Polymerizations cyclic sulfides

Cyclic sulfides polymerize via anionic and cationic mechanisms. 

Polymerization of cyclic amines proceeds usually with high activation energies, due to the high strength of the C—N bond. To attain significant rates (k 10-3 mol-1 1 s 1) requires temperatures from 30 to 100 °C for azetidines and about 200 °C for the less strained bicyclic six-membered amines quinuclidine and triethylenediamine 9). Cyclic sulfides polymerize more rapidly than amines but much slower than ethers of a comparable structure (e.g. for 3,3-dimethylthietane k, = 2 10-2 mol-1 1 s at 35 °C, which is at least 104 times lower than for 3,3-dimethyloxetane (cf. Adv. Polymer Sci. 37 (1980)). 

The 3- and 4-membered cyclic sulfides, referred to as thiiranes (also episulfides) and thie-tanes, respectively, are easily polymerized by both cationic and anionic initiators, such as... 

Although there are no reported polymerizations of simple cyclic sulfides of ring size 5 and higher, polymerizations of 1,3,5-trithane (the cyclic trimer of thioformaldehyde) (LX), disulfides such as l-oxa-4,5-dithiacycloheptane (LXI), and trisulfides such as norhonene trisulfide (LXII) have heen achieved [Andrzejewski et al., 1988 Baran et al., 1984 Moore et al., 1977 Zuk and Jeczalik, 1979]. 

A similar reaction of 2,2,4,4-tetrafluoro-l,3-dithietane 69 at elevated temperature resulted in formation of the mixture of products 70-72 along with some polymeric materials (Equation 10) <2005JFC(126)1332>. The proposed mechanism assumes a single electron transfer from electron-rich quadricyclane to the electron-deficient cyclic sulfides <2005JFC(126)1332>. 

Table 3. Cyclic oligomer formation during the cationic polymerization of cyclic sulfides in CH2C12 as solvent...

Again, these reactions, depending on the structure of monomer, may be reversible or irreversible. If resulting branched or macrocyclic onium ions are not reactive, i.e., they can not re-form the original active species either by intramolecular cyclization or by reaction with the next monomer molecule, then these reactions lead to termination (such a situation exists in the polymerization of cyclic sulfides or amines cf., Section 1II.D.E.). 

The factors, governing the reactivities of branched onium ions are of complex nature and will not be discussed here. It has been shown, however, that in the polymerization of cyclic sulfides and cyclic amines the formation of branched onium ions of the type ... 

The results show that the presence of bulky substituent on a polymer chain may effectively inhibit the termination proceeding by this mechanism. The results presented at this point may be summarized as follows chain transfer to polymer is a general feature of cationic ring-opening polymerization although for different systems the contribution of this reaction may vary only in some systems this process results in termination (These systems involve, e.g., cyclic amines (3- and 4-membered) and cyclic sulfides (3- and 4-membered) and the contribution of the reaction is reduced for substituted chains. 

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] ... 

The other onium salts (sulfonium or ammonium salts) are less often applied due to their lower reactivity. They can neither initiate the polymerization of cyclic ethers or acetals nor be displaced by less nucleophilic ligands (cyclic sulfides and amines respectively). 

Cyclic sulfides are known to be less basic than linear sulfides. Because the equilibrium constant K (Eq. (38)) is small, trialkylsulfonium ions cannot be used as initiators for the polymerizar tion of cyclic sulfides l. The relation between cyclic and linear amines resembles that of sulfides rather than etha s. In some cases, however, ammonium ions have been successfully used as initiators, e. g. in the polymerization of conidine or l-benzylaziridine l. 

Equilibria like (62) are strongly shifted to the side of oxonium ions e. g. the calculated heat of formation of the triethyloxonium ion from ethyl cation and diethyl ether is as high as 128 kcal mole" in the gas phase. This would give no chance for a single carbenium ion to exist at the usual polymerization conditions of the majority of heterocycles cyclic sulfides and amines are even stronger nucleophiles than cyclic ethers (cf., however, Ihe discussion on cyclic acetals below). 

The polymerization of cyclic sulfides and amines provided, although tentatively, some information on kp and kp. The equality k kp was found (the Kq value measured for an initiator used was further applied in the determimtion of a in polymerization studies) in the polymerization of 3,3-dimethylthietane in CgHsNOa and qualitatively for l-phenylmethyl-2-methyl-aziridine Small differences were found between kp and k (the macrocation being less... 

Table 12. Rate constants of propagation and activation parameters in the cationic polymerization of cyclic sulfides, amines and iminoethers at 0 °C... 

Other evidence stems from H-NMR studies of the polymerization of cyclic sulfides . 

Thietane is a superior inhibitor of corrosion of iron in 10% hydrochloric acid and its effectiveness is said to be due to partial polymerization on the surface of the iron. Addition of chloride ion reduces the inhibition, possibly by inducing ringopening with the formation of sulfhydryl groups. The cyclic sulfide also has been considered as an odorant for natural gas and its absorption by organic soil and clay have been determined. Stabilization of methylchloroform and trichloroethylene by thietane, 2-methylthietane, 3-hydroxythietane, and two spirothietane derivatives has been claimed. Phosphorus and tin derivatives of 3,3-bis-hydroxy-methylthie-tane are reported to be light stabilizers for poly(vinyl chloride), and the dibutyl-tin derivative is a catalyst for the polymerization of aliphatic isocyanates. Mercury and zinc compounds derived from phenylmercury or phenylzinc hydroxide and 3-... 

Trialkyloxonium ions are strong alkylating agents and initiate the polymerization of heterocyclic monomers by simple alkylation of the most nucleophilic site of the monomer. The initiation occurs quantitatively and without side reactions when stable anions are used. Consequently, these initiators are very useful for kinetic measurements. The initiation was followed directly by NMR spectroscopy in the case of THF (41). cyclic sulfides (42). and cyclic esters of phosphonic acid such as 2-methoxy-2-oxo-l,3,2-dioxaphosphorinane (43). 

From a series of kinetic studies with cyclic sulfides, amines (47. 81. 89). and cyclic esters of phosphonic acid (90). it was shown that in a given series of monomers with the same parent heterocycle, the ratio k /kj. increases with the bulkiness and the number of substituents on the monomer. Although the absolute rate of polymerization decreases also, the presence of substituents apparently retards the termination more than the propagation. A few examples are presented below. 

Thus, we first discuss thermodynamics, paying attention to features that are important for polymer synthesis (e.g., dependence of equilibrium monomer concentration on polymerization variables) then we describe kinetics and thermodynamics of macrocyclization, trying to combine these two related problems, usually discussed separately. In particular we present the new theory of kinetic enhancement and kinetic reduction in macrocyclics. Thereafter, we describe the polymerization of several groups of monomers, namely cyclic ethers (oxiranes, oxetanes, oxolanes, acetals, and bicyclic compounds) lactones, cyclic sulfides, cyclic amines, lactams, cyclic iminoethers, siloxanes, and cyclic phosphorus-containing compounds, in this order. We attempted to treat the chapters uniformly we discuss practical methods of synthesis of the corresponding polymers (monomer syntheses and polymer properties are added), and conditions of reaching systems state and reasons of deviations. However, for various groups of monomers the quality of the available information differ so much, that this attempt of uniformity can not be fulfilled. 

Heterocycles containing sulfur atoms are usually less strained than the corresponding ethers. Thus, the 5-membered cyclic sulfide-tetrahydrotiophene (THT) does not polymerize in contrast to THF (cf. additional discussion on THF on p. 23). 

The same phenomenon was observed in the polymerization of cyclic sulfides 4). Goethals has shown that in the polymerization of propylene sulfide linear polymer is formed first which then degrades to a mixture of cyclic oligomers, mostly to the cyclic tetramer 25). Thus, in several systems it was shown conclusively that cyclic... 

We have shown in the previous sections that in certain systems (e.g. polymerization of cyclic formals studied by Schulz 7 8) and Y amashita 20>) one can use kinetic enhancement to obtain higher proportions of macrocyclics. This is mostly due to the enhanced contribution of the end-to-end closure. There are systems (THF2- 3), cyclic sulfides 25y) in which kinetic ring-depression was observed due to a slow rate of cyclization and thus the equilibrium concentration of the rings was attained only slowly. These two extreme cases are depicted in Fig. 3.9. 

In some systems, mostly in the polymerization of cyclic sulfides and amines, the formation of polymeric onium ions is irreversible, i.e. the reactivity of these structures is so low that further propagation on these species does not proceed ... 

Polymerization of cyclic sulfides and amines can be initiated not only by the usual cationic and cationogenic compounds like Lewis and protonic acids, carbenium and oxonium salts and esters of strong acids, but also by alkyl halides, which are active enough to induce polymerization of some azetidines 9 10). 

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