High-temperature polymer Diels-Alder reaction

After the catalytic formation of a basis substrate, a radical mechanism (also referred to as the pyrolytic mechanism) takes place with a reduced growth rate. This second mechanism is active in the bulk of the formed polymer, close to the fluid interface, and becomes more important at high temperatures. The Diels-Alder or concerted path reactions also give a major contribution. 

A -sulfinylacetamide 297 in greater than 90% yield when a catalytic amount of methyltrioxorhenium is employed. Futhermore, the hetero-Diels-Alder adduct is highly soluble in both chlorinated and ethereal solvents. A detailed investigation of the retro-Diels-Alder reaction of 298 by thermogravimetric analysis revealed an onset temperature of 120 °C and complete conversion of bicycle 298 to pentacene 296 at 160 °C, which are temperatures compatible with the polymer supports typically used in electronics applications. The electronic properties of these newly prepared OTFTs are similar to those prepared by traditional methods. Later improvements to this chemistry included the use of A -sulfinyl-/< r/-butylcarbamate 299 as the dienophile <2004JA12740>. The retro-Diels-Alder reaction of substrate 300 proceeds at much lower temperatures (130 °C, 5 min with FlTcatalyst 150 °C, Ih with no catalyst). 

The Durham route to polyacetylene 103, 104) involves the metathesis polymerization of 1 to give a soluble but thermally unstable high polymer (Scheme 5.2). Slowly at room temperature, or more rapidly at 80 °C, the polymer undergoes a retro-Diels-Alder reaction. This reaction results in elimination of a substituted benzene and formation of amorphous polyacetylene. An enormous weight loss accompanies the conversion, but high-density films were produced with no apparent voids. The kinetics of the transformation reaction were extensively studied (JOS). 

By choosing the appropriate monomers (J06), the temperature at which the retro-Diels-Alder reaction takes place can be controlled (Scheme 5.3). When 1 and 2 were copolymerized, the precursor polymer became thermally stable at room temperature (79) yet still could be converted to polyacetylene at 80 °C. Long runs of polyenes that would make the polymer intractable were avoided by the inclusion of the saturated 2-1-2 photoproduct, which is stable at room temperature. At higher temperatures, the diene was regenerated, and the conversion to polyacetylene proceeded to completion. The diene regeneration and the retro-Diels-Alder reaction are so highly exothermic that the homopolymer of 2 can char during the transformation. 

Furyl derivatives 76, with an allylether or allylamine-type linkage to a methylenecyclopropane framework, readily undergo high pressure-promoted intramolecular cycloaddition" to give spirocyclopropane tricyclic products 77. No cycloaddition reaction occurred at ambient pressure and the products were mostly tar and polymers. Lewis acid catalysis was only marginally successful (Scheme 7.18). At 1.0 GPa and a slightly elevated temperature (60-70 °C) the intramolecular Diels-Alder reaction occurs readily and is exo-diastereo-selective. To quantify the pressure effect on the kinetics the volumes of activation were determined. 

The Diels-Alder reaction with maleic acid esters can be easily performed [25]. However, when it is carried out at high temperature ( 250°C), partial decarboxylation takes place, with formation of ketone-type dirnCTS [26], yielding polyfunctional derivatives (Fig. 4.9) that could be good monomeric units for polymer synthesis [27]. 

This result proves that well-defined structures with low degree of heterogeneity of the multiarm star-shaped polymers can be synthesized. Moreover, the method reported herein can also provide a synthetic pathway for the introduction of block copolymers synthesized via different polymerization routes (RAFT, ROP, etc.) onto the anthracene-end-functionalized multiarm star-shaped polymers. Although the Diels-Alder cycloaddition between anthracene and maleimide derivatives has proven to provide good results in the formation of complex architectures, the major drawback of this method remains the requirement of high temperature and relatively long reaction times. 

The evolution of the gaseous product in this polymerization makes the initial Diels-Alder reaction irreversible. This factor probably contributes to the ease with which high molecular weight polymers are obtained. Recently, two groups of workers have described the polymerization of diacelylenes with bis(cyclo-pentadienones) (79,21,27). An example is shown in Eq. (111-15). The reactants were heated together in a sealed tube at 300°C for 50 hours with toluene as a solvent. Relatively high molecular weight products were obtained (27). The polymers were also notable in that they possessed decomposition temperatures of 470°-550°C in air (27). Other examples are listed in Table 111.1. 

Three unique di-diene have been utilized by Meek (9, 10) in the preparation of Diels-Alder copolymers. Reaction of 1,8-diphenyl-octatetraene with ftts-maleimides in refluxing chloroform for several days affords high yields of polymers with softening temperatures well over 300°, but low intrinsic viscosities. The copolymerization of 1,5-di(9-anthryl)-l,4-pentadiene-3-one and anthralazine with fo s-maleimides employs the diene nature of anthracene to obtain polymers with... 

While metallocenes are ubiquitous in organometallic and polymer chemistry, few such complexes have been reported to catalyze the Diels-Alder process in high enantioselectivity [127,128,129]. Thebis(tetrahydroindenyl)zirconium tri-flate 60 and the corresponding titanocene are electrophilic to the extent that they catalyze the low-temperature cycloadditions of acrylate and crotonate imides with cyclopentadiene with good diastereoselectivity and excellent enantioselec-tion (Scheme 48). The reactivity of 60 is noteworthy since the corresponding reaction using the crotonyl imide with highly reactive catalysts 31a or 44 requires temperatures of -15 and 25 °C, respectively. 

Click reactions are a variety of condensation reactions in which Cu(I) is used as a catalyst for the synthesis of oligomers or polymers. The starting reactants are typically allgme-azides (125), but thiol-enes (TEC) and other alkenes (Diels-Alder) are also used (128). Several of their properties—selectivity, high yield, mild conditions, for example, room temperature, and fast reactions that proceed via minimal steps— make them desirable for use in green chemistry, a relatively new field that has imde-rgone explosive development in recent years (125,126,128,161). 

The hydrocarbon resins can be produced by a simple thermal polymerization process (48-50) or by Lewis acid catalyzed reaction (51). The thermal process is carried out at a high temperature in the range of 200-280°C and a reactor pressure above 300 psig. At temperatures below 200°C, the Diels-Alder polymers are formed. They are not desirable in most resins because they are insoluble in aromatic solvents. If reaction temperature exceeds 280°C, decomposition of the resins would occur. 

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