Thietanes polymerization

Thus, in the polymerization of unsubstituted thietane, polymerization stops at limited conversion ( 20% in CH2C12 at 20° C) while for 2,2-diethylthietane nearly quantitative conversion (96% in CH2C12 at 20° C) to polymer can be obtained. 

The soluble (II) was produced in about 60% yield in polymerizations carried out at —3°C whereas the crosslinked polyether (I) was obtained from polymerizations at 30°C. These authors conclude that under these experimental conditions, the thietane group polymerized more rapidly than the oxetane group but are careful to note that this does not prove that in general thietanes polymerize more rapidly than oxetanes. Instead... 

The tertiary sulfonium ion produced is too stable to start a new thietane polymerization. 

Higher conversions in thiirane polymerizations, however, proceed with chain scission transfer mechanism under the influence of BF3 (C2H5)20 [192]. This is indicated by a change in the molecular weight distribution, a bimodal character. When the reaction is complete there is a marked decrease in the average molecular weight of the polymer. When thietane polymerizes with triethyl-oxonium tetrafluoroborate initiation in methylene chloride, the reaction terminates after only limited conversion [193]. This results from reactions between the reactive chain ends (cyclic sulfonium salts) and the sulfur atoms on the polymer backbone. In propylene sulfide polymerization, however, terminations are mainly due to formations of 12-membered ring sulfonium salts from intramolecular reactions [193]. 

The ratio fep/fet has been taken as a measure of the living character of the polymerization. This ratio is greatly affected by the presence and the nature of substituents on the thietane ring. Substituents decrease the reactivity of the active species toward the sulfide funrtion but, as shown in Table 4, this deaease is more pronounced for the termination reaction than for the propagation reaaion. Thus, thietane polymerizes... 

The occurrence of a termination reaction in the thietane polymerization leads to the formation of branched polymer stmctures, and therefore it is not possible to control the molecular weight or the MWD, except for the 3,3-diethyl derivative, which shows a relatively high living character. 

The occurrence of an intermolecular termination reaction in the polymerization of thietane has been used to prepare star-shaped segmented copolymers. Oxonium ions are excellent initiators for the polymerization of cyclic sulfides and therefore addition of thietane to a living polyTHF solution results in the formation of a block copolymer. Since the thietane polymerization gives the termination reaction described above, the end result of this sequential monomer addition is a... 

Similar results have been obtained by combining the living cationic polymerization of vinyl ethers and the ROP of thietane. In this case, the thietane acts as growing species stabilizer for the vinyl ether polymerization by reversible formation of an a-alkoxy-thietanium ion. This ion can, however, also be attacked at an endocyclic methylene by another thietane molecule leading to an alkyl-thietanium ion. This ion is incapable of reacting with a vinyl ether but is the active species for the thietane polymerization. Also in this case the end product is a star-shaped segmented polymer. 

Thietene is a liquid that polymerizes within an hour at room temperature. However, thietane, also a liquid, is more stable it exists as a puckered structure, similar to that of azetidine (note the sulfur atom and hence its lone-pair electrons occupy more space than those of oxygen). 

Thietanes all have, to varying degrees, the tendency to polymerize. The rate of polymerization is mainly dependent on the conditions. In sunlight the process takes place at a relatively slow rate. The strained polycyclic thietanes, such as 6-thiobicyclo[3.3.1]heptane polymerize spontaneously when photon or proton initiated. ... 

The photocatalyzed polymerization of thietanes makes it very difficult to utilize them in other photochemical reactions, and often special handling and reaction procedures are required. 

In one case, polymerization could be prevented by gradual extraction of the thietane from an inert solvent, such as n-pentane or petroleum ether, into the reaction medium. ... 

Cationic polymerization of thietanes initiated by trityl salts mechanism of the initiation reaction. ... 

Fluorothioketones are more difficult to polymerize. There are two reasons. First, agents that promote polymerization also catalyze dimerization to dithi-etanes, which is a very fast reaction. Second, ceiling temperature of polymerization is low with the result that polymer decomposes back to monomer as it is being isolated. However, poly(hexafluorothioacetone) can be formed at very low temperatures by initiation with dimethylformamide or BF3 etherate, even though at — 78° C the only product isolated is 2,2,4,4-tetrakis(trifluoromethyl)-l,3-di-thietane. 

The block copolymer of ethylene oxide and 3,3-dimethylthietane shows useful properties of complexing halogen and heavy metal salts (79MI51402). Thietanes can be polymerized with methylmagnesium iodide as well as with a variety of electrophiles such as methyl sulfate, trimethyloxonium tetrafluoroborate, triethylaluminum, boron trifluoride and phosphorus trifluoride (67IC1461, 67MI51400). Thietane (210) has been patented as a stabilizer for poly(vinyl chloride) (73USP3767615). 

Sulfur is an interesting atom because of the specific characteristics of the simple substance and compounds containing sulfur. Further, their functions are indispensable for current technologies. Studies on sulfur compounds are valuable not only in chemistry but also in biochemistry as well. Many cyclic monomers containing sulfur such as cyclic disulfides, thiiranes, thietanes, and arene dithiols were reported to form cyclic polymers by the ring-opening polymerization. The natural source of cyclic polymers is believed to be liquid, elemental sulfur [177-179]. 

Thietane-3-ol 21a yielded the polymeric product poly(3-hydroxythietane) 22a. The same results were obtained when thietane 21h was treated with aqueous sodium hydroxide and the polymeric substance 22b was obtained... 

Thus under the given conditions, the thietane ring is more reactn. than oxetane [48]. On the other hand, the polymerization of 1,3-oxathiolane... 

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 cyclic sulfides that can be polymerized by cationic mechanism include 3-membered rings, thiiranes, and 4-membered rings, thietanes. Five-membered rings do not undergo polymerization, although the 6-membered sulfur analog of 1,3,5-trioxane, namely 1,3,5-trithiane, has been reported to polymerize [155], Polymerization of thiiranes and thietanes is practically irreversible. 

As in the case of thiiranes, also in the cationic polymerization of 4-mem-bered cyclic sulfides, thietanes, chain transfer to polymer effectively competes with propagation. Intramolecular chain transfer, leading to formation of branched structures is well documented in these systems, because branched ions has been observed directly by H NMR [160] ... 

In cationic ring-opening polymerization, there are not too many examples of the systems in which ratios of kplk, are known. In the polymerization of substituted aziridines and substituted thietanes the ratios of rate constants of chain transfer to polymer to the rate constants of propagation have been measured and at least the value obtained for polymerization of N-/-butylaziridine (1.2-104) [260], indeed indicates the living character... 

The simplest systems involve copolymerization of structurally related pairs of comonomers, polymerizing irreversibly. Copolymerization of different oxetanes [294], thietanes [295], azetidines [296], and oxazolines [297] was studied, the results were interpreted in terms of simple four-parameter copolymerization scheme and the corresponding reactivity ratios for some systems were determined. 

---