Inclusion compounds crystalline, polymerization

Crystalline inclusion compounds containing unsaturated monomers are effective reactive systems for the production of linear polymers (1-6). This process belongs to the wider class of solid state polymerization, but possesses some specific features which make it worthy of a separate description. Throughout this article, the polymerization in inclusion compounds will be referred to as "inclusion polymerization" (other names currently used in the scientific literature are channel, canal or tunnel polymerization), and the terms "clathrate" will be used as synonymous with "inclusion compound". When there is no risk of confusion, the more general term of "adduct" will be used for clathrate in principle a... 

It has long been known that highly crystalline forms of polybutadiene with melting points higher than conventional crystalline polybutadiene are available via radiation polymerization of thiourea inclusion compounds (1. Crystallographic studies of the polymerization of butadiene derivatives in inclusion complexes have been reported (53). Recently, butadiene derivatives in layer perovskite salts have been polymerized to singlecrystal polymers which have been structurally characterized (54). 

C SPs. The assembly belongs to the class of clathrates (inclusion compounds) that have interest for separation processes. Single chains are confined in small channels (diameter — 6 A) for a wide variety of crystalline adducts. These systems suggests that smoothed-out effects assist rhe formation of host-guest polymeric assemblies in addition to localized interactions. The nonlocalized effects may arise from the suppression of... 

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 further seminal studies. Brown and White" developed a strategy for irradiating solid inclusion compounds to initiate polymerization, and then extracting the host with hot solvent to recover the crude polymer. Crystalline polymers of 2,3-dimethylbutadiene (Figure 13a),... 

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. 

Another possibility for irradiation-induced solid state polymerization is that in mono- or multilayers. Thus acrylates or methacrylates with different long-chain ester groups are polymerized by UV light, y-radiation, or electron-beam radiation [65-67], The majority of the examples given in the literature for irradiation-induced bulk polymerization deal with monomers in the liquid state as pure compounds. Some examples are given for polymerization in the presence of inclusion compounds or related polymer matrices (see Refs. [60,68-72]). Another possibility has been described as photopolymerization of an oriented liquid crystalline acrylate [73]. 

Pure tra i-l,4-polyisoprenes as well as 1,4-polybutadienes can be synthesized by polymerization in inclusion compounds [266-269]. As typical hosts for this dienes, the inclusion compounds or clathrates of urea, thiourea, or perhydrotriphenylene [PHTP Eq. (36)] are used [270,271]. The host forms the frame of the crystal and the guest is placed in the cavities existing in the lattice. Polymerization is generally started by subjecting the inclusion compound to irradiation with a-, y-, or x-rays and proceeds by a radical mechanism [272,273]. Also, free radical initiators such as di-/cr/-butylperoxide could be used [274]. Inclusion in urea yields crystalline trans-, A polymers, whereas trans-lA-polyisoprene obtained in PHTP is amorphous. There is no trace of, A-cis units or of 1,2, 3,4, and cyclic units. The reason for the amorphous product is the presence of a substantial number of head-to-head and tail-to-tail junctions in addition to head-to-jail junctions [275, 276]. 

There are several ways of isolating molecules, in addition to dilution in appropriate solvents. For instance, extremely long PDA chains can be diluted in their monomer single crystal by exploiting the peculiar polymerization mechanism [91] of this class of polymers. In the case of CPs blended with non-conjugated macromolecules (polyethylene, polymethylmethacrylate, etc.) or inclusion crystalline compounds [92], the interaction between molecule and environment is usually strongly suppressed, but at the expense of the sample optical density, in a way that may easily challenge the common sensitivity of time-resolved techniques. 

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