Diels-Alder crosslinking

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]. 

Canadell J., Fischer H., de WithG., vanBenthemR.A.T.M. (2010), Stereoisomeric effects in thermo-remendable polymer networks based on Diels-Alder crosslink reactions , J. Polym. ScL, Part A Polym. Chem., 48, 3456-3467. 

Scheme 25 Diels-Alder crosslinking of polyene sequences in PVC degradation...

Literature articles, which report the formation and evaluation of difunctional cyanoacrylate monomers, have been published. The preparation of the difunctional monomers required an alternative synthetic method than the standard Knoevenagel reaction for the monofunctional monomers, because the crosslinked polymer thermally decomposes before it can revert back to the free monomer. The earliest report for the preparation of a difunctional cyanoacrylate monomer involved a reverse Diels-Alder reaction of a dicyanoacrylate precursor [16,17]. Later reports described a transesterification with a dicyanoacrylic acid [18] or their formation from the oxidation of a diphenylselenide precursor, seen in Eq. 3 for the dicyanoacrylate ester of butanediol, 7 [6]. 

An example of the second type of modification is the application of the Diels-Alder cycloaddition reaction to polders and copol ers containing pendant or backbone furan moieties. The use of bis-dienophiles such as propiolic acid and its esters or bis-maleimides provides a means of crosslinking based on multiple bridging by the double interchain lycloadditions. The thermal reversibility of these reactions allows the return to the original linear structure (thermoplastic material) by simply heating the gel. 

The Diels-Alder reaction can be employed to obtain thermosetting polyimides. If BMI (the bisdienophile) and the bisdiene react nonstoichiometrically, with bismaleimide in excess, a prepolymer carrying maleimide terminations is formed as an intermediate, which can be crosslinked to yield a high temperature resistant network. 

Cyclopentene is readily available as a byproduct in the ethylene production. Norbornene 2-ethylhexyl carboxylate is obtained by the Diels-Alder reaction of 2-ethylhexyl acrylate with cyclopenta-diene (5). Norbornene isobornyl carboxylate, norbornene phen-oxyethyl carboxylate, and other related monomers are synthesized according to the same route. Polymers obtained from these esters exhibit excellent properties in terms of controlling the crosslinking density, the associated product modulus, and the glass transition temperature (Tg), thus allowing tailoring the properties of elastomers, plastics and composites. Other suitable monomers are summarized in Table 1.1 and sketched in Figure 1.2. 

Monomers are shown in Table 2.1 and in Figure 2.1. The most important monomer is norbornene. Norbornene is in made from di-cyclopentadiene (DCPD) and ethene by a Diels-Alder reaction. 5-Tri-ethoxysilyl-2-norbomene is used for crosslinkable compositions (2). 

BMIs have been copolymerized with unsaturated monomers such as ortho-dl-lylphenol [67,68] and orf/zo-propenylphenoxy oligomers. In any case, the first step has been identified as an ene reaction between the allylic or the propenyl double bond followed by a Diels-Alder reaction (Fig. 18). Identification of such a complex structure is very difficult and can be performed only for low conversion ratios. However, the chemistry was intensively studied because the final network can exhibit better mechanical properties than the pure crosslinked nadimides. 

Although this work was carried out with a model compound, the results are quite consistent with the mechanisms suggested by the BP team [86] which was working within a bisnadimide. However, the work of Meador et al. [42] presents some diverging conclusions by minimizing the role of the reverse Diels-Alder reaction in the crosslinking mechanism. 

The metathesis polymerisation of dicyclopentadiene, an inexpensive monomer (commercially available cyclopentadiene dimer produced by a Diels-Alder addition reaction containing ca 95 % endo and ca 5 % exo form), leads to a polymer that may be transformed into a technically useful elastomer [144-146, 179] and thermosetting resin [180,181]. The polymerisation has characteristics that make it readily adaptable to the reaction injection moulding ( rim ) process [182], The main feature of this process comes from the fact that the polymerisation is carried out directly in the mould of the desired final product. The active metathesis catalyst is formed when two separate reactants, a precatalyst (tungsten-based) component and an activator (aluminium-based) component, are combined. Monomer streams containing one respective component are mixed directly just before entering the mould, and the polymerisation into a partly crosslinked material takes place directly in this mould (Figure 6.5) [147,168,183-186],... 

