Polymerization of thietans

The study of the polymerization of thietan and its substituted derivatives has been undertaken relatively recently [131—134]. Detailed kinetic experiments have been reported by Goethals et al. [33, 130, 135—137] and, as we shall see, the results parallel closely those foimd in the polymerization of THF. Most of the work has been carried out using 3,3-dimethylthietan, 

Molecular weight data was also in agreement with this proposed termination mechanism. 

It is known that the basicity of cyclic sulphides is generally less than that of linear sulphides [133, 138] depending upon substitution, and confirmation of these suggestions was achieved when 1-ethyl-3,3-dimethyl trimethylene sulphonium tetrafluoroborate, 

When t = 0, [M] = [M] o and hence, K = —In [P ] q. However, if initiation is instantaneous then [P ] = [I]q, the initial concentration of initiator salt. Thus the equation above becomes 

Polymerizations were followed by a nuclear magnetic resonance technique and Rp was obtained from the slope of the tangent at various points on the time/conversion curves. A plot of In (l p /[M] [I] o) against A yields a straight line of slope, fej, and ordinate intercept. In fep. A similar analysis has been used by Penczek and Kubisa [139, 140] in the polymerization of 3,3-bis(chloromethyl)oxetan (Section 7.3) and a summary of the results obtained by Goethals and Drijvers is shown in Table 9. 

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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. 

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

Inorgamc halides. Hydrochl[Pg.31]

The cationic polymerization of heterocyclic monomers has been reviewed, and the polymerization of thietanes has been effected by the use of Lewis acids... 

Anionic polymerization of thietane and various 2-, 3- and polysubstituted thietanes has been achieved with alkali metals (Li, Na, K, Cs, Rb), naphthyl sodiumn-butyllithium, °" 1,4-dilithio-l, 1,4,4-tetraphenylbutane, and a thiolate anion.Treatment of 3-chlorothietane with aqueous sodium thiocyanate is said to give polymeric material.Polymerization of thietane has been effected with Grignard reagents.Thietane and substituted thietanes have been polymerized with dialkyl zinc reagents.A copolymer has been obtained by treating 2-methylthietane and styrene with -butyllithium a block copolymer has been derived from thietane and isoprene. " Copolymers of thietane and 3,3-dimethylthietane with pivalolactone have been reported. ... 

No polymerization of thietane 1-oxide could be effected by either cationic or anionic catalysts. ... 

If kp > fej, i.e. if initiation is virtually instantaneous, then this scheme is identical to that used by Goethals and Drijvers [33] for the polymerization of thietans (Section 7.2), and a similar analysis of experimental data is possible. However, if initiation is slow, as it is in this case, and compares with the rate of propagation, then the rate of change of the concentration of active centres takes a modified form, i.e. 

The cationic polymerization of thietane is generally accepted to proceed in the same manner as that of the oxetanes. The most detailed study was carried out by Goethals et al. [42] on thietane and methyl thietanes in CH2CI2 with Et3 0 BF4 as initiator. The initiation reaction was immediate and quantitative [68]. For these monomers propagation also occurred by nucleophilic attack of the monomer on the a-carbon of the cyclic sulphonium ion. For example, the propagation reaction for... 

Fig. 5. Time-conversion cui-ve for the polymerization of thietane, a-MT, DMT, DET and 3-ethyl-3-butylthietane (EBT) in CH2CI2 at 20°C [42],...

Oxetanes are rather basic materials and are inert to attack by bases. However, as stated above, anionic polymerization of thietanes has been reported [67], Significantly, in anionic polymerizations initiated by organo-lithium compounds the propagating end is a carbanion rather than a thiolate species viz. 

Termination evidently does not occur as readily in anionic polymerization of thietanes as it does in cationic polymerization. Organo-lithium initiated polymerizations of thietanes lead to living polymers, which have been used to prepare ABA block copolymers of dienes and cyclic sulphides [69, 70]. Since the anionic polymerization of thietanes proceeds via a carbanion, thietEines can initiate vinyl polymerization and their polymerization can be initiated by vinyl monomers. Kinetic parameters of such polymerizations have not yet been reported. 

Before leaving Et3 0 BF4 initiated polymerizations of THE one other study needs to be mentioned. In polymerizations of thietanes above we saw that free sulphonium ions did not propagate very much more rapidly than ion pairs (Section 3.2.1). Sangster and Worsfold [88] have found that in THE polymerizations with BE4 counter-ions in CH2CI2 the free ion rate is only a factor of 7 greater than that of the ion pair rate. 

In much of the early work basicity emerged as the most important factor relating relative reactivity of the cyclic ethers. It is still considered an important factor in many polymerizations. We have already seen that during polymerizations of thietanes, the relative basicity of the sul-phonium ion of both the monomers and the polymers plays an important role. In this case steric hindrance was also important. 

A suicide also occurs if the atom carrying the cation in the polymer is more basic than the same atom in the monomer, and in this case it occurs by transfer to polymer. An example of this is the polymerization of thietanes with (C2H5)30 BFf,... 

The mechanism of the cationic polymerization of thietanes differs from the mechanism described for the thiiranes by the fact that the reaction between the active species and a sullin atom of the polymer chain appears to be irreversible so that it is a real termination reaction. As a consequence, the polymerizations of thietanes stop before all monomer is consumed and there is no formation of cyclic oligomers. The maximum conversions obtained depend on the amoimt of initiator used and on the ratio of the rate constant of propagation, fep, to the rate constant of the termination reaction, kt (Scheme 34). 

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... 

Sulfur-containing heterocycles have rarely been used in ROP even though they are interesting for a variety of biomedical applications. Heterocycles higher than four membered cannot be polymerized by an anionic mechanism. While the cationic polymerization of thietanes leads to branched polymers or crosslinked networks, higher sulfur heterocycles were never used in the synthesis of hyperbranched polymers by ROP, since the synthesis of linear polymers is already... 

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