Vinyl ether oxiranes

Vinyl ether oxiranes 112 form bicyclic systems stereospecifically. The regio- and stereospecific synthesis of 113 from 112 has been described under mild conditions (Eq. 122). ... 

The virtues of photoinitiated cationic polymerization are rapid polymerization without oxygen inhibition, minimal sensitivity to water, and the ability to polymerize vinyl ethers, oxiranes (epoxides), and other heterocyclic monomers (see Table 10.7) that do not polymerize by a free radical mechanism. 

Ionic Polymerization Ionic chain polymerizations can also be initiated with the aid of Vis/UV light although, as in the case of free-radical photopolymerization, the light serves only as an initiating tool. Most studies on ionic photopolymerization have focused on cationic polymerization, which proceeds rapidly and is not inhibited by oxygen [18,21-24]. Moreover, cationic photopolymerization is apt to polymerize monomers such as vinyl ethers, oxiranes (epoxides), and other heterocyclic compounds that do not polymerize via a free-radical mechanism (Table 3.9). 

Among the monomers given in Scheme 1, several possess sufficient nucleophilicity to undergo cationic polymerization (7,8), namely 2-vinyltetrahydrofiiran 1, the alkenylfiirans 2, the furfurylidene ketones 3, 2-furyl oxiranes 4 and the furfuryl vinyl ethers 6, Moreover, furfural and its 5-methyl derivative can act as comonomers in certain cationic copolymerizations. 

Thus, 2-furfuryl vinyl ether 6a is extremely sensitive to cationic activation (16) because of its very pronounced nucleophilic character, but the polymerization is accompanied by some gel formation due to abundant alkylation of the furan rings pendant to the macromolecules. This structural anomaly is not encountered with the 5-methylated monomer 6b (16) precisely because electrophilic substitutions take place predominantly at C5 and are therefore impossible with this monomer. A similar difference of phenomenolo was observed with the 2-fiiryl oxiranes 4a and 4b (17). 

Gree and co-workers have found that bicyclic epoxy alcohols when treated with diethylamino sulfur trifluoride (DAST) can undergo a ring expansion via C-C bond cleavage of the oxirane ring to provide fluorinated cyclic vinyl ethers (Table 17) <20020L451>. 

All cationically polymerizable monomers can be potentially used in this process however, the main study has been focused so far on the most reactive oxirane and vinyl ethers [4], Alkoxysilane derivatives - the most common acid-sensitive monomers for the synthesis of siloxane materials through the use of sol-gel methods - were not used extensively. Only a few examples of their application in photo-activated cross-linking can be noted, mainly in co-reaction with oxirane sites [5]. Typically, alkoxysilanes are subjected to an acid- or base-catalyzed process involving hydrolysis of an =SiOR group and then condensation of the formed silanol with another molecule bearing an =SiOH or =SiOR function to give a siloxane linkage [6]. It was of interest to combine the properties of cross-linked silicone materials with the ones provided by sterically overloaded... 

Radical addition of triphenylgermane to vinyl oxiranes proceeded in the presence of triethylborane to yield 4-triphenylgermyl-2-buten-l-ol derivatives in good yield. Iodo acetals, prepared by iodoetherification of vinyl ethers with the allylic alcohol, underwent radical cychzation to give 2-alkoxy-4-vinyltetrahydrofurans which were converted into 4-vinyltetrahydro-2-furanones by Jones oxidation (Scheme 11.22) [38]. 

Poly (ethyl vinyl ether) (PEVE) macromonomers were also prepared using initiator 5 bearing an allylic function [148]. This reactive group remained intact during the polymerization and could be transformed into the corresponding oxirane by peracid oxidation. 

A prepolymer that contains calixarene derivatives was reported. The material ean also include methacrylate, vinyl ether, propargyl ether, oxetane, oxirane, or spiro ortho ester groups. Such materials were synthesized... 

There are three t)q)es of eompositions that eure by eationie meehanism. One of them uses aryldiazonium salts to initiate the reaetion. The seeond one utilizes onium salts and the third one organometallie eomplexes. The most prominent ones are those that eure with the aid of onium salt photoinitiators. Many cationic curable compositions consist of mixtures of compounds with oxirane rings. They may also be mixtures of vinyl ether. In addition, some compositions contain both, epoxides and vinyl ethers. More recent compositions might also include silicone based monomers with epoxide groups. Thus, Crivello and Lee described a synthesis of a series of silicon-epoxy monomers that undergo rapid and efficient photoinitiated cationic polymerizations. Such compounds can be prepared by direct hydrosilylation of olefmic epoxides. 

The synthesis of dichloronorcarane from cyclohexene by the chloroform-base-PTC method has been improved further as has the preparation of a-halogeno-aP-unsaturated ketones via em-dihalogenocyclopropanes by employing trimethylsilyl vinyl ethers rather than ethyl vinyl ethers. The formation of gem-difluorocyclo-propanes proceeds in high yield (60— 90%) when chlorodifluoromethane is treated with halide ion and an epoxide in the presence of an olefin. The epoxide-halide ion combination is employed to produce a base of sufficient strength, and in sufficient concentration, to maximize the production of difluorocarbene oxiran and chloro-methyloxiran afford the most suitable bases when treated with chloride ion (Scheme 4). 

Various basic (nucleophilic) initiators can be used to initiate anionic polymerization, including covalent or ionic metal amides, alkoxides, hydroxides, cyanides, phosphines, amines, and organometallic compounds such as butyllithium. The polymerization is initiated by addition of the initiating species to the monomer, typically a vinyl compound with electron-withdrawing moieties. In anionic ROPs, cyclic monomers with electron-deficient carbon atoms such as amides (lactams) and esters (lactones), ethers (oxiranes), or Leuchs anhydrides are used as monomers (Scheme 13). 

The appearance of sulfonium salts as end groups in polymers produced using these photoinitiators supports this conclusion Among those monomers which undergo facile polymerization with dialkylphenacylsulfonium and dialkyl-4-hydroxyphenyl-sulfonium salts include oxiranes, oxetanes, thiiranes, vinyl ethers and s-trioxane. Polymerizations of tetrahydrofuran, e-caprolactone, a-methylstyrene, and N-vinyl-2-pyrrolidone are very sluggish and often fail using these photoinitiators. 

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