Monomers, Diels-Alder type

CpC2 gives the monomer CpC. It was suggested that one of these dimers might be the result of a Diels-Alder type reaction. 

Early reports by Kirchhoff, Hahn, Tan and Arnold describe the use of polybenzocyclobutene monomers in Diels-Alder type polymerizations [10,13]. For the most part, these early studies focused on the thermal characterization of mixtures of polybenzocyclobutenes and bismaleimides. Several reports describe the thermal analysis of mixtures of the bisbenzocyclobutene 41 with either the bismaleimide 44 or the diacetylenes 42 and 43 as shown in Fig. 19 [13,79,81, 90-93]. 

Diels-Alder mechanism. It was found that those polymers which had the least weight loss in the isothermal aging study were derived from those monomers which by DSC polymerized to the greatest extent by a Diels-Alder mechanism. It was inferred therefore that if similar considerations apply to the copolymers of bismaleimides and bisbenzocyclobutenes then these materials too might be the result of predominantly Diels-Alder type polymerizations. To the extent to which these conclusions are correct, the enhanced thermal stability of the bisbenzocyclobutene/bismaleimide copolymers is likely to be due to their being Diels-Alder polymers of some sort. 

Diels-Alder Polymerization Diels-Alder type polymerization of a bisdienophile monomer (30) and bisdiene monomers (31, 32a,b) by using chiral Lewis acidic catalysts (33-35) affords optically active polymers [68]. For instance, the polymerization of 30 in CH2C12 with 32b by using 33 as a catalyst at -30°C gives a polymer with structure 36 showing molecular rotation of [ ]D +243°. When CHC13 or tetrahydrofuran is used as solvent, the polymers with only low optical activity are produced. 

A more recent, and particularly successful, variant of the multifunctional coupling process to ladder polymers is the use of repetitive cycloaddition reactions that start from bifunctional monomers, for instance dienes and dienophiles (Diels-Alder-type cycloadditions) [2-4]. The fact, that both chains are generated simultaneously in a concerted process constitutes the important progress associated with such a route. 

The chemistry of the l,3-dithiole-2,4,5-trithione oligomer system 374 derived from monomers 375 and 376 (Scheme 49) has been the subject of a review <2006CHE423>. This system easily depolymerized (heat or UV irradiation at 253 nm) and was effective in Diels-Alder-type cycloadditions. Reactions with unsaturated compounds constituted an efficient method for synthesis of functionalized l,3-dithiole-2-thiones containing substituted 1,4-dithiin rings and are examples of reactivity of thione or sulfenyl substituents (stmctures 374-376) attached to the 1,3-dithiole ring carbon atom. 

For styrene, the conversion of monomer per hour rises from —0.1% at 60°C to about 14% at 140°C. Thus, the effect has to be encountered, especially for polymerizations at higher temperatures. Furthermore, when a styrene-based monomer is to be purified by distillation, the addition of inhibitors and distillation at reduced pressure is advisable in order to avoid the distillate from becoming viscous. Another difficulty occurring during distillation is the formation of polymer in the column, which can also be prevented by distilling in vacuo. The initiation of a styrene-based monomer is assumed to involve a (4-1-2) cycloaddition of the Diels-Alder type with a subsequent hydrogen transfer from the dimer to another monomer molecule ... 

The Japanese species Theondla swinhoei has yielded dimers of these 4-methylenesterols, bisconicasterone and bistheonellasterone, which are formally adducts of the Diels-Alder type of 4-methylene-steroid monomers, conicasterone and theonellasterone (Kho et al., 1981 Kobayashi et al, 1992d Inouye et al, 1994). 

Schliiter et al. have isolated the beltene derivative 20, which was developed as byproduct when synthesizing the ladder-type polymer 21 by a repeated Diels-Alder reaction (Fig. 11). The monomer 19 can act both as diene and as dienophil [41]. The iterative Diels-Alder method is however limited by the decreasing solubility of the products of higher molecular masses. 

The use of porphyrinic ligands in polymeric systems allows their unique physio-chemical features to be integrated into two (2D)- or three-dimensional (3D) structures. As such, porphyrin or pc macrocycles have been extensively used to prepare polymers, usually via a radical polymerization reaction (85,86) and more recently via iterative Diels-Alder reactions (87-89). The resulting polymers have interesting materials and biological applications. For example, certain pc-based polymers have higher intrinsic conductivities and better catalytic activity than their parent monomers (90-92). The first example of a /jz-based polymer was reported in 1999 by Montalban et al. (36). These polymers were prepared by a ROMP of a norbor-nadiene substituted pz (Scheme 7, 34). This pz was the first example of polymerization of a porphyrinic macrocycle by a ROMP reaction, and it represents a new general route for the synthesis of polymeric porphyrinic-type macrocycles. 

Generally, at least in theory, an important aspect of cation-radical polymerization, from a commercial viewpoint, is that either catalysts or monomer cation-radicals can be generated electrochem-ically. Such an approach deserves a special treatment. The scope of cation-radical polymerization appears to be very substantial. A variety of cation-radical pericyclic reaction types can potentially be applied, including cyclobutanation, Diels-Alder addition, and cyclopropanation. The monomers that are most effectively employed in the cation-radical context are diverse and distinct from those that are used in standard polymerization methods (i.e., vinyl monomers). Consequently, the obtained polymers are structurally distinct from those available by conventional methods although the molecular masses observed so far are still modest. Further development in this area would be promising. 

