Oxonium polymerization

Organometallic compounds asymmetric catalysis, 11, 255 chiral auxiliaries, 266 enantioselectivity, 255 see also specific compounds Organozinc chemistry, 260 amino alcohols, 261, 355 chirality amplification, 273 efficiency origins, 273 ligand acceleration, 260 molecular structures, 276 reaction mechanism, 269 transition state models, 264 turnover-limiting step, 271 Orthohydroxylation, naphthol, 230 Osmium, olefin dihydroxylation, 150 Oxametallacycle intermediates, 150, 152 Oxazaborolidines, 134 Oxazoline, 356 Oxidation amines, 155 olefins, 137, 150 reduction, 5 sulfides, 155 Oxidative addition, 5 amine isomerization, 111 hydrogen molecule, 16 Oxidative dimerization, chiral phenols, 287 Oximes, borane reduction, 135 Oxindole alkylation, 338 Oxiranes, enantioselective synthesis, 137, 289, 326, 333, 349, 361 Oxonium polymerization, 332 Oxo process, 162 Oxovanadium complexes, 220 Oxygenation, C—H bonds, 149... 

Cationic ring-opening polymerization is the only polymerization mechanism available to tetrahydrofuran (5,6,8). The propagating species is a tertiary oxonium ion associated with a negatively charged counterion ... 

It is possible to balance all of these thermodynamic, kinetic, and mechanistic considerations and to prepare well-defined PTHF. Living oxonium ion polymerizations, ie, polymerizations that are free from transfer and termination reactions, are possible. PTHF of any desired molecular weight and with controlled end groups can be prepared. 

Initia.tlon. The basic requirement for polymerization is that a THF tertiary oxonium ion must be formed by some mechanism. If a suitable counterion is present, polymerization follows. The requisite tertiary oxonium ion can be formed in any of several ways. 

Hall and Steuck polymerized 2 with a variety of Lewis and Bronsted acids or oxonium salts. The best conditions for the polymerization proved to be the use of phosphorus pentafluoride in methylene chloride solution at -78 °C. Yields of methanol-insoluble polymers ranging from 68 to 84% were obtained with inherent viscosities of 0.26—0.33 dl/g. Lower or higher temperatures gave lower yields. Tetra-hydrofuran as solvent at —78 °C gave 68-92% yields of materials having inherent viscosities of 0.12-0.14 dl/g. No incorporation of tetrahydrofuran into the polymer occurred. 

A special case of the internal stabilization of a cationic chain end is the intramolecular solvation of the cationic centre. This can proceed with the assistance of suitable substituents at the polymeric backbone which possess donor ability (for instance methoxy groups 109)). This stabilization can lead to an increase in molecular weight and to a decrease in non-uniformity of the products. The two effects named above were obtained during the transition from vinyl ethers U0) to the cis-l,2-dimethoxy ethylene (DME)1U). An intramolecular stabilization is discussed for the case of vinyl ether polymerization by assuming a six-membered cyclic oxonium ion 2) as well as for the case of cationic polymerization of oxygen heterocycles112). Contrary to normal vinyl ethers, DME can form 5- and 7-membe red cyclic intermediates beside 6-membered ringsIl2). 

Chojnowski and co-workers have studied the polymerization of octamethyltetrasila-l,4-dioxane, a monomer more basic than cyclosiloxanes, which is capable of forming more stable oxonium ions, and thus being a useful model to study the role of silyloxonium ions.150-152 In recent work, these authors used Olah s initiating system and observed the formation of oxonium ion and its transformation to the corresponding tertiary silyloxonium ion at the chain ends.153 The 29Si NMR spectroscopic data and theoretical calculations were consistent with the postulated mechanism. Stannett and co-workers studied an unconventional process of radiation-initiated polymerization of cyclic siloxanes and proposed a mechanism involving the intermediate formation of silicenium ions solvated by the siloxane... 

Studies on the cationic polymerization of cyclic ethers, cyclic formals, lactones and other heterocyclic compounds have proliferated so greatly in the last few years that a detailed review of the evidence concerning participation of oxonium and analogous ions in these reactions cannot be given here. Suffice it to say that there is firm evidence for a few, and circumstantial evidence for many such systems, that the reactive species are indeed ions and there appears to be no evidence to the contrary. A few systems will be discussed in sub-sections 3.2 and 4.4. 

In the present context the term cationic polymerization refers to reactions in which compounds with C=C bonds are added to carbenium ions, R R"R" C+, with the reformation of the carbenium ion after each addition, and the eventual formation of polymers in this way. The polymerizations via oxonium ions are excluded, as are pseudo-cationic polymerizations (Plesch 1988). 

In the standard procedure the neutralised polymer is purified and hydrolysed and the hydrolysate is examined for ethanol. The absence of ethanol indicates that no tert.-oxonium ions containing a polymeric moiety and no polymeric oxycarbenium ions could have been present in the reaction mixture. 

In the initial step of the polymerization, a cyclic oxonium ion is formed by transfer of an alkyl group from the initiator to the cyclic ether. Propagation occurs by SN2 attack of a monomer molecule at a ring a-methylene position of the cyclic tertiary oxonium ion, followed by opening of the oxonium ring and formation of a new cyclic oxonium ion. 

