Poly thermal elimination reactions

In Situ Analysis of the Thermal Elimination Reaction in the Synthesis of Poly(p-phenylene vinylene)(PPV) and PPV Derivatives... 

Abstract In situ spectroscopy is an important tool to characterize polymers synthesized via a precursor route. Highly conjugated polymers such as po y(p-phenylene vinylene) (PPV) and PPV derivatives are commonly prepared from a precursor polymer because the final polymers are very insoluble and intractable. Preparation in the precursor form enables the polymer materials to be cast as films. The PPV polymers are obtained from the precursor forms using a thermal elimination reaction. The exact conditions of the reaction are important as they influence the properties of the resultant polymer. The details of this thermal elimination reaction have been analyzed using thermal gravimetric analysis (TGA) coupled with infrared analysis of the evolved gas products. In situ infrared spectroscopy of the precursor films during thermal conversion to the polymers has provided further details about the elimination reaction. We have characterized PPV synthesized from a tetrahydrothiophenium monomer (sulfonium precursor route) and via the xanthate precursor route. PPV derivatives under study include poly(2,5-dimethoxy-p-phenylene vinylene) and poly(phenoxy phenylene vinylene). 

In Table 1 the experimental data (weight average of molar mass, polydispersity index, decomposition temperature and heat of decompositioiO of polymers with varius add groups — OR (see equ.l) and different stereochemistry are listed. It can be seen, that the decomposition temperature decreases from ca. 390 °C for poly(emfo,endo-2,3-diacetoxy-5-norbomene), polymer 1 b, down to ca. 270 C for poly(e >,exo 5-norbomene-23-bis(methylcarbonate)), pofymer 3 a. As erqpected, the polymers from the exo,eco-isomers decomposed at lower temperatures as the endo,endo- sova.cK because the thermal elimination reaction is a typical syn-elimination ... 

RAFT end groups are known to be unstable at very high temperatures (>200 °C). Thermal elimination has been used as a means of trithiocarbonate end group removal. For ps430,4W direct elimination is observed (Scheme 9.54). For poly(butyl acrylate)464 the major product suggests a hoinolysis/backbiting/ i-scission reaction is involved (Scheme 9.55). 

This unexpected result may be related to the increase in TOC on fraction G3 and may be further evidence of the polymerization phenomenon discussed earlier. However, this hypothesis must be carefully considered because of our limited knowledge of pyrolysis mechanisms. The possibility of phenol formation during the thermal fragmentation process from elimination reactions followed by cycliza-tion of poly conjugated chains has been suggested by Bracewell (22) and should be investigated. 

Elimination of carbon dioxide from carboxyl, water from alcoholic hydroxyl, carboxylic acid from alkanoate, and hydrogen chloride from chlorine side groups or chain ends are typical thermal decomposition reactions in the temperature range 250-350°C. Hydrogen chloride is an important product of poly(vinyl chloride) because every second carbon atom of the hydrocarbon polymer chain is chlorine substituted. But hydroxyl, alkanoate and free carboxylic acid groups normally occur only at the ends of the macromolecular chains in customary plastics, thus the contribution of their elimination to the volatile pyrolysis products is negligible. 

In addition to the depolymerization reaction discussed earlier, other reactions may be favoured. These are elimination from a side chain and cyclization. For example, propylene is eliminated from the side chain of poly (isopropyl acrylate) as shown in Scheme 1.51(a), leaving poly(acrylic acid). This occurs in all polymers having ester side groups with P-hydrogens available to form a six-membered transition state, as shown. Thus both acrylate and methacrylate polymers will undergo this reaction and, since depolymerization is the dominant thermal-degradation reaction in methacrylates, elimination of alkenes is more important in the poly(acrylates). 

Figure 5. Three-dimensional temperature-infrared frequency-intensity plot of the reaction byproducts of the thermal elimination from xanthate poly(2,5-dimethoxy phenylene vinylene) precursor...

The thermal elimination process can be applied to most substituted groups in vinyl polymers by controlled pyrolysis at 600 to TOO C, producing poly vinylene compoimds, e.g., by the splitting off of acetic acid from poly(vinyl acetate). By careful temperature control, one can achieve bifunctional reactions and/or intramolecular cyclizations. This has been developed commercially at relatively high temperatures, in the case of the polymerization of methacrylamide above 65°C, to yield a polymer with a substantial proportion of imide groups ... 

The important feature is the formation of polyene sequences by successive eliminations. These absorb visible light, causing the polymer initially to become yellow and eventually black. Similar reactions can occur with many polymers, e.g. poly(vinyl alcohol), poly(vinyl acetate) and cellulose and its esters. If the elimination reactions are allowed to go to high conversions, the polymer is typically transformed into a carbonaceous char, which may be quite thermally stable. Much of fire retardant chemistry is aimed at increasing char yields in polymer thermolysis. 

COT have been described, but an alternative route for synthesis of long conjugated polyenes has been known for many years. A typical example is the base-catalyzed thermal elimination of hydrogen chloride from poly(vinyl chloride). Unfortunately, synthesis of well-characterized polyacetylene by this route was not successful. The first synthesis of high quality, well-characterized polyacetylene films was reported by Edwards and Feast [66]. The process [67-74] consists of three reaction steps as shown in Fig. 7.8. 

A. Torres-Filho and R. W. Lenz, Electrical, thermal, and photoconductive properties of poly(phenylene vinylene) precursors I. Laser-induced elimination reactions in precursor solutions, J. Polym. Sci. Part B Polym. Phys. 57 959 (1993). 

Poly(arylene vinylenes). The use of the soluble precursor route has been successful in the case of poly(arylene vinylenes), both those containing ben2enoid and heteroaromatic species as the aryl groups. The simplest member of this family is poly(p-phenylene vinylene) [26009-24-5] (PPV). High molecular weight PPV is prepared via a soluble precursor route (99—105). The method involves the synthesis of the bis-sulfonium salt from /)-dichloromethylbenzene, followed by a sodium hydroxide elimination polymerization reaction at 0°C to produce an aqueous solution of a polyelectrolyte precursor polymer (11). This polyelectrolyte is then processed into films, foams, and fibers, and converted to PPV thermally (eq. 8). 

Polymeranalogous reactions considered above may be referred to as intramolecular condensation transformations since they are accompanied by elimination of low-molecular products. On the other hand, PCSs can be obtained via polymeranalogous transformations, principally intramolecular polymerization reactions . Thermal and chemical cyclization of poly(acrilonitrile) (PAN) is an example of processes of this type. It was demonstrated by a number of researchers216-225 that thermal transformations of PAN follow the scheme ... 

The second step is the thermal conversion of borylaminoborazine into poly(borylaminoborazine). Continuous elimination of parent alkylamine is the main by-product dining the thermal polycondensation of 2,4,6-trialkylaminoborazine (see earlier). We expected the continuous elimination of the starting 5-alkylam i noborane during the thermal polycondensation of borylborazine. However, the alkylami noborane is a liquid, which requires that the thermal polycondensation must be performed in vacuo to continuously remove the evolving B(NHR)3 from the reaction mixture. This procedure also precludes any competing polycondensation reaction from B(NHR)3. 

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