Thiourea/dimethylbutadiene-inclusion

Figure 18 (a) Sketch of the thiourea/dimethylbutadiene-inclusion complex structure and electron density map of the cross-section of the... 

When butadiene and 2,3-dimethylbutadiene are included in the channels of urea and thiourea, respectively, 1,4 addition invariably results to yield polymers with chemical and stereo regularities (Scheme 39). Note that addition in the 1,2 fashion is prevented sterically by the narrow channel. Similarly, high selectivity was obtained when butadiene, vinyl chloride, and styrenes were polymerized in the channels of cyclophosphazenes. Syndiotac-tic polymer alone is obtained from vinyl chloride included in urea channels this is apparently the first example of inclusion polymerization of a vinyl polymer in which control is exerted over the steric configuration of the developing tetrahedral carbon atom (Scheme 39). Highly isotactic polymer is obtained from 1,3-pentadiene when it is included in a perhydrotriphenylene matrix (Scheme 39). Note that addition could occur at either end (i.e., Q to... 

A topochemical condition for polymerization is the proper approach of successive monomers at the growing chain-end within the channels. In this respect, conjugated dienes like butadiene, isoprene, etc. possessing reactive atoms in terminal positions, are very suited to inclusion polymerization. However, even bulkier monomers such as substituted styrenes or methyl methacrylate can polymerize if the space available inside the channels permits a favorable orientation and/or conformation of the monomer. The most studied examples are butadiene, vinyl chloride, bromide and fluoride, and acrylonitrile in urea 2,3-dimethylbutadiene and 2,3-dichlorobutadiene in thiourea butadiene, isoprene, cis- and trans-pentadiene, trans-2-methylpentadiene, ethylene and propylene in PHTP butadiene, cis- and trans-pentadiene, cis- and trans-2-methylpentadiene in DCA and ACA butadiene, vinyl chloride, 4-bro-mostyrene, divinylbenzene, acrylonitrile and methyl methacrylate in TPP. 

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. 

In an early example of polymerization within a solid inclusion compound, Clasen (in 1956) observed that 2,3-dimethylbutadiene guest molecules within the thiourea tunnel structure undergo spontaneous polymerization over... 

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... 

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. 

Historically, Clasen [61] reported the first example of inclusion polymerization of 2,3-dimethylbutadiene in thiourea. It took months to obtain polymers because he used no initiators. Brown and White described [62] inclusion complexes using urea or thiourea with various low molecular weight compounds, including some monomers, such as butadiene, styrene, 2,3-dimethylbutadiene, and acrylonitrile. They obtained polymers by irradiating the inclusion complexes of the monomers with 7-rays or X-rays. 

Inclusion polymerization in PHTP has a wider range of feasibility butadiene and 2,3-dimethylbutadiene polymerize in PHTP, whereas butadiene polymerizes only in urea, and 2,3-dimethylbutadiene polymerizes only in thiourea. 

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