Sequential cation-free radical

Oxman et al., smdied controlled, sequentially curable cationic/free radical hybrid photopolymerization of diepoxide/acrylate hybrid material with the aid of photodifferential scanning calorimetry. The polymerizations were carried out in the presence of various concentrations of either ethyl-4-dimethylamino benzoate or 4-tert-butyl-N,N,-dimethylaniline as electron donors and camphoquinone/diphenyliodonium hexafluoroantimonate as the sensitizing system. The results showed that the free-radical acrylate reactions always precede the cationic epoxy polymerizations. 

In addition to cationic cyclizations, other conditions for the cyclization of polyenes and of ene-ynes to steroids have been investigated. Oxidative free-radical cyclizations of polyenes produce steroid nuclei with exquisite stereocontrol. For example, treatment of (259) and (260) with Mn(III) and Cu(II) afford the D-homo-5a-androstane-3-ones (261) and (262), respectively, in approximately 30% yield. In this cyclization, seven asymmetric centers are established in one chemical step (226,227). Another intramolecular cyclization reaction of iodo-ene poly-ynes was reported using a carbopaUadation cascade terminated by carbonylation. This carbometalation—carbonylation cascade using CO at 111 kPa (1.1 atm) at 70°C converted an acycHc iodo—tetra-yne (263) to a D-homo-steroid nucleus (264) [162878-44-6] in approximately 80% yield in one chemical step (228). Intramolecular aimulations between two alkynes and a chromium or tungsten carbene complex have been examined for the formation of a variety of different fiised-ring systems. A tandem Diels-Alder—two-alkyne annulation of a triynylcarbene complex demonstrated the feasibiHty of this strategy for the synthesis of steroid nuclei. Complex (265) was prepared in two steps from commercially available materials. Treatment of (265) with Danishefsky s diene in CH CN at room temperature under an atmosphere of carbon monoxide (101.3 kPa = 1 atm), followed by heating the reaction mixture to 110°C, provided (266) in 62% yield (TBS = tert — butyldimethylsilyl). In a second experiment, a sequential Diels-Alder—two-alkyne annulation of triynylcarbene complex (267) afforded a nonaromatic steroid nucleus (269) in approximately 50% overall yield from the acycHc precursors (229). 

Chain-growth polymerization involves the sequential step-wise addition of monomer to a growing chain. Usually, the monomer is unsaturated, almost always a derivative of ethene, and most commonly vinylic, that is, a monosubstituted ethane, 1 particularly where the growing chain is a free radical. For such monomers, the polymerization process is classified by the way in which polymerization is initiated and thus the nature of the propagating chain, namely anionic, cationic, or free radical polymerization by coordination catalyst is generally considered separately as the nature of the growing chain-end may be less clear and coordination may bring about a substantial level of control not possible with other methods. ... 

Monomers containing rings or double bonds can be polymerized by chain polymerization, which is also known as addition polymerization. (It should be contrasted with Step polymerization.) The chain reaction involves the sequential steps of initiation, propagation and termination. Initiation is the process by which active centres are formed these may be free radicals, anions or cations. The free radical chain polymerization of a vinyl monomer is illustrated below. 

In chain reactions the different types of monomers can be added subsequently to an active chain end. The most important techniques here are sequential living polymerization techniques, such as anionic or cationic polymerization. Certain metallocenes can be used in coordination polymerization of olefins leading to stereo block copolymers, like polypropylene where crystalline and amorphous blocks alternate with each other due to the change of tacticity along the chain [34]. In comparison to living polymerization techniques, free radical and coordination polymerization lead to rather polydisperse materials in terms of the number of blocks and their degree of polymerization. 

In systems where the monomer is highly polarized, radical polymerization does not take place, e.g. aldehydes and ketones are polymerized only by anionic and cationic initiators. Ionic polymerizations are very susceptible to impurities which act as poisons (not the case with free radical initiations). Therefore the system must be scrupulously clean and dry. However, despite these drawbacks commercial polymers such as the thermoplastic elastomeric block copolymers of butadiene/styrene are made. This is possible because the lifetime of the anionic intermediate is long (several hours) and sequential polymerization can take place. 

The features and detail of the IPN kinetics were also studied in other works [274-276]. The kinetics of thermally initiated cationic epoxy polymerization and free radical acrylate photopolymerization were investigated in [277]. It was found that the preexistence of one polymer has a significant effect on the polymerization of the second monomer. The reaction kinetics and phase separations were studied for sequential IPNs in [278]. The kinetics of IPN formation was studied for IPNs based on PDMS-cellulose acetate butyrate [279]. All these and other works [280-282] confirm the general regularities of the reaction kinetics and its connection with phase separation in forming systems. 

We proposed the sequential single-electron transfer mechanism for ethylene biosynthesis based on precedents of cyclopropylamine oxidations by enzymes such as cytochrome P450 (55, 40) and monoamine oxidase (59,40). A syllogism underlying this postulate is that the electrochemical oxidation of ACC must have a close mechanistic relationship to the enzymatic oxidation because each gives the same products with the same stereochemistry. This mechanism was modified slightly by theoretical considerations (5) (eq 10). Key intermediates are 1, the cyclopropylamine radical cation, and 2, the "half-opened radical." The presence of the latter species explains, by free rotation about the -CH2-CH2 bond, the loss of stereochemistry observed with deuterated substrates. It is further consistent with cyclopropylcarbinyl... 

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