In general, crosslinks, by whatever mechanism, were considered to consist of C—C bonds, although there was speculation (e.g., by Rozhdestvenskii (24)), that oxygen bridges might also be involved. Razuvaev and co-workers (23) have recently proposed a Diels-Alder type of cycliza-tion between neighboring polyene chains as a mechanism of crosslinking. 

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. 

Inoue K, Yamashiro M, Iji M (2009) Recyclable shape-memory polymer poly(lactic acid) crosslinked by a thermoreversible Diels-Alder reaction. J Appl Polym Sci 112 876-885... 

The first successful examples of enantioselective Diels-Alder reactions catalyzed by chirally modified Lewis acids were reported by Koga [85]. The catalysts were prepared from menthol and AlEt2Cl [86]. Alumina-supported chiral menthoxy aluminum derivatives (64, 65, 66, 67) have been prepared by simple mixing of (-)-menthol, AlEt2Cl, and alumina in toluene under reflux. The reaction of methacrolein with cyclopentadiene (Eq. 20) was conducted with 67 as catalyst at -50 °C and afforded 81 % conversion with 31 % ee [87] Koga reported 57 % ee at -78 °C by use of an homogeneous catalyst [85]. Solid catalyst 69, prepared from silica gel-supported proli-nol 68 and AlEt2Cl (Eq. 21) is also an active catalyst in the same reaction, but with low enantioselectivity [87]. When the same catalyst was attached to crosslinked polystyrene (70) the ee in the reaction was lower [88]. 

Kobayashi et al. developed chiral Lewis acids derived from A -benzyldiphenylproli-nol and boron tribromide and used these successfully as catalysts in enantioselective Diels-Alder reactions [89]. The corresponding polymeric catalyst 71 was prepared and used for the Diels-Alder reaction of cyclopentadiene with methacrolein [90]. Different polymeric catalysts 72, 73, 74 were prepared from supported chiral amino alcohols and diols fimctionalized with boron, aluminum and titanium [88,90]. In these polymers copolymerization of styrene with a chiral auxiliary containing two polymerizable groups is a new approach to the preparation of crosslinked chiral polymeric ligands. This chiral monomer unit acts as chiral ligand and as a crosslink. 

Crosslinking by chemical bond formation was tried with maleic anhydride. It is known that the addition of dienophiles such as maleic anhydride or acrylamide reduces color formation ( discoloration ) in acrylic fibers. A reasonable explanation for this phenomenon has been given by Marien It is based on a Diels-Alder reaction of the dienophile with cis-dienic structures created during the oligomerization of the... 

Note that such cis-dienic structures can be present only if intermolecular oligomerization of nitrile groups has taken place.) The Diels-Alder product interrupts conjugation, thus reducing color formation. In the particular case of maleic anhydride, and under the relatively rough conditions of stabilization, this type of reaction should lead to crosslinking, by cross-anhydridization ... 

Various types of chirally modified Lewis acids have been developed for asymmetric Diels-Alder cycloadditions. Some of these, including Ti-TADDOLates, have been attached to crosslinked polymers [11]. A recent example of this approach involved polymeric monoliths 103 containing TADDOL subunits (Scheme 3.29). The treatment of 103 with 71X4 afforded Ti-TADDOLates, which were used for the asymmetric Diels-Alder reachon of cyclopentadiene 104 and 105. The major product obtained in this reachon was the mdo adduct with 43% ee [58]. The supported Ti-catalysts showed an exhaordinary long-term stabihty, being achve for at least one year. 

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