Other Oxetanes

Goethals, in a review published in 1965, listed 33 cationically polymerized differently substituted oxetanes8). In most cases the corresponding monomer was synthesized, and its ability to polymerize was confirmed by routine experiments using BF3 initiator. Visual observations were made (i.e. viscous oil, amorphous solids, crystalline) but no details as to yields and molecular weights of polymers prepared under truly reproducible conditions were given. 

Recently Kops described in more detail the polymerization of a 2-substituted oxetane, 2-methyloxetane 40). By the use of (C2Hs)30 PFf initiator and CH2C12 solvent at —80 °C, Kops obtained reasonable yields (30-60%) of polymers with Mn as high as 3 105. 

According to 13C-NMR analysis, the polymers were not uniform, containing both head-to-head and head-to-tail units, and the proportion of the latter increased with decreasing temperatures  

This result indicates that opening of the 4-membered ring is nonspecific and both O—C2 and O—C4 bonds can be broken. 

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The field of metal-catalyzed copolymerization of oxetanes and C02 will continue to flourish, due not only to the versatility of the reaction but also to the aliphatic polycarbonate products being important components of thermoplastic elastomers that, in turn, have huge potential in medical applications such as sutures, drug-delivery systems, body, and dental implants, and tissue engineering. The exploration of other oxetane monomers (Figure 8.17) such as 3,3-dimethyloxetane and 3-methoxymethyl-3-methyloxetane, will surely provide a multitude of applications... 

Applications. PBCMO is used especially as material resistant in corrosive atmospheres at moderately elevated temperatures. Useful applications are found as adhesives, coatings, sheeting, lining pipes, tanks, etc. It must also be noted that BCMO and other oxetanes are also used to prepare polyglycols which are used in polyurethanes, polyesters, and polyamide-type elastomers. 

Other oxetanes have also been found to have ansoathstio activity with rats, but the tmde does was about the same aa the effective doso. " Capillary rupture waa observed. The additional ozetanes thus teateci were trimethylene oxide, 3>methyloxetane, 2,3- and 3,3-dimetliyl oxetone and S-ethyloxetatie. 

Oxetane monomers and telechelics. 3-Bromomethyl-3-methyloxetane, BrOx, served as the starting material for the fluorous and other oxetane monomers described in sections 10.2-10.5 (Figure 10.1). The synthesis of BrOx is conveniently carried out on a large scale from l,l,l-tris(hydroxymethyl) ethane as shown in Equations 10.1 and 10.2 [23-25]. Fluorous and other oxetane monomers were prepared from BrOx by variations of the Williamson synthesis (Equation 10.3) in a manner similar to that described by Malik [26,27]. 

Other Oxetane Elastomers. The 1M)-butadiene monoxide (BMO) copolymer elastomer (Run 6, Table II) containing 6.5% BMO gave inferior properties (Table VIII) to the TMO-AGE elastomer with 4.4% AGE described in the previous section. Actually the properties were even slightly inferior to a 98-2 TMO-AGE copolymer vul-canizate (Table VIII). Thus AGE appears about three times more effective (on a weight basis) in conferring sulfur vulcanizability than BMO. On a molar basis, the difference is even greater, i.e., AGE is five times more effective. This same difference between BMO and AGE was previously observed on comparing PO-BMO elastomer with PO-AGE elastomer. 

A 96-4 TMO-AGE copolymer prepared under conditions to make it reasonably uniform gave an interesting sulfur-curable elastomer. Preliminary vulcanizate data on this elastomer show good tensile and tear properties, a low Tg (-75 C.), high resilience, and good heat resistance. Further development of this family of interesting elastomers requires a lower cost route to TMO. Copolymerizations of TMO with other epoxides and oxetanes as well as the polymerization of other oxetanes are also described. 

In the NTC region, back-bitiag reactioas appear to be respoasible for the formation of cycHc ethers (60,165—170). la additioa to oxetanes and tetrahydrofurans, tetrahydropyrans, oxiranes, and others are also observed (60,96,169) the tetrahydrofurans are favored. 0-Heterocycle yields of 25 to 30% have been reported for / -pentane (165,171). Conjugate and other olefins are also prominent products ia this regioa (60,169—172). 

