Cycloaddition vinyl monomers

In the photopolymerization of methacrylamide by benzoin methyl ether, chain-transfer to monomer has been found to be important, and benzalde-hyde is reported to be an inefficient photoinitiator of methyl methacrylate polymerization unless benzophenone and triethylamine are present. Acetophenone has been found to sensitize the cycloaddition of maleic anhydride to 7-oxabicyclo[2.2.1]heptan-5-one-2,3-dicarboxylic anhydride, , a-hydroxy-acetophenone derivatives have been found to be non-yellowing initiators, and h.p.l.c. has been used to determine residual carbonyl photoinitiators in u.v.-cured resins. In the emulsion-polymerization of methyl methacrylate using an aromatic ketone and a continuous or intermittent laser, the former conditions were found to be similar to those under continuous u.v. irradiation. The dependence of the polymerization rate and average chain-length on the absorbance of the initiator used in the photoinitiated polymerization of vinyl monomers has been studied. Interestingly, irrespective of all conditions, maximum conversion is observed when initiator absorbance is 2.51. "... 

Hence, the symmetric vinyl monomer should polymerize most actively on contact ion pairs. First, the presence of a counterion should eliminate or at least weaken the effect of the local symmetry principle and at the same time that of the spin exclusion principle. Secondly, on the base of Epiotis results [58], it may be expected that the polar character of the carbon-metal bond should favor cycloaddition. However, this also follows from the formulation of the correspondence principle if the transition to a more polar bond is considered to be a transition to a more asymmetric bond. 

The existence of the assumed local maximum on curve 2 is due to the fact that the mutual donor-acceptor interaction between the monomer and the active center is probably most effective when the asymmetry of the former and the carbon-metal bond polarity (pseudosymmetry) in the latter supplement each other to a certain asymmetry which is attained, e.g. in the interaction between the symmetric vinyl monomer and the contact ion pair. It should be noted that the requirement of a certain asymmetry of the vinyl reagent is an indispensable general condition of cycloaddition [24, 25]. 

Vinyl monomers containing P-lactam groups were prepared by ketene-imine [2 - - 2] cycloaddition followed by modification of the side chains, and by free radical polymerization gave polyacrylate P-lactams (Scheme 4.41). Cationic polymerization of dimethylketene catalyzed by aluminum tribromide with tetra- -butylammonium bromide in dichloromethane... 

As was mentioned, cycloaddition of unactivated hydrocarbons, namely, that of cyclopentadiene, has practical significance. 5-Vinyl-2-norbomene is produced by the cycloaddition of cyclopentadiene and 1,3-butadiene546,547 [Eq. (6.96)] under conditions where side reactions (polymerization, formation of tetrahydroindene) are minimal. The product is then isomerized to 5-ethylidene-2-norbomene, which is a widely used comonomer in the manufacture of an EPDM (ethylene-propylene-diene monomer) copolymer (see Section 13.2.6). The reaction of cyclopentadiene (or dicyclopentadiene, its precursor) with ethylene leads to norbomene548,549 [Eq. (6.97)] 550... 

Polymerizable indazole derivatives have received some attention with respect to the preparation of redox polymers and metal ion chelating polymers (73MI11102). Monomers such as 3-vinyl-4,7-dihydro-1//-4,7-indazoledione (63) are readily prepared by 1,3-dipolar cycloaddition of vinyldiazomethane to 1,4-benzoquinones. Crosslinked, low swelling, recyclable redox polymers have been prepared from these monomers. 

A facile synthesis of enantiopure tricyclic furyl derivatives employing 4-vinyl-2,3-dihydrofuran via Diels-Alder cycloaddition reaction was reported <02TL7983>. A new capture-ROMP-release procedure for chromatography-free purification of N-hydroxysuccinimide Mitsunobu reactions was reported by Hanson, who used a Mitsunobu reaction to capture a variety of alcohols onto a norbomenyl A-hydroxysuccinirmde monomer. Treatment of this monomer under ROM-polymerization then generated a water-soluble polymer that was readily separable from other by-products. Subsequent reaction with hydrazine was utilized to release the O-alkylhydroxylamines in good purity from the water-soluble polymer <020L1007>. 

