Topochemical polymerization crystals

The reactivity of the DSP crystal was thoroughly interpreted in terms of the topochemical concept proposed by Schmidt, in which potentially reactive double bonds are oriented parallel to each other and separated by approximately 3.5-4.2 A. The reaction proceeds with a minimum of atomic and molecular motion (2 ). The reactive double bonds in most of the topochemically polymeric crystals thus far found are related to the center of symmetry (centrosymmetric ct-type crystal) and dimerize to give highly crystalline polymers containing cyclobutanes with a 1,3-trans configuration in the main chain. 

Fig. 17 Schematic diagram showing the molecular packing and topochemical polymerization of diacetylenes [30] in crystals. There are two idealized packing arrangements either (a), (b) or (c), (d) in crystals and each has two modes of polymerization. The polymerization in the solid state is said to occur smoothly when, v = 240 400 pm and y = 45° (Baughman, 1974 Baughman and Yee, 1978).

Keywords Crystal engineering Solid-state photoreaction Topochemical polymerization Controlled radical polymerization Dimerization Isomerization Topotactic reaction... 

Later, Tieke reported the UV- and y-irradiation polymerization of butadiene derivatives crystallized in perovskite-type layer structures [21,22]. He reported the solid-state polymerization of butadienes containing aminomethyl groups as pendant substituents that form layered perovskite halide salts to yield erythro-diisotactic 1,4-trans polymers. Interestingly, Tieke and his coworker determined the crystal structure of the polymerized compounds of some derivatives by X-ray diffraction [23,24]. From comparative X-ray studies of monomeric and polymeric crystals, a contraction of the lattice constant parallel to the polymer chain direction by approximately 8% is evident. Both the carboxylic acid and aminomethyl substituent groups are in an isotactic arrangement, resulting in diisotactic polymer chains. He also referred to the y-radiation polymerization of molecular crystals of the sorbic acid derivatives with a long alkyl chain as the N-substituent [25]. More recently, Schlitter and Beck reported the solid-state polymerization of lithium sorbate [26]. However, the details of topochemical polymerization of 1,3-diene monomers were not revealed until very recently. 

The polymerization proceeds under photo- [49,50],X-ray [51], and y-ray [52] irradiation in the dark in vacuo, in air, or even in water or organic solvent as the dispersant (nonsolvent) for the crystals, similar to the solid-state polymerization of diacetylene compounds [ 12]. The process of topochemical polymerization of 1,3-diene monomers is also independent of the environment surrounding the crystals. Recently, the thermally induced topochemical polymerization of several monomers with a high decomposition and melting point was confirmed [53]. The polymer yield increases as the reaction temperature increases during the thermal polymerization. IR and NMR spectroscopies certified that the polymers obtained from the thermally induced polymerization in the dark have a stereoregular repeating structure identical to those of the photopolymers produced by UV or y-ray irradiation. 

We can learn the structural change in the crystals during topochemical polymerization by powder X-ray diffraction measurements. X-ray diffraction profiles continuously changed during the polymerization of 1 under the irradiation of an X-ray beam (Fig. 5) [51]. The reflections shifted and approached the reflection position of the polymer. This suggests that the polymerization... 

Molecular alignment in the monomer crystals is controlled by several intermolecular interactions, such as strong and weak hydrogen bonds, leading to the formation of various types of stereoregular polymers via a topochemical polymerization process. This approach to the stereocontrol of polymers differs from other conventional ways in the control of the propagating chain end using catalysts or additives in solution polymerization. 

Thus, it has been demonstrated that the combination of weak interactions is useful for the design of topochemical polymerization to yield a new type of stereoregular polymer. The weak intermolecular interactions are tolerant of a variety of crystal structure formations and induce a different molecular stack-... 

The topochemical polymerization of 1,3-diene monomers based on polymer crystal engineering can be used not only for tacticity but also for the other chain structures such as molecular weight [ 102], ladder [84] or sheet [ 103] structures, and also polymer layer structures using intercalation reactions [ 104-107]. Some mechanical and structural properties have already been revealed with well-defined and highly or partly crystalline polymers [ 108-111 ]. A totally solvent-free system for the synthesis of layered polymer crystals was also reported [112]. 

Topochemical Polymerization The chiral crystalline environment of a monomer itself can be a source of asymmetric induction in solid-state polymerization [69-72], Prochiral monomers such as 37 give enantiomorphic crystals, one of which can be preferentially formed by recrystallization with a trace amount of optically active compounds. Photoir-... 

Fig. 32 (a) Topochemical polymerization mechanism for a typical diacetylene monomer, (b) Crystal structure of a typical poly diacetylene... 

A great majority of polymerizations are simultaneously affected by many physical and chemical factors, and their course is the result of a superposition of these effects. Only in rare cases does one of these factors dominate and the polymerization is formally simplified. In topochemical polymerizations, the growth of macromolecules is governed by forces in the crystal lattice of the monomer. Solid-state polymerization of trioxane (trioxacyclohexane) is a typical example of topochemical polymerization. 

Crystalline poly(oxymethylenes) with peculiar morphologies were produced via y-initiated topochemical polymerization of trioxane. Depending on the crystal modification of the trioxane, porous poly(oxymethylene) crystals, showing cylind-... 

Some important aspects of topochemical polymerizations can be understood by inspection of Eq. (1), All reactivity comes about by very specific rotations of the monomers and by 1,4-addition of adjacent units and an extended, fully conjugated polymer chain is formed. The unique feature of the topochemical polymerization of diacetylenes is the fact that in many cases the reaction can be carried out as a single phase process. This leads to macroscopic, defect-free polymer single crystals which cannot be obtained, in principle, by crystallization of ready-made polymers by conventional methods. Thus, polydiacetylenes are ideal models for the investigation of the behaviour of macromolecules in their perfect three dimensional crystal lattice. 

Owing to the mechanism of the topochemical reaction the polyconjugated polymer chain is of exceptional purity and stereochemical regularity. Polydiacetylene crystals are thus ideally suited to study the inherent optical and electrical properties of polyconjugated chains. These unique features have attracted considerable attention and in recent years the topochemical polymerization of diacetylenes has developed to... 

Fig. 1. Scheme of a topochemical polymerization. Transformation of a monomer single crystal into the polymer single crystal... 

The gradual conversion of the monomer crystal into the equivalent polymer crystal can be considered a special type of phase transition. In some cases the topochemical polymerization is accompanied by an additional structural phase transition. This behaviour is most often observed in monomer structures with a comparatively moderate reactivity where only a partial conversion can be achieved. Here, the side group packing is rearranged either spontaneously or by thermal annealing. This process... 

The topochemical polymerization of diacetylenes proceeds with retention of molecular packing of the crystal structure. The reactivity strongly depends on... 

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