Diels-Alder crosslinking polymers

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. 

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]. 

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. 

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],... 

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... 

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. 

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. 

The mechanism of the crosslinking reaction has been postulated as (a) dissociation of the terminal cyclopentadiene-N-arylma1-eimide Diels-Alder adduct to the monomeric precursors, which immediately react to form an adduct which initiates the homopolymerization of the undissociated terminal norbornene rings to form a saturated polymer (j>). 

Bis(citraconimidomethyl)benzene, commercial name Perkalink 900, has been introduced and functions exclusively as a reversion resistor. It is understood to react via a Diels-Alder reaction to form a six-membered ring on the polymer chain (Figure 9.8). The ultimate crosslink is thermally stable and replaces sulfur crosslinks that disappear during reversion (Rubber Chemicals, 1998). 

The Diels-Alder reaction was performed with cyclopentadiene and methacroleine (Scheme 56) in presence of 15 mol% of the oxazaborolidine and oxazaborolidinone catalysts derived respectively from supported aminoalcohols and from V-sulfonylamino acid polymers. The oxazaborolidine and oxazaborolidinone catalysts were formed in situ by action of BH3, BH2Br, BHBr2 or BBrs. The diastereoselectivity was excellent in favour of the exo adduct and yields from 65 to 99%. It is noteworthy that higher loading of chiral catalyst site in the polymer, lower exo selectivity and enantioselectivity. The diastereoselection depended not only on the nature of the supported ligand, the crosslinker but also on the borane and the solvent. Results are summarized in Table 6. 

This paper describes novel approaches to the exploitation of both furan monomers and a specific facet of furan reactivity in order to synthesize either conjugated oligomers incorporating the heterocycle in their backbone, or polymeric structures which can be crosslinked and returned to linear structures through the reversible chemistry of the Diels-Alder reaction. The first family of compounds showed interesting features in terms of conductivity, luminescence, mesogenic character and photoactivity. The second class of materials owes its interest to the possibility of recycling otherwise intractable polymers, e.g. tires, thanks to a simple thermal process. 

Furan, Furfural, Hydroxymethylfurfural, Furan polymers. Chain polymerizations. Step-growth polymerizations, Furan polyesters, Furan polyamides, Furan polyurethanes, Furan-containing conjugated oligomers, Diels-Alder reaction, Dendrimers, Reversible crosslinking... 

Method of synthesis maleic anhydride and diamines are reacted in the presence of catalyst such as triethylamine, these are further cured to form crosslinked polymers. Thermal curing is promoted by the presence of radical or ionic initiators. BMI can also be synthesized by Diels-Alder reaction (see ref.) Jiang, B Hao, J Wang, W Jiang, L Cai, X, Eur. Polym. J., 37, 463-70,2001. 

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