Ring-Opening Polymerizations of Cyclic Sulfides

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. 

The propagation reaction probably involves nucleophilic attacks at the a-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. 

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

Two mechanisms of formation of sulfoniumions are possible (1) by approaches to the catalyst s electron-accepting sites, (2) by abstraction of hydrides by methyl cations.  

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Aliphatic polysulfides with two or more carbon atoms per monomeric unit are accessible through ring opening polymerization of cyclic sulfides or by the addition of thiol groups onto vinyl groups. In these cases, the anionic polymerization of cyclic sulHdes differs substantially from that of cyclic ethers. The ethyl anion attacks the carbon atom in cyclic ethers. But in the ethyl lithium initiated polymerization of propylene sulfide, a lithium ethane thiolate is first formed, and its anion then starts the polymerization of propylene sulfide ... 

Lastly, mention will be made of some methods for the formation of block copolymers containing poly(thioalkylene) sequences by the ring opening polymerizations of cyclic sulfides. 

The key initiation step in cationic polymerization of alkenes is the formation of a carbocationic intermediate, which can then interact with excess monomer to start propagation. We studied in some detail the initiation of cationic polymerization under superacidic, stable ion conditions. Carbocations also play a key role, as I found not only in the acid-catalyzed polymerization of alkenes but also in the polycondensation of arenes as well as in the ring opening polymerization of cyclic ethers, sulfides, and nitrogen compounds. Superacidic oxidative condensation of alkanes can even be achieved, including that of methane, as can the co-condensation of alkanes and alkenes. 

Another preparative technique involves the ring-opening polymerization of cyclic polysulfides.56 For example, l-oxa-4,5-dithiacycloheptane polymerizes very rapidly at room temperature when treated with an aqueous sulfide (reaction (11)) ... 

PROGRESS IN RING-OPENING POLYMERIZATION OF CYCLIC ETHERS AND CYCLIC SULFIDES... 

Studies of ring-opening polymerization of cyclic ethers and cyclic sulfides are underway in many laboratories throughout the world, A search of eleven leading polymer science journals, made in the course of preparing the present review, revealed 109 papers on these subjects just in the period 1980-1982 (1). This chapter makes no attempt to be comprehensive in its treatment of this active area of research. Instead, discussion is limited to important advances of three kinds ... 

Scheme 12 Mechanism of ring-opening polymerization of cyclic allyl sulfide (44a). Reproduced from Moad, G. Rizzardo, E. Thang, S. H., Radical addition-fragmentation chemistry in polymer synthesis. Polymer 2008, 49,1079-1131." ...

Onium Ions. Trialkyloxonium ions (R O A ) became the conventional initiators for the cationic ring-opening polymerization of all classes of heterocycles (cyclic acetals, ethers, sulfides, lactones, phosphates, and amines). They are prepared by two methods developed by Meerwin (38) and Olah (39). Another more general and convenient synthesis method was recently developed by Penczek et aL (40) ... 

Stille and Empen [61] reported on ring-opening polymerization of other cyclic sulfides to give poly(alkylene sulfide). Stille and Empen [61] found that... 

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. 

A wide variety of cyclic monomers have been successfully polymerized by the ring-opening process [Frisch and Reegan, 1969 Ivin and Saegusa, 1984 Saegusa and Goethals, 1977]. This includes cyclic amines, sulfides, olefins, cyclotriphosphazenes, and IV-carboxy-oc-amino acid anhydrides, in addition to those classes of monomers mentioned above. The ease of polymerization of a cyclic monomer depends on both thermodynamic and kinetic factors as previously discussed in Sec. 2-5. 

Alkyllithiums react quite differently with cyclic sulfides compared to the normal nucleophilic ring-opening reaction with epoxides (35,36). Ethyllithium reacts with 2-methylthiacyclopropane to generate propylene and lithium ethanethiolate. The resulting lithium ethanethiolate is capable of initiating polymerization of 2-methylthiacyclopropane. 

There are a number of heterocyclic monomers, for example, epoxides, cyclic sulfides, lactones and lactides, lactams, cyclic carbonates, and cydosUoxanes, which can be polymerized by ring-opening reactions many of them can he polymerized by an anionic as well as by a cationic mechanism. They cannot all be covered here, but there are a number of monographs and reviews on this subject [181-185]. 

Cyclic monomers that have been polymerized via ring-opening encompass a variety of structures, such as alkanes, alkenes, compounds containing heteroatoms in the ring oxygen [ethers, acetals, esters (lactones, lactides, and carbonates), and anhydrides], sulfur (polysulfur, sulfides and polysulfides), nitrogen [amines, amides (lactames), imides, N-carboxyanhydrides and 1,3-oxaza derivatives], phosphorus (phosphates, phosphonates, phosphites, phosphines and phosphazenes), or silicon (siloxanes, silaethers, carbosilanes and silanes). For the majority of these monomers, convenient polymerization conditions have been elaborated, that result in the controlled synthesis of the corresponding polymers [1-13]. 

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