Precursors polymer synthesis

The emphasis in the approaches to boron nitride [10043-11 -5] BN, precursors has been concentrated on cycHc compounds. There have been recent reports of trimethylsilyl-substituted aminoboranes being evaluated as B—N precursors. These are linear borylamines containing up to four boron atoms. Compounds were also synthesized with free —NH2 groups amenable to condensation with either dihaloboranes or dihaloborazines (65) and offering suitable monomers for linear B—N polymer synthesis and borazine-ring-linking appHcations. 

A drawback to the Durham method for the synthesis of polyacetylene is the necessity of elimination of a relatively large molecule during conversion. This can be overcome by the inclusion of strained rings into the precursor polymer stmcture. This technique was developed in the investigation of the ring-opening metathesis polymerization (ROMP) of benzvalene as shown in equation 3 (31). 

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). 

Diphenol/thiophenol is one of the most important polymer precursors for synthesis of poly(aryl ethers) or poly-(aryl sulfides) in displacement polymerizations. Commonly used bisphenols are 4,4 -isopropylidene diphenol or bisphenol-A (BPA) due to their low price and easy availability. Other commercial bisphenols have also been reported [7,24,25]. Recently, synthesis of poly(aryl ethers) by the reaction of new bisphenol monomers with activated aromatic dihalides has been reported. The structures of the polymer precursors are described in Table 2. Poly(aryl ether phenylquinoxalines) have been synthesized by Connell et al. [26], by the reaction of bisphenols containing a preformed quinoxaline ring with... 

A potential drawback of all the routes discussed thus far is that there is little control over polydispersity and molecular weight of the resultant polymer. Ringopening metathesis polymerization (ROMP) is a living polymerization method and, in theory, affords materials with low polydispersities and predictable molecular weights. This methodology has been applied to the synthesis of polyacctylcne by Feast [23], and has recently been exploited in the synthesis of PPV. Bicyclic monomer 12 [24] and cyclophane 13 [25) afford well-defined precursor polymers which may be converted into PPV 1 by thermal elimination as described in Scheme 1-4. 

The careful control of electronic properties is, of course, a key motivation of such structural changes the so-called band-gap tuning being a particularly important concern. Efficiency of synthesis and structural homogeneity of the products are essential ingredients of such an approach since failure to achieve e.g. quantitative transformation of precursor polymers or to couple benzene units exclusively in a para-fashion interrupts the extensive -conjugation and hampers a reliable structure-propcrty-relalion. 

In a classical multi-step route the critical point is to conduct (he ring closure quantitatively and regioseleclively. In the synthesis of I.PPP, the precursor polymer 13 is initially prepared in an aryl-aryl coupling from an aromatic diboronic acid and an aromatic dibromoketone. 

The synthesis of tailor-made star-shaped polymers can be performed in several ways by means of a plurifunctional organometallic initiator, or by reacting a living precursor polymer with a plurifunctional reagent, to build the centra] body, or by block copolymerization involving a diunsaturated monomer (Scheme 3). 

This idea was realized impressively in 1991 with the first synthesis of a soluble, conjugated ladder polymer of the PPP-type [41]. This PPP ladder polymer, LPPP 26, was prepared according to a so-called classical route, in which an open-chain, single-stranded precursor polymer was closed to give a double-stranded ladder polymer. The synthetic potential of the so-called classical multi-step sequence has been in doubt for a long time in the 1980s synchronous routes were strongly favoured as preparative method for ladder polymers. 

The synthetic route represents a classical ladder polymer synthesis a suitably substituted, open-chain precursor polymer is cyclized to a band structure in a polymer-analogous fashion. The first step here, formation of the polymeric, open-chain precursor structure, is AA-type coupling of a 2,5-dibromo-1,4-dibenzoyl-benzene derivative, by a Yamamoto-type aryl-aryl coupling. The reagent employed for dehalogenation, the nickel(0)/l,5-cyclooctadiene complex (Ni(COD)2), was used in stoichiometric amounts with co-reagents (2,2 -bipyridine and 1,5-cyclooctadiene), in dimethylacetamide or dimethylformamide as solvent. 

In this paper we present results on the polymer redox elimination reaction used in the synthesis of the polymers in Figure 6. Preliminary results on electrochemical redox elimination on precursor polymers are also presented. A mechanism of the polymer elimination reaction is proposed. Related recent experimental observations at other laboratories that can be described within the framework of the scheme of Figure 4 are discussed. 

Figure 13 shows the irreversible conversion of a nonconjugated poly (p-phenylene pentadienylene) to a lithiun-doped conjugated derivative which has a semiconducting level of conductivity (0.1 to 1.0 S/cm) (29). Obviously, the neutral conjugated derivative of poly (p-phenylene pentadienylene) can then be reversibly generated from the n-type doped material by electrochemical undoping or by p-type compensation. A very similar synthetic method for the conversion of poly(acetylene-co-1,3-butadiene) to polyacetylene has been reported (30), Figure 14. This synthesis of polyacetylene from a nonconjugated precursor polymer containing isolated CH2 units in an otherwise conjugated chain is to be contrasted with the early approach of Marvel et al (6) in which an all-sp3 carbon chain was employed.

Three kinds of PAV films was prepared using methoxy pendant precursors. The chemical structures and synthetic route of the PAV films used in this study are shown in Fig. 19. The details of synthesis of the methoxy pendant precursors have been described in refs. 29 and 30. The precursors were soluble in conventional organic solvents, for example, chloroform, dichloromethane, benzene and so on. The precursor polymer thin films were spin-coated on fused quartz substrates from the chloroform solutions. The precursor films were converted to PAV films by the heat-treatment at 250 0 under a nitrogen flow with a slight amount of HC1 as a catalyst. This method provided high performance PAV films with excellent optical quality. 

Several applications of hyperbranched polymers as precursors for synthesis of crosslinked materials have been reported [91-97] but systematic studies of crosslinking kinetics, gelation, network formation and network properties are still missing. These studies include application of hyperbranched aliphatic polyesters as hydroxy group containing precursors in alkyd resins by which the hardness of alkyd films was improved [94], Several studies involved the modification of hyperbranched polyesters to introduce polymerizable unsaturated C=C double bonds (maleate or acrylic groups). A crosslinked network was formed by free-radical homopolymerization or copolymerization. 

Polymer Synthesis and Modification. The condensation reaction between either BTDA or BDSDA and ODA was performed in DMAc at room temperature under a nitrogen atmosphere. ODA (0.004 mole) was added to a nitrogen-purged glass septum bottle with 7 ml DMAc. One of the dianhydrides (0.004 mole) was then added to the diamine solution with an additional milliliter of DMAc resulting in 15-25 wt% solids depending upon the monomer combination. The resulting solution was stirred for 20-24 hours to form the poly(amide acid), a polyimide precursor. For the modified polyimides, anhydrous cobalt(II) chloride (0.001 mole) was added as a solid within one-half hour after the dianhydride. 

The synthesis of well defined, ring shaped macromolecules has been independently achieved reacently in three different laboratories.U 13 A linear "living" dlcarbanlonlc precursor polymer is reacted with a stoichiometric amount of an efficient electrophilic coupling agent, such as dlbromo-p-xylene or dime thyldichlorosi lane. The reaction has to be carried out at... 

Thus, the ease of polymer synthesis, the facile handling of soluble precursors, the ready thermal or chemical conversion to the final polyimide structures, and the often unprecedented polymer properties that can be obtained collectively make polyimides highly desirable for a wide range of applications. 

How might a self-replicating polymer come to be How might it maintain itself in an environment where the precursors for polymer synthesis are scarce How could evolution progress from such a polymer to the modern DNA-protein world These difficult questions can be addressed by careful experimentation, providing clues about how life on Earth began and evolved. 

SYNTHESIS OF NOVEL SILANOL POLYMERS AND COPOLYMERS BY A SELECTIVE OXIDATION OF SI—H BOND