Polymerization, inclusion reactions hosts

Allcock and coworkers " have carried out extensive work on inclusion polymerization within cyclotriphosphazene host structures (Figure 15), reporting numerous stereoselective polymerization reactions. For example, polymerization of 1,4-divinylbenzene (4-vinylstyrene) in its bulk phase produces an insoluble cross-linked polymer, whereas polymerization of 1,4-divinylbenzene guest molecules within the inclusion compound formed with host molecule J (Figure 15) produces a soluble, linear noncross-linked polymer. According to the authors, this inclusion polymerization reaction represents one of the few ways of producing... 

Noteworthy also is the reaction between L42 and Cul in the presence of KI. The crystal structure determination reveals the formation of hitherto unknown exocyclic Cu g2- clusters, which are linked by L42 to generate a polymeric ID array of composition pC2(Cu4I6)L42]n 42". Within the crown ether cavities of L42, two potassium ions are trapped in an endocyclic manner. Also, upon inclusion of the K+ ions the crown ring shrinks leading to opening of opposite aromatic rings in the host calixarene unit, a rare behavior for this class of compound.162... 

The real occurrence of polymerization inside the channels was demonstrated in several ways. It does not occur, at least for a number of monomers, when there is a simple mixture of host and monomer without the formation of a clathrate or when the monomer is placed in the presence of solid substances unable to form inclusion compounds. Even in cases when polymerization does take place the structure of the polymer formed outside the channels differs from that obtained in proper conditions. The reaction rate is very temperature and pressure dependent and has a sheirp drop-off point beyond which reaction ceases. The boundaur y conditions for polymerization correspond to those which delimit the field of thermodynamic stability of the monomeric clathrate, determined by vapor pressure measurements or by DSC. This coincidence enables us to state that the two phenomena, monomer inclusion and polymerization, are strictly related. In addition, in some typical cases a structural change from monomer to polymer was directly observed inside the channels by X-ray analysis. 

Inclusion compounds allow the realization of copolymerization in the crystal state (1-6). This is a further difference with respect to typical solid state reactions. Both block- and statistical copolymers can be obtained the former involves a two-step process, with subsequent inclusion and polymerization of two different monomers (21) the latter requires the simultaneous inclusion of two guests. This phenomenon has a much wider occurrence than thought at first, especially when a not very selective host such as PHTP is used. Research with this host started with mixtures of 2-methylpentadiene and 4-methylpentadiene, two almost exactly superimposable molecules (22), but was successfully extended to very dissimilar monomers, such as butadiene and 2,3-dimethylbutadiene. 

Finally, we wish to comment briefly on a recent development in inclusion polymerization. As already discussed, this reaction can be carried out on the pure clathrate or in the presence of an excess monomer. Consequently, the vapor pressure of a volatile monomer during polymerization ranges from the decomposition pressure of the clathrate to the vapor pressure of the saturated solution of the host in the guest, which is generally very close to that of the pure liquid monomer. For example, the vapor pressure... 

Molecular motions are much smaller in channels than in solution at the same temperature. This indicates that the polymerization may proceed slowly. But the neighboring monomers are located very near, suggesting that the reaction may proceed rapidly. This is how the reversed effects work during the propagation reaction. It is known that inclusion polymerization smoothly occurs at low temperatures in the chaimels of urea, thiourea, and perhydrotriphenylene. However, in the case of steroidal hosts, we observed a deerease in polymerization rates. For example. in one case, the polymerization reached a saturated state after 1 month. 

Fig. 6 Conceptual comparison of the processes for enzymatic reaction of proteins (a) and inclusion polymerization (b). The chiral and sequential earbon chains express their information through their architectures (A), host-guest compounds (B), and reactions of the guests (C). (Vie-w this art in color at www.dekker.com.)...

The study on inclusion polymerization by using steroidal hosts led us to the concept of molecular information and expression, as follows. It is theoretically considered that molecular information at a nanometer level may originate from sequential and chiral carbon chains, like proteins and steroidal molecules. As shown in Fig. 6a, the proteins express their molecular information through noncovalent bonds by the following processes molecular architecture (Fig. 6A). host-guest compounds (Fig. 6B), and reactions of the included guest components (Fig. 6C). Similarly, the steroidal molecules express their information through the noncovalent processes (Fig. 6b). Therefore, inclusion polymerization corresponds to one step of the expression process of molecular information that the sequential and chiral carbon-chains store. 

Although the thiourea tunnel structure can accommodate guest molecules containing a diverse array of functional group types, reported studies of reactions in thiourea inclusion compounds have primarily concerned polymerization reactions. Classical work in this field ] reported polymerization reactions of 2,3-dimethylbutadiene 2,3-dichlorobutadiene 1,3-cyclohexadiene cyclohexadiene monoxide iso-butylene and vinylidene chloride. In all but the latter two cases, high-melting, crystalline polymers were obtained. There have been many subsequent reports of polymerization reactions carried out within the thiourea host structure. A computational study investigating the size requirements of polymers within the thiourea tunnel structure has also been reported. 

Chatani et al used X-ray diffraction to follow the structural changes associated with the polymerization reactions of 2,3-dichlorobutadiene and 2,3-dimethylbutadiene in the thiourea host structure and 1,3-butadiene in the urea host siructure. The crystal structures of the monomer/thiourea and polymer/thiourea inclusion compounds were found to be very similar, and in both cases the host structure is distorted from the... 

The importance of the size of the host tunnel in controlling the stereoregularity of the product obtained in inclusion polymerization reactions has been demonstrated by Miyata and coworkers, who investigated the polymerization of 2,3-dimethylbutadiene in the host structures formed by several DCA derivatives (see Figure 1) with a variety of tunnel dimensions. The results are summarized in Table 1. The relative sizes of the tunnels in the inclusion compounds... 

Recently, Cataldo and co-workers reported polymerization reactions of isoprene and 3-melhyl-1,4-pentadiene in DCA inclusion compounds, and compared the polymerization reactions of 2,3-dimethylbutadiene within the DCA and thiourea host structures. The polydimethylbu-tadiene prepared by inclusion polymerization in the DCA host structure was found to have a lower degree of regularity and crystallinity than that prepared by polymerization in the thiourea host structure. 

Table 1 Stmctural details of samples of polydimethylbutadiene produced by inclusion polymerization reactions in different soM host structures based on daivatives of deoxycholic acid. ...

---