Polyelectrolytes and soluble polymers containing triarylamine monomers have been applied successfully for the indirect electrochemical oxidation of benzylic alcohols to the benzaldehydes. With the triarylamine polyelectrolyte systems, no additional supporting electrolyte was necessary [91]. Polymer-coated electrodes containing triarylamine redox centers have also been generated either by coating of the electrode with poly(4-vinyltri-arylamine) films [92], or by electrochemical polymerization of 4-vinyl- or 4-(l-hydroxy-ethyl) triarylamines [93], or pyrrol- or aniline-linked triarylamines [94], Triarylamine radical cations are also suitable to induce pericyclic reactions via olefin radical cations in the form of an electron-transfer chain reaction. These include radical cation cycloadditions [95], dioxetane [96] and endoperoxide formation [97], and cycloreversion reactions [98]. 

Among these reactions, the Cu(l)-catalyzed azide-alkyne cycloaddition (CuAAC) is the most widely used. This reaction has been implemented for the preparation of segmented block copolymers from polymerizable monomers by different mechanisms. For example, Opsteen and van Hest [22] successfully prepared poly(ethylene oxide)-b-poly(methyl methacrylate) (PEO-b-PMMA) and PEO-b-PSt by using azide and alkyne end-functionalized homopolymers as the click reaction components (Scheme 11.2). Here, PEO, PSt, and PMMA homopolymers were obtained via living anionic ring-opening polymerization (AROP), atom transfer radical polymerization (ATRP), and postmodification reactions. Several research groups have demonstrated the combination of different polymerization techniques via CuAAC click chemistry, in the synthesis of poly(e-caprolactone)-b-poly(vinyl alcohol) (PCL-b-PVA)... 

Beyond these common families of monomers, a variety of other monomer classes have been successfully polymerized by RAFT, including isoprene, 2- and 4-vinylpyridine, and acrylonitrile. An elegant approach to increase the range of functional monomers was introduced by Hawker and coworkers, who described the RAFT polymerization of 4-vinyl-l,2,3-triazole monomers. These ttiazole derivatives are easily synthesized via a copper-catalyzed (1 + 3] dipolar cycloaddition, and present an entirely new class of functional monomers (Figure 21.1) [10]. 

On the other hand, the joint polymerization of a series of electron-accepting and electron-donating monomers leads to alternating copolymers, mostly as a mixture with the head-to-head cycloaddition products. Electron-accepting maleic anhydride, fumaric ester, sulfur dioxide, or carbon dioxide in combination with electron-donating butadiene, isobutylene, vinyl ether, and p-dioxene or vinyl acetate belong to this series. 

The mechanism of ROMP has been extensively investigated, and it has been shown that the reaction proceeds by a series of [2+2]-cycloadditions and retro [2-h2]-cycloadditions as shown in Scheme 2. Once polymerization is complete, the initially formed polymer still contains a metal alkylidene, so a reagent such as benzaldehyde or ethyl vinyl ether is added to destroy this metal alkylidene. If a second batch of monomer is added before the metal alkylidene is destroyed, then a block copolymer will be formed. This... 

Figure L (A) This chart outlines the Diels-Alder dimerization of 2 styrene monomers (or similar vinyl aromatics) to form AH and the subsequent possible reactions of AH to give oligomers or to produce free radicals. The [2 + 2] cycloaddition of 2 monomer units to form the 1,4-airadical Mt also is shown. (B) The reactions of 1,4-diradical that convert it to oligomers such as dicyclobutanes (DCB) or to monoradicals. ( See Refs. 5,6, and 7.)...

Kitagawa and coworkers have recently described an interesting but different host-guest-polymerization concept to synthesize cross-linked polymers such as polystyrene, methylmethacrylate, and vinylacetate with pseudo-crystallinity in non-photochemical route [57]. In order to achieve this, they have first incorporated the cross-linker 2,5-divinyl-benzene-1,4-dicarboxylate (DVTP) into the porous CP [Cu(DVTP)(triethylenediamine)o.5] (51). The host framework containing porous channels with dangling vinyl groups provides a suitable environment for radical polymerization of these monomers as shown in Fig. 31. This is obviously different from photopolymerization by [2+2] cycloaddition reaction. 

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