Polymerization of Oxepanes

Oxepanes are polymerized by various cationic initiators like (C2H5)3C BF4 , (C2H5)3C -SbCl6 , BF3-epichlorohydrin, and SbCl6-epichlorohydrin. The reactions take place in chlorinated solvents, like piethylene chloride. The rates of these reactions, however, are quite slow. In addition, these polymerizations are reversible. The rates of propagation of the three cyclic ethers, oxetane, tetrahydrofuran, and oxepane, at 0 °C fall in the following oider  

The cationic pohmierizations of cyclic acetals are different from the polymerizations of the rest of the cyclic ethers. The differences arise from greater nucleophilicity of the cyclic ethers as compared to that of the acetals. In addition, cyclic ether monomers, epirane, tetrahydrofuran, and oxepane, are stronger bases than their corresponding polymers. The opposite is true of the acetals. As a result, in acetal polymerizations, active species like those of 1,3-dioxolane may exist in equilibrium with macroalkoxy carbon cations and tertiary oxonium ions. By comparison, the active propagating species in polymerizations of cyclic ethers, like tetrahydrofuran, are only terdaiy oxonium ions. The properties of the equilibrium of the active species in acetal polymerizations depend very much upon polymerization conditions and upon the structures of the individual monomers. 

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The polymerization of oxepane has provided very simflar results , Goethals has observed that in the polymerization of 3,3-dimethylthietane k, is also larger in CH2CI2 than in C6HsN02 >. 

More recently, the application of H-NMR confirmed that in the polymerization of oxepane the transfer to pobmiei (reaction (81)) has no kinetic importance ). There is, however, no evidence that in the polymerization of other cydic ethers, particularly more strained ones like oxe-tane and 3,3-dimethyloxetane, a portion of tertiary oxonium ions is not in the form of the dormant , non-reactive spedes. A few papers have reported the formation of larger rings, apparently by unimolecular back-biting, which may reduce the concentration of active spedes and eventually give wrong values of kp , if the method used for the determination of [P ] does not discriminate between two kinds of oxonium ions given in Eq. (81). 

Kinetic parameters for ring-opening polymerization of oxepane" [118]... 

S. Lundmark, M. SjOUng, A.C. Albertsson, Polymerization of oxepan-2,7-dione in solution and synthesis of block copolymers of oxepan-2,7-dione and 2-oxepanone, J. Macromol. Sd. A 28 (1991) 15-29. 

Ionic dissociation equilibria similar to those in THF polymerizations were observed in the cationic polymerization of oxepane. Parallel measurements of propagation and transfer rate constants have been conducted. ... 

For several monomers, propagation rate constants have been determined (the extensive list can be found in Reference 3). For polymerization of THF, the values between 3 x 10 and 1 X 10 mor ls" are given by different authors for different conditions for polymerization of oxepanes values between lO" and 10 morMs" and for polymerization of oxetanes values between 10" and ImoPMs" (at 0 °C in different solvents at different monomer and initiator concentrations). 

Polymerization is frequently observed as a side reaction in acid-catalyzed reactions, and under appropriate conditions many oxetanes can be quantitatively transformed into high molecular weight linear polyethers with useful properties. Polymerization is very general for oxetanes and is closely related to Lewis acid-catalyzed polymerization of oxiranes, THF and oxepane, but oxetanes generally polymerize much more rapidly than THF and oxepane and at a rate similar to oxiranes (72MI51300). 

NMR studies of the polymerization of THF, oxepane, 3,3-dimethylthietane, 2-methoxy-2-oxo-l,3,2-dioxaphosphorinane and 2-methyl-2-oxazoline revealed that the structures of the growing qj ies are the corresponding onium ions these structures are given in Table 7 (earlier, the first reaction products, formed upon initiation, were observed in the polymerization of THF ). 

Fig. 14. The propagation step in the polymerization of styrene (counter-ion, sodium). Variation of fegpp mole" sec" ) with inverse square root of concentration of active centres. Solvents ( ) tetrahydrofuran 25°C [16] (O) tetrahydropyran 30°C [106] (A) oxepane 30°C [105] ( ) dioxane 25°C [104].

More recent results indicate that in the polymerization of the 7-membered cyclic ether oxepane (Ox), both intra- and intermolecular ionizations have to be... 

Processes that do not terminate were described for the polymerization of five- and seven-membered cyclic ethers THF (4, 91) and oxepane (93). Under proper working conditions both cyclic acetals, namely 1,3-dioxolane and 1,3-dioxepane (94. 95). as well as bicyclic acetals or 1,6-anhydro-l,3,4-tri-0-benzyl-a-D-glucopyranose (96) can polymerize under living conditions. 

ROP offers an alternate approach to the synthesis of polyanhydrides used for medical applications. Albertsson and coworkers prepared adipic acid polyanhydride from cyclic adipic anhydride (oxepane-2,7-dione) using cationic (e.g., AICI3 and BFjTCjHj) ), anionic (e.g., CHjCOO K and NaH), and coordination-type inhibitors snch as stannous-2-ethylhexanoate and dibutyltin oxide [24,25]. ROP takes place in two steps (1) preparation of the cyclic monomer and (2) polymerization of the cyclic monomers [26]. 

How do the rates of oxepane polymerization compare to those of oxetane and tetrahydrofuran What affects these rates ... 

A ( )5, and solvent-complexed ion pairs, A (+) soi Measurements were made in hexamethyl cyclotriphosfamide, HMPA dimethyl glycol, DME tetrahydrofuran, THF 3-methyl tetra-hydrofuran MeTHF, tetrahydropyran, THP oxepane, OX and 1,4-dioxane, DIO. (After B. J. Schmitt and G. V. Schulz.) Polymerization of styrene. 

Cationic Ring-opening Polymerization and Copolymerization of Oxepane Initiated by Acylium Perchlorate Groups on Carbon Black. Journal of Polymer Science, Polymer Chemistry Edition, 26, 2715-2724... 

Kinetic poiymerizabiiity depends on the actual conditions of polymerization, having no influence on thermodynamic poiymerizabiiity. There are a few systems for which equilibrium constants of dissociation (Kd) were determined in order to establish the proportions of macroion pairs and macroions. Then, the rate constants of propagation for these species were determined and in this way the kinetic reactivity of various forms of active species could be compared. In the cationic polymerization of THF, oxepane (OXP), and more recently conidine, it has been shown that This was explained... 

This has been shown for cationic polymerization of five-membered and seven-membered oxepane. The... 

In contrast, in the cationic polymerization of cyclic ethers (e.g. THF or seven-membered oxepane), the dissociation equilibrium constants have considerably higher values, depending on the solvent polarity Ro-lO molh (CH2CI2) and - 10 molT (CH3NO2, QH5NO2) with SbClg as a counterion (see Table 8 in Ref [3a]). 

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