Bisbenzocyclobutenes readily react with molecules which contain sites of reactive unsaturation such as bismaleimides [10,13, 31, 32]. This is in essence, a novel type of Diels-Alder polymerization in which the bis-diene is latently embodied within two benzocyclobutene moieties. The properties of these polymers depends strongly on the mole ratio of the monomers and when it is equimolar, can result in some exceptionally tough high Tg resins [33, 34]. 

There are many examples of AB monomers in the polymer literature [104]. In particular there are examples of the Diels-Alder reaction being used in successful AB type approaches to polymer syntheses. There are however several practical problems associated with this type of approach. If the diene or dienophile is highly reactive, the synthesis and purification of the monomer can be very difficult. Furthermore, these reactive monomers can be difficult to store without some partial advancement of the molecular weight. If on the other hand the diene or dienophile is not very reactive under moderate conditions, then low molecular weight polymers are obtained. If more stringent conditions for polymerization are employed, the retro Diels-Alder reaction and other side reactions can become more important and therefore lead to the formation of a low molecular weight polymer. 

The use of benzocyclobutene as the source of the diene in a Diels-Alder polymerization offers a unique solution to the problems described above. Benzocyclobutene containing monomers can be stored indefinitely at room temperature without concern for further advancement of the molecular weight. It is only when benzocyclobutene is heated to temperatures of approximately 200 °C that the reactive diene, o-quinodimethane, is formed at a significant rate and enters into reaction with the dienophile. The only requirement of the dienophile is that it must be stable at these temperatures and not undergo reaction with itself. The most common dienophiles that have been successfully used in the formation of polymers from AB type benzocyclobutene monomers have been acetylenes, olefins and maleimides. 

As before, the homopolymer forming reaction involved the generation of an o-quinoidimethane from benzocyclobutene which in turn reacted with the double bond of a maleimide functionality on a second monomer to give chain extended AB material via a Diels-Alder pathway. The polymers prepared to date using this type of approach have been shown to have excellent thermal and mechanical properties and have been considered as potential candidates for use as matrix resins for advanced composites. This area has been investigated by the researchers at both the Air Force Wright Laboratories and the Dow Chemical Co. 

This obstacle can be overcome by moving electron withdrawing substituents away from the double bond and increasing the reactivity of double bond by positioning it in a strained ring. This is achieved using bicyclic monomers. The monomers are readily obtained from the Diels-Alder reactions of substituted olefins with cyclopentadiene. This route is effective also for fluorinated monomers. These types of monomers undergo a ROMP with a variety of one component and two-component initiator systems. 

The polymerization of cyclopentadiene represents only one type of monomer which can be employed in a Diels-Alder polymerization. In theory, a polymerization reaction should be successful if a monomer which contained both a diene portion and a dienophilic portion were subjected to polymerization conditions. The polymerization of cyclopentadiene represents a special case of this reaction type. Another type... 

The simplest monomer containing both a diene portion and a dienophilic portion is 2-vinylbutadiene (4, 3). This monomer polymerizes in refluxing cyclohexane presumably by a Diels-Alder reaction to give an insoluble polymer, but the possibility of some vinyl type addition polymerization which would crosslink segments exists. 

The most successful polymerizations carried out by using a Diels-Alder step-growth reaction are those which generate a highly reactive A-B monomer in situ by the reaction of a bismaleimide with cyclopenta-dienone (12), 2-pyrone (6, 13), or thiophene dioxide (5) derivatives. The intermediate 1 1 adduct loses carbon monoxide, carbon dioxide, or sulfur dioxide, respectively, all to generate the same type of reactive AB monomer, which is converted rapidly to polymer. High molecular weight polymers are obtained (Reaction 4). 

In a second novel approach, Mullen and co-workers further developed an inter-molecular repetitive Diels-Alder procedure for the generation of dendritic and hy-perbranched poly(phenylene)s. Hereby, they applied the concept of reacting A2B-type monomers, in this case monomers containing both cyclopentadienone (dien-... 

Other workers have reported the homopolymerization of certain substituted bismaleimides in solution by successive 2+2 cycloaddition reactions, and they have appropriately defined this type of process as a true photopolymerization i.e. a polymerization in which every chain-propagating step involves a photochemical reaction. Another such example is the solid-phase 2+2 cyclopolymerization of divinyl monomers.— By contrast, the polymers described above result from a combination of photocycloadditions and Diels-Alder cycloadditions. 

Gandini and co-workers described a unique double click strategy related to the preparation of monomers based on vegetable-oil derivatives bearing furan heterocycles appended through thiol-ene click chemistry, and their subsequent polymerisation via the Diels-Alder (DA) polycondensation between furan and maleimide complementary moieties (i.e., a second type of click chemistry). Details about the DA reaction, its mechanism, applications and the reason why it is classified as a click reaction can be found in Chapter 7. 

Several other highly efficient coupling reactions have been reported to be of click in nature since they fulfilled all or some of the click chemistry criteria. Several types of metal-free click reactions have been developed. The more prominent ones are thiol-ene, thiol-yne, thiol-para-fluoro, hetero Diels-Alder (HDA) coupling reactions, and pyridyl disulfide exchange reactions (Scheme 8.6) [155-162]. Each of them requires specific initiators, monomers, and/or polymerization conditions. Therefore, the synthetic route should be carefully designed to prepare the desired functional polymers or solid substrates. 

We believe that S P did not need to interpret the thermosetting of their bis(Cp)s to a vinyl-type addition but to Cp Diels-Alder crosslinking. The approach to re-mending plastics through precursor monomers of type 6 proved to be quite versatile in that one could prepare a wide variety of plastics ranging from brittle solids to stretchable rubbers by simply varying the number of carbons and heteroatoms in the tether, as well as the crosslinking temperature [17]. 

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