olefinic crystals to dimerize has been widely applied to the topochemical photocycloaddition polymerization of conjugated diolefinic compounds, so called "four-center type photopolymerizations" (7,8). All the photopolymerizable diolefin crystals are related to the center of symmetry mode (centrosymmetric -type crystal) and thus give polymers having cyclobutanes with a 1,3-trans configuration in the main chain on irradiation. 

In conclusion, the four-center photopolymerization is a novel type of topochemical reaction which is crystal-lattice controlled with respect to the whole set of elementary processes44 including initiation, propagation and crystallization of polymer. 

Based on the results of crystalline-state depolymerization, a reversible topochemical process, which is a monomer crystal lattice-controlled photopolymerization and a polymer crystal lattice-controlled thermal depolymerization, is established65. ... 

Several new hetero- and homo-adduct polymers were prepared photochemically from the unsymmetrically substituted diolefin crystals. In almost all of the topochemical [2+2] photopolymerizations where the homo-adduct polymers were produced, a single type of dimer was formed, followed by further polymerization. The photoreaction was completed at the dimer in some crystals. Such remarkable differences in the photochemical behavior in these crystals with similar chemical structures, can be explained not by their intrinsic chemical reactivity, but by the difference in the molecular arrangement in the crystals. 

Such kaleidoscopic topochemical behaviour of 2 OPr is exemplified in Scheme 6 (Hasegawa, 1992). 2 OPr crystallizes with a-type packing [2 OPr(a)] from 1-propanol solution but into the jS-type [2 OPr(j3)] from a mixture of ethanol and water. On photoirradiation using a filter (>410 nm), the a-type homo-adduct dimer (2 OPr-a-dimer) and the jS-type heteroadduct dimer (2 OPr- -dimer) are produced from 2 OPr(a) and 2 OPr(j3), respectively, both in nearly quantitative yield. The dimer crystal 2 OPr-a-dimer as-prepared is entirely stable on photoirradiation whereas, on further photoirradiation without a filter, 2 OPr-)8-dimer is converted nearly quantitatively into a highly strained [2.2] paracyclophane (2 OPr-cyclo). The photostable 2 OPr- i -dimer crystal is, however, transformed into a highly photoreactive crystal complex (2 OPr-a-dimer-PrOH) if 2 OPr-a-dimer is recrystallized from 1-propanol solution. The crystal (2 OPr-a -dimer-PrOH) photopolymerizes into a crystalline a-type homo-adduct linear polymer. Judging from their X-ray diffraction patterns, recrystallized l3-type dimer (2 OPr-)8-dimer) has a different crystal structure from that of the crystal... 

There are several alternative methods for the synthesis of optically active polymers from achiral or racemic monomers that do not involve polymerization catalysts. Optically active polymers have been formed from achiral dienes immobilized in a chiral host lattices [ 106]. In these reactions, the chiral matrix serves as a catalyst and can be recovered following the reaction. For example, 1,3-penta-dienes have been polymerized in perhydrotriphenylene and apochoUc acid hosts, where asymmetric induction occurs via through-space interactions between the chiral host and the monomer [107,108]. The resultant polymers are optically active, and the optical purities of the ozonolysis products are as high as 36%. In addition, achiral monomers have been found to pack in chiral crystals with the orientations necessary for topochemical soHd-state polymerization [109]. In these reactions, the scientist is the enantioselective catalyst who separates the enantiomeric crystals. The oligomers, formed by a [27H-27i] asymmetric photopolymerization, can be obtained in the enantiomeric pure form [110]. 

This reaction, called a four-center photopolymerization, is a typical example of topochemical reactions used to prepare polymer crystals.5 The changes in higher-order structure during the reaction are shown in Table 2.5 . Various polydiacetylene crystals have also been prepared by solid-state photopolymerization of diacetylene monomer crystals, such as 1,6-dicarbazoyl-2,4-hexadiene. These syntheses have attracted considerable interest, since they can lead to organic materials of high conductivity or of nonlinear optical properties. 

Polymerization of self-assembled LMOG molecules within the fibers of a molecular gel locks the scaffold-like structures and can make new stable materials [99,100]. Photopolymerization in the gels of diacetylenic cholesteryl derivatives, such as 63X-n [88,89], has been successfully employed to access organic nanowires with potential electrical conductivity, which could be used in molecular electronics. This strategy was based on the known propensity of diacetylenes to undergo topochemical soUd-state 1,4-addition reactions leading to conjugated polymers [101]. 

Monomer B5A can be photopolymerized at room temperature in its Sg phase to near completion with little volume change. The smectic liquid crystalline structure is retained and locked into the resulting polymeric network upon polymerization. This can be attributed to the topochemical control exerted by the LC monomer matrix on the as-formed polymer a quasi-topochemical polymerization reaction is involved (B5A-RT and PB5A-RT has almost the same unit cell parameters). Fixation of the LC structure is realized through cross-linking, which severely restricts molecular motion and relaxation. The LC structure is maintained up to very high temperatures. This could extend the application temperature of... 

Thus the photopolymerization of bifunctional monomers, in suitable crystal structures, might lead to topochemically controlled reactions. In order to test this possibility we studied the solid-state photochemistry of a,co-butadienes [19] and butatrienes [20]. In these systems it was found that the C4 dimers are as predicted from the crystal structures of the monomers however, the polymers formed did not show the stereoregularity expected to result from lattice control. 

The topochemical diacetylene photopolymerization is apphcable to various organized structures, including Langmuir-Blodgett films, liposomes, vesicles, and self-assembled monolayers (SAMs) on metal oxide or graphite surfaces. Scheme 3.14 depicts an assembly of diacetylene molecules and the subsequent photopolymerization at 254 nm. At ambient temperatures, the polymerization proceeds as a chain reaction by 1,4-addition, and results in alternating ene-yne polymer chains with exclusive trans selectivity. The quantum yield for initiation is low (about 0.01) [60]. 

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