Cyclic thioether, polymerization

A superior and relatively versatile procedure for the synthesis of unsymmetrical dialkyl thioethers, which avoids the unattractive direct use of thiols, utilizes the stable l-alkylthioethaniminium halides, which are readily obtained from thioacet-amidc [32] (Scheme 4.4). The reaction has also been used for the synthesis of alkyl aryl thioethers from activated aryl halides [33], but it cannot be used for the synthesis of cyclic thioethers, as polymeric sulphides are formed from a,co-dihaloalkanes. A similar sequence to that which leads to the thioethers has been used for the synthesis of S-alkyl thioesters [34] (see 4.1.26). 

In an extreme case the reactivity difference between the two monomers is positively utilized to obtain block copolymers. For example, Goethals recently polymerized IBVE with CF3S03H in the presence of thietane (a cyclic thioether) [86]. Because the thioether is much less reactive than vinyl ethers, it cannot polymerize and serves as a nucleophilic additive in the first-phase vinyl ether polymerization [64], but once IBVE has been completely polymerized, the cyclic monomer now polymerizes from the living end to form block polymers. 

The mixed-valence copper complexes with polymeric chain structures have been described. Reaction of the cyclic thioether ligand tet-... 

A further interesting example is WO 96 16959 A1 (6 Jun 1996) granted to Borealis A/S. This describes a polymerization system based on a new type of platinum metal complex containing cyclic thioether ligands, activated by alumoxanes. High molar ratios of alumoxane to platinum are, however, still required and the activities are unremarkable. 

The formation of rings that contain a thioether linkage does not appear to be catalyzed efficiently by Ru, even when terminal olefins are present. On the other hand, molybdenum appears to work relatively well, as shown in Eqs. 30 [207] and 31 [208]. Under some conditions polymerization (ADMET) to give poly-thioethers is a possible alternative [26]. Aryloxide tungsten catalysts have also been employed successfully to prepare thioether derivatives [107,166,169]. Apparently the mismatch between a hard earlier metal center and a soft sulfur donor is what allows thioethers to be tolerated by molybdenum and tungsten. Similar arguments could be used to explain why cyclometalated aryloxycarbene complexes of tungsten have been successfully employed to prepare a variety of cyclic olefins such as the phosphine shown in Eq. 32 [107,193]. 

When copolymers are prepared by ring opening polymerization of chiral monomers such as cyclic ethers and thioethers, lactones, lactams, N-carboxy anhydrides, etc., the chirality of the monomers is often retained in the repeating groups incorporated into the copolymers. A few stereoregular copolymers prepared this way have been analyzed by nmr spectroscopy, but much remains to be done in this area. 

---