Asymmetric polymerization solid state

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-... 

It is typically on the order of several hundred nanometers. In practice the minimum thickness for polymeric membranes is 50gm or greater, which is far more than one would expect from (6.53). This is apparendy due to the fact that these membranes hydrate in the bulk, thus increasing the dielectric constant. They also form a hydrated layer at the solution/membrane interface (Li et al 1996) which affects their overall electrochemical properties and selectivities. Macroscopic ISEs use relatively thick membranes ( 500jU.m). In contrast, it is desirable to use thin membranes in the construction of asymmetric solid-state potentiometric ion sensors, in order to make their preparation compatible with the thin-layer preparation techniques. 

It is possible to exactly identify and characterize the radical species and chain structures of the reaction intermediates, which are determined by their different reactive or unreactive chain ends. The reactive intermediates are best described by diradical (DR), asymmetric carbene (AC) and dicarbene (DC) oligomer molecules of different lengths. The respective singlet (S = 0), triplet (S = I) or quintet (S = 1) states and their roles in the polymerization process are investigated in detail by solid state spectroscopy. A one-dimensional electron gas model is successfully applied to the optical absorption series of the DR and AC intermediates as well as on the different stable oligomer SO molecules obtained after final chain termination reactions. 

However, the most interesting products could be obtained upon the radiolysis of butadiene derivatives included in a host matrix ( - ) For a number of monomers with a non-centrosymmetric molecular structure it could be demonstrated that y-irradiation leads to stereoregular, optically active polymers in a direct asymmetric induction. Especially these studies indicate that apart from polymerization in solution using optically active catalyst systems (A), the solid state polymerization represents a suitable method to obtain stereoregular polybutadienes. 

Further possibilities are opened up for investigation of fluoropolymers by two-dimensional methods. Thus, solid-state F COSY can be used to study potential spin exchange (see Ref. 17 for a nonpolymeric example), which may arise from chemical exchange (rare in polymeric systems, but conceivable for internal rotation of C—CF3 groups in asymmetric environments) or from spin diffusion. 

The halide bridging still persists in dialkyltin dichlorides and dibromides, but is somewhat weaker and is of an asymmetric nature, either as in (20) or (21). Diphenyltin dichloride and methylphenyltin dichloride have weak tetrameric associated structures the two terminal tin atoms are tetracoordinate, with the middle two tin atoms being hexacoordinate. Dicy-clohexyltin dichloride has essentially a one-dimensional polymeric structure with a live-coordinate tin atom, but with an additional weak tin chlorine interaction (22). Both bis(2-biphenylyl)tin dichloride and 5,5-dichloro-10,ll-dihydrodibenzo[fc,/]stannepin (23) are monomeric in the solid state. 

A variety of monomers can be trapped in the inclusion spaces at the molecular level and polymerized under suitable conditions. Such a reaction is called inclusion polymerization. " The study of inclusion polymerization started soon after the discovery of a honeycomb structure of urea inclusion compoxmds. The early study aimed to obtain highly stereoregular and asymmetric polymers in the spaces. Further studies brought about a profound understanding of the space effects from various viewpoints. Now. inclusion polymerization is classified between bulk or solution polymerization and solid state polymerization. In other words, it may be situated as low-dimensional and space-dependent polymerizations. Such a polymerization closely relates to supramolecular chemistry from a viewpoint of molecular information and expression. 

Oxidative Poiymerization of Naphthoi Derivatives. Oxidative polymerization of 2-naphtol (109) and 1,5-dihydroxynaphthalene (110) has been done using enzyme catalysts. Solid-state polycondensation of 2,6-dihydroxynaphthalene with FeCls catalyst (111) has been accomplished. Asymmetric oxidative coupling polymerization of 2,3-dihydroxynaphthalenes and their derivatives was achieved by chiral copper catalysts (112-115). 

Farina etal. also reported a breakthrough result in 1967 which was the first example of an asymmetric polymerization reaction in the solid state, in which optically active isotactic tra i-l,4-polypentadiene was formed by irradiation of the inclusion compound containing trans-... 

Several groups were attracted by the mechanistic aspects of asymmetric synthesis from pro-chiral monomers, or the selective polymerization of one enantiomeric form of a racemic mixture, and the corresponding catalytic systems. Increased solid state thermal stability was expected from asymmetry but better interpretation of the properties of biological molecules by studying more simple synthetic models is surely one of the most frequent motivations in the field. 

Abstract. The asymmetric synthesis of chiral polymers by topochemically controlled polymerization in chiral crystals is discussed. Following a short survey of topochemical polymerization in the solid state and some comments on chiral crystals, we present the requirements for the performance of asymmetric polymerization based on [2+2]-photocycloaddition. The planning and execution of two successful polymerizations of this sort are described. In the first, we start with a chiral non-racemic monomer and obtain optically active cyclobutane oligomers. The optical yields of the dimer and trimer were quantitative on the scale of N.M.R. sensitivity. In the second reaction we start with a racemate, and by the processes of crystallization in a chiral structure and solid-state reaction we generate an optically active polymer, in the absence of any outside chiral agent. The possible application of this novel method to additional systems is discussed. 

What are the requirements for an absolute asymmetric polymerization First, we need an achiral or,chiral-racemic monomer crystallizing in a chiral structure in the latter case this entails disorder in the crystal with respect to the chiral handle. Second, the solid-state polymerization must be such that both the initiation and propagation steps are controlled by the crystal lattice (topochemical control) for this to be so requires a structure in which adjacent reactant centers are the correct distance apart and have the correct relative orientation [10]. 

Probably the first example of asymmetric induction in the solid-state reaction of a chiral was that of polymerization. Farina et al [26, 34] polymerized by 7-irradiation trans.trans-pentadiene included in channels of a resolved sample of perhydrotriphenylene, and obtained an optically active product. 

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