Kinetic study of this reaction usually requires sampling the polymerizing mixture and analyzing for the concentrations of the various reaction species at different polymerization times. Vofsi and Tobolsky in 1965 reported the use of radioactively tagged initiator (10), while Saegusa amd coworkers in 1968 developed a "phenoxy end-capping" method in which the oxonium ion is trapped with sodium phenoxide and the derived phenyl ether at the polymer chain end quantitatively determined by UV spectrophotometry (11). 

Polymerization Equilibria. As mentioned earlier, esters of strong acids, e.g. trifluoromethane sulfonic acid ("triflates"), are excellent initiators for the polymerization of THF. With such initiators, however, a complication arises. In addition to the normal propagation i depropagation equilibria of oxonium ions, Smith and Hubin postulated that the macroion ( ) may also convert into a corresponding nonpolar macroester ( ) by attack of the anion (14). ... 

In nonpolar media, on the other hand, the newly formed oxonium ion will either quickly convert to the corresponding soluble ester, or it will precipitate, since monomeric or short-chain oligomeric oxonium salts have low solubility in such media. The soluble ester is structurally similar to the initiator and may add another THF molecule. The resulting oxonium ion will again revert to the ester or precipitate. In fact, precipitates are generally observed durina the eairly stages of polymerization in media of low polarity. They have been isolated and characterized as monomeric or short chain oligomeric oxonium salts (17). 

We observe the a-methylene carbons of the methyl tetrahydro-furanium ion, the a-carbons of the two types of propagating chain heads, the macroion and the macroester (17). The observation of the a-methylene carbon resonances of the acyclic tertiary oxonium ion provides a direct proof of chain transfer reaction in THF polymerization. 

As an example of a cyclic ether copolymerization, we will briefly discuss the polymerization of THF with OXP initiated with methyltriflate. The homopolymerizations of both cyclic monomers follow a similar mechanism, and both were found to proceed via macrooxonium ion and/or the macroester mechanism depending on the polarity of the polymerization medium. There should then be 8 possible end-groups, i.e. two types of methoxy tails having a penultimate THF or OXP unit, respectively, two covalent macroesters, and four different oxonium ion propagating chain heads two from a THF oxonium center attached to penultimate THF or OXP units, and two from an OXP oxonium center attached to THF and OXP penultimate units (Scheme III). ... 

In the macroester region, we have a small signal due to OXP-ester only, no THF macroester was observed under these conditions. On the other hand, in the oxonium region there are resonances due to the THF-macroions only but not to OXP-macroions. Therefore, in this particular polymerization system we have identified 4 out... 

Slomkowski S, Szymanski R, Hofman A (1985) Formation of the intermediate cyclic six-membered oxonium ion in the cationic polymerization of P-propiolactone initiated with CHsCO SbFs. Makromol Chem 186 2283-2290... 

Many polymerizations exhibit a maximum polymerization rate at some ratio of initiator to coinitiator [Biswas and Kabir, 1978, 1978 Colclough and Dainton, 1958 Taninaka and Minoura, 1976]. The polymerization rate increases with increasing [initiator]/[coinitiator], reaches a maximum, and then either decreases or levels off. Figure 5-1 shows this behavior for the polymerization of styrene initiated by tin(IV) chloride-water in carbon tetrachloride. The decrease in rate at higher initiator concentration is usually ascribed to inactivation of the coinitiator by initiator. The inactivation process in a system such as SnCl4-H20 may involve hydrolysis of Sn—Cl bonds to Sn—OH. There is experimental evidence for such reactions when comparable concentrations of coinitiator and initiator are present. However, the rate maxima as in Fig. 5-1 are observed at quite low [initiator]/[coinitiator] ratios where corresponding experimental evidence is lacking. An alternate mechanism for the behavior in Fig. 5-1 is that initiator, above a particular concentration, competes successfully with monomer for the initiator-coinitiator complex (V) to yield the oxonium salt (VI), which... 

Ring-opening polymerizations are generally initiated by the same types of ionic initiators previously described for the cationic and anionic polymerizations of monomers with carbon-carbon and carbon-oxygen double bonds (Chap. 5). Most cationic ring-opening polymerizations involve the formation and propagation of oxonium ion centers. Reaction... 

Propagation in the cationic polymerization of cyclic ethers is generally considered as proceeding via a tertiary oxonium ion, for example, for the polymerization of 3,3-bis(chlor-omethyl)oxetane (R = CH2C1)... 

A variety of initiator systems, of the types used in the cationic polymerization of alkenes (Sec. 5-2a) can be used to generate the tertiary oxonium ion-propagating species [Dreyfuss and Dreyfuss, 1969, 1976 Inoue and Aida, 1984 Penczek and Kubisa, 1989a,b]. 

Strong protonic acids such as trifluoroacetic, fluorosulfonic, and trifluoromethanesulfonic (triflic) acids initiate polymerization via the initial formation of a secondary oxonium ion... 

This type of initiation is limited hy the nucleophilicity of the anion A derived from the acid. For acids other than the very strong acids such as fluorosulfonic and triflic acids, the anion is sufficiently nucleophilic to compete with monomer for either the proton or secondary and tertiary oxonium ions. Only very-low-molecular-weight products are possible. The presence of water can also directly dismpt the polymerization since its nucleophilicity allows it to compete with monomer for the oxonium ions. 

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