Ring-Opening Polymerizations of Cyclic Sulfides

Ring-opening polymerizations of cyclic sulfides can be carried out by anionic, cationic, and coordinated mechanisms [187-189]. These polymerizations are easier to carry out then those of the oxygen analogs, because the sulfur-carbon bond is more polarizable. On the other hand, due to the larger size of the sulfur atoms the rings are less strained than in the oxygen compounds. As a result, the sulfur analog of tetrahydrofuran fails to polymerize. In cationic polymerizations, the propagating species are sulfonium ions [189, 190] and in anionic ones the sulfide anions. Goethals and Drigvers proposed the following cationic mechanism for the polymerization of dimethyl thietane [189]:

1. The initiation mechanism with triethyl fluoroborate consists of alkylation of the monomer molecule and formations of cyclic sulfonium ions. The reaction occurs instantaneously and quantitatively:

2. The propagation reaction probably involves nucleophilic attacks at the x-carbon atom of the cyclic sulfonium ions by the sulfur atoms from other monomer molecules:

The existence of the sulfonium ions among the propagating species was confirmed with NMR studies [191].

3. Termination is presumed to occur through formations of unreactive sulfonium ions.

Two mechanisms of formation of sulfonium ions are possible: (1) by approaches to the catalyst's electron accepting sites, (2) by abstraction of hydrides by methyl cations [190]:

There are indications of a "living" chain-growth mechanism in boron trifluoride diethyl etherate initiated polymerizations of propylene sulfide [192] at conversions of 5-20%. In these early stages of polymerization the molecular weight corresponds to that calculated for typical "living" polymers. This is believed to take place through formations of stable sulfonium ions:

Higher conversions in thiirane polymerizations, however, proceed with chain scission transfer mechanism under the influence of BF₃•(C₂H₅)₂O [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]. When the polymerizations of cyclic sulfides are carried out with anionic initiators, many side reactions can occur. On the other hand, common anionic initiators, like KOH yield optically active polymers from optically active propylene sulfide [194]. An example of a side reaction is formation [192] of propylene and sodium sulfide in sodium naphthalene-initiated polymerizations. Such reactions are very rapid even at -78°C. A similar reaction was shown to take place with ethyllithium [195]:

Other side reactions that occur in butyllithium-initiated polymerizations are cleavages of the polysulfides [192]:

High molecular weight polymers can be prepared from ethylene sulfide with a diethylzinc-water catalyst [196]. The polymers form in two steps. Initially insoluble crystalline polymers form at room temperature with a high catalyst to monomer ratio. These product polymers, that contain all of the catalyst act as seeds for further polymerizations. Though the final polymers are insoluble, the molecular weights are estimated to be high. At a conversion of 20% the molecular weights are believed to be about 900,000 [196]. When diethylzinc is prereacted with optically active alcohols, optically active poly (propylene sulfide)s form [197-199]. Cadmium salts are also very effective catalysts for polymerization of thiiranes. The polymers of substituted thiiranes have high stereoregularity.

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