Polymerization of oxetanes

Oxetanes (or oxacyclobutanes) are preferably polymerized in a solvent to maintain stirring and temperature control. It is necessary to carefully purify both the solvent and the monomer, because impurities interfere with attainment of high molecular weights. 

Two initiating systems were most often used for the initiation of oxetane polymerization, namely BF3 and its complexes with ethers, and the R3A1/H20 system. 

It was established that the polymerization of oxetane (OX) cannot be initiated with BF3 alone in the absence of water or other proton donating compounds 12,13). The same phenomenon was observed later for the polymerization of BCMO where the addition of controlled amounts of water increased the overall polymerization rate 14,15). Thus, the necessity of water cocatalysis is well documented for OX-BF3 and BCMO-BF3 systems, the rates and yields of initiation in these systems are however unknown. 

Water plays also an important role in R3A1 initiated polymerizations, described in terms of a cationic process 16). Kinetic studies on the BCMO-R3A1-HzO system revealed that the rate of polymerization depends on the concentration of added water, i.e., the rate increases until the ratio [H2OJ/[(i-C4H9)3Al] approaches unity and than decreases for [HzO] [(i- H Al]1718). 

When the concentration of [(i-C4Hg)3Al] was changed at constant [HzO], similar effect was observed, namely the rate of polymerization increased until the 1 1 ratio was reached and then remained constant18). 

These results indicate that the true initiator is formed in this system by the reaction of alkylaluminum compounds with water. The initiator is most effective when the molar concentration ratio of both components is equal to 1. Considering that the molecular weights depend on water concentration, it was deduced that approximately ten molecules of water are required to start one chain. This means that an average initiator molecule results from the reaction of approx, ten molecules of HzO with approx, the same number of A1R3 molecules. 

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The chemistry of polymerization of the oxetanes is much the same as for THE polymerization. The ring-opening polymerization of oxetanes is primarily accompHshed by cationic polymerization methods (283,313—318), but because of the added ring strain, other polymerization techniques, eg, iasertion polymerization (319), anionic polymerization (320), and free-radical ring-opening polymerization (321), have been successful with certain special oxetanes. 

Figure 8.3 Two mechanisms have been proposed for the cationic ring-opening polymerization of oxetanes, the ACE route and the AMM [16]...

Lewis Acid Driven Anionic Polymerization of a Monomer with High Cationic PolymerizabiHty Ring-Opening Polymerization of Oxetane with Aluminum Porphyrin in the Presence of a... 

As described in the previous sections, the living anionic polymerizations of epoxides and methacrylic esters initiated with aluminum porphyrins 1 [67] are dramatically accelerated upon addition of sterically hindered Lewis acids such as 3 [68,69], where the monomers are coordinated to and activated for nucleophilic attack by the Lewis acids. Successful extension of this method is the living anionic polymerization of oxetane [70]. 

After the completion of the first-stage polymerization of oxetane ([oxe-tane]o/[l (X=Cl)]o/[3e]o=50/l/3) at room temperature, 150 equiv of oxetane with respect to 1 were newly added to the system, whereupon the second-stage polymerization ensued, and the Mn of the polymer was increased from 2900 (Mw/Mn=1.09) to 8300 (Mw/Mn=1.22), retaining the narrow MWD. 

PMMA-polyoxetane olock copolymer demonstrates that the polymerization of oxetane initiated with 1 (X=C1) in the presence of 3e is anionic,... 

A part of the present work was presented at the 43rd Annual Meeting of the Society of Polymer Science, Japan (May 1994, Nagoya) Polymer Prepr. Jpn. 1994,43, 171. Concurrently, Amass et al. reported the polymerization of oxetane with (TPP)AICI (1) alone at 55 C Amass AJ, Perry MC, Riat DS, Tighe BJ, Colclough E, Steward M (1994) J Eur Polym J 30(5) 641... 

Mixing of oxetane (15 mmol) with 4 (0.3 mmol) in CH2CI2 (3 ml) in the absence of 1 under similar conditions resulted in a polymerization of oxetane, where the monomer conversion was stopped at 61.4% (2h), affording a broad MWD polymer (Mn 10, Mw/Mn=1.93) together with some cyclic oligomers... 

The oligomer formation during the polymerization of oxetanes has been reininves-tigated by two groups recently. Drey fuss and Dreyfuss (21) found that with oxonium salts or with ethyl trifluoromethanesulfonate as initiator, oxetane forms not only cyclic tetramer and polymer as reported by Rose, but also cyclic trimer. The amount and type of oligomer obtained depends on counter ion, temperature and solvent. Polymeri-... 

Degradative transfer is not limited to radical processes. It can occur in many ionic polymerizations which are not exactly living. Degradative transfer here is, of course, harder to prove an ideal standard is missing. Nevertheless, degradative transfer has been proved, for example, during the cationic polymerization of oxetane [23],... 

This group usually leads to anionic or coordinate polymerization which are not covered by this review. Nevertheless, polymerizations of oxetanes and THF, known to proceed exclusively by a cationic mechanism, have also been induced by various organometallic initiators. In many cases, these initiators lead to higher molecular weight polymers, probably reacting fast and first with impurities that could, if not destroyed, lower the molecular weight by chain transfer. 

The equaibrium according to Eq. (92) involving oxazolinium cations was apparently the first observation of the equilibrium between ionic and covalent species in the cationic polymerization of heterocyclic monomers A similar collapse, being however irreversible, has earlier been postulated for the cationic polymerization of oxetane propagating with C(N02)3 anions and of styrene propagating with CIO anions ... 

Other evidence for the formation of macrocycles by similar medhanisins comes from the studies of the polymerization of oxetane and 3,3-dimethyloxetane. It was demonstrated by Rose h and confirmed by Dreyfuss, Wotsfold and Goethals ) that cyclic trimers and tetramers and higher cyclic oligomers are formed from these two monomers in polymerization. 

Rose also derived heats of polymerization. The values he obtained were 20.0 kcalmole for polymerization in methyl chloride solution at —20°C and 19.3 kcalmole" for polymerization in a mixture of ethyl chloride and methyl chloride at —9°C. It is at once a bit unfortunate that this elegant piece of work was carried out as early as 1956 before catalysts leading to less complicated kinetics were discovered, and a tribute to Rose s careful work that he was able to sort out the many complications involved. No other detailed kinetic study of the polymerization of oxetane was made until many years later. In 1971 Saegusa et al. [52], reported a kinetic study of the polymerization of oxacyclobutane initiated by a BF3—THF complex. 

The mechanism of the polymerization of oxetanes has been extensively investigated by means of kinetics, trapping and coupling experiments and NMR studies. It is now generally accepted that the reaction is initiated by electrophilic attack of the initiator on the ring oxygen atom and propagated by an 5n2 type chain mechanism, as shown below in equation (32) (74802699). 

Fig. 9.U Scheme of the cationic ring-opening polymerization of oxetanes.

J.V. Crivello, R. Falk, and M.R. Zonca, Photoinduced cationic ring-opening frontal polymerizations of oxetanes and oxiranes. J. Polym. Sci. A Polym. Chemi. 2004. 42(7), 1630-1646. 

Polymerization of oxetane in nitro-benzene is initiated by nitroform but not by tetranitromethane although the latter initiates the polymerization on addition of car-... 

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