SolubiHty parameters of 19.3, 16.2, and 16.2 (f /cm ) (7.9 (cal/cm ) ) have been determined for polyoxetane, po1y(3,3-dimethyl oxetane), and poly(3,3-diethyloxetane), respectively, by measuring solution viscosities (302). Heat capacities have been determined for POX and compared to those of other polyethers and polyethylene (303,304). The thermal decomposition behavior of poly[3,3-bis(ethoxymethyl)oxetane] has been examined (305). 

Properties have been determined for a series of block copolymers based on poly[3,3-bis(ethoxymethyl)oxetane] and poly [3,3-bis(methoxymethyl)oxetane]- (9-tetrahydrofuran. The block copolymers had properties suggestive of a thermoplastic elastomer (308). POX was a good main chain for a weU-developed smectic Hquid crystalline state when cyano- or fluorine-substituted biphenyls were used as mesogenic groups attached through a four-methylene spacer (309,310). Other side-chain Hquid crystalline polyoxetanes were observed with a spacer-separated azo moiety (311) and with laterally attached mesogenic groups (312). 

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. 

Krespan ° has prepared a number of macrocycles, having both aza- and oxa-linkages in them, based on the 3,3-dimethyleneoxetane unit (see also Sect. 8.4 and Eq. 8.12). Typically, 3,3-bis(chloromethyl)oxetane is treated with a diol as shown in Eq. (3.40), in the presence of base. Once the bicyclic system is formed, further treatment with other nucleophiles (e.g., ammonia) can lead to opening of the 4-membered ring. 

The mechanism of the Patemo-Biichi reaction is not well understood, and while a general pathway has been proposed and widely aceepted, it is apparent that it does not represent the full scope of reactions. Biichi originally proposed that the reaction occurred by light catalyzed stimulation of the carbonyl moiety 1 into an excited singlet state 4. Inter-system crossing then led to a triplet state diradical 5 which could be quenched by olefinic radical acceptors. Intermediate diradical 6 has been quenched or trapped by other radical acceptors and is generally felt to be on the reaction path of the large majority of Patemo-Biichi reactions. Diradical 6 then recombines to form product oxetane 3. 

Oxetanes are present in several biologically active natural compounds as, for example, the taxol ring skeleton. An interesting method used to obtain this particular ring is the thermal [2 -i- 2] cycloaddition reaction. Longchar and co-workers reported a novel [2-1-2] cycloaddition of /1-formil enamides 5, often used in other cycloaddition and condensation processes, with acetylenic dienophiles 6 under microwave irradiation (in a domestic oven) to afford ox-etenes 7 in 80% yields [29]. This reaction was directed towards the synthesis of D-ring annelated heterosteroids (Scheme 2). 

This dimerization is so rapid that ketene does not form P-lactones with aldehydes or ketones, except at low temperatures. Other ketenes dimerize more slowly. In these cases the major dimerization product is not the P-lactone, but a cyclobutanedione (see 15-61). However, the proportion of ketene that dimerizes to p-lactone can be increased by the addition of catalysts such as triethylamine or triethyl phosphite. Ketene acetals R2C=C(OR )2 add to aldehydes and ketones in the presence of ZnCl2 to give the corresponding oxetanes. ... 

The dimerization of the parent ketene gives the P-lactone. One molecule of ketene reacts across the C=C bond as a donor and the other molecule reacts across the C=0 bond as an acceptor. This is similar to the concerted [2+2] cycloaddition reaction between bis(trifluoromethyl)ketene and ethyl vinyl ether to afford the oxetane (Scheme 26) [127], A lone pair on the carbonyl oxygen in the ketene molecule as a donor activates the C=C bond as the alkoxy group in vinyl ether. 

It is clear from Fig. 8 and Table 1 that the angle 0 does indeed decrease as expected if the n-pair models and rule 1 were applicable. Moreover, the hydrogen bond non-linearity 9 decreases along the series B = oxirane, oxetane, 2,5-dihydrofuran. On the other hand, the values of 9 for oxirane- -ClF and 2,5-dihydrofuran- -ClF (included in Fig. 8) reveal that the halogen bond shows little propensity to be non-linear. 

The reaction of carbonyl compounds to olefins often yields products difficult to obtain synthetically by other routes. The excellent yields obtainable under proper conditions make this reaction of definite preparative interest. Examples of some synthetic applications of oxetane formation follow ... 

Schuster and co-workers<11 16) have provided one of the best examples where both diradicals and zwitterionic species are probably involved. The dienone (14), when photolyzed in 1,1,2-trimethylethylene, forms only the oxetane (15) all other photoreactions are eliminated. This finding confirms that n w and not states are involved in the dienone photorearrangements. 

The other photochemical reactions of simple carbonyls mentioned earlier in this chapter—type I cleavage (a-cleavage) and oxetane formation—will be discussed in Chapter 4. 

In Chapter 3 we discussed two photochemical reactions characteristic of simple carbonyl compounds, namely type II cleavage and photoreduction. We saw that photoreduction appears to arise only from carbonyl triplet states, whereas type II cleavage often arises from both the excited singlet and triplet states. Each process was found to occur from discrete biradical intermediates. In this chapter we will discuss two other reactions observed in the photochemistry of carbonyls, type I cleavage and oxetane formation. 

Aryl ketones are often used to effect cis and tram isomerization of olefins.(118-ia0) Although this, in some cases, can be viewed as an energy transfer process where the ketone triplet transfers its energy to the olefin, which then isomerizes, the failure of noncarbonyl sensitizers of comparable triplet energy to isomerize the olefins suggests that a process other than energy transfer may be involved. Schenck and Steinmetz<118) suggested that isomerization results from decomposition of a biradical carbonyl-olefin adduct similar to that involved in oxetane formation ... 

The photocycloaddition of an aldehyde or ketone with an olefin to yield an oxetane was reported by Paterno and Chieffi in 1909. 58> Contemporary studies on the synthetic utility and mechanistic features were initiated nearly 50 years later by Biichi et al. 59) Two review articles summarizing synthetic aspects of Paterno-Biichi reactions have been published 6.12)) and mechanistic studies have been reviewed several times. 6,38,60-62) The reaction involves the addition to olefin of a photo-excited carbonyl moiety. This circumstance makes it advantageous to review this reaction before a discussion of olefin-olefin additions, because the solution photochemistry of carbonyl compounds is probably better understood than any other aspect of organic photochemistry. Many of the reactions of carbonyl compounds have been elucidated during studies of the important phenomena of energy transfer and photosensitization. 63-65)... 

Cyclobutane has not been polymerised cationically (or by any other mechanism). Thermochemistry tells us that the reason is not thermodynamic it is attributable to the fact that the compound does not possess a point of attack for the initiating species, the ring being too big for the formation of a non-classical carbonium ion analogous to the cyclopropyl ion, so that there is no reaction path for initiation. The oxetans in which the oxygen atom provides a basic site for protonation, are readily polymerizable. Methylenecyclobutane polymerises without opening of the cyclobutane ring [72, 73]. 

On the other hand, in cyclic ethers (alkene oxides, oxetans, tetrahydrofuran) and formals the reaction site is a carbon-oxygen bond, the oxygen atom is the most basic point, and, hence, cationic polymerization is possible. The same considerations apply to the polymerization of lactones Cherdron, Ohse and Korte showed that with very pure monomers polyesters of high molecular weight could be obtained with various cationic catalysts and syncatalysts, and proposed a very reasonable mechanism involving acyl fission of the ring [89]. 

The cationic ring-opening polymerization of epichlorohydrin in conjunction with a glycol or water as a modifier produced hydroxyl-terminated epichlorohydrin (HTE) liquid polymers (1-2). Hydroxyl-terminated polyethers of other alkylene oxides (3 4), oxetane and its derivatives (5 6), and copolymers of tetrahydrofuran (7-15) have also been reported. These hydroxyl-terminated polyethers are theoretically difunctional and used as reactive prepolymers. 

The strained rings of epoxides and oxetanes are susceptible to nucleophilic attack. In this section we discuss the reactions of these oxygen heterocycles with nitrogen oxides and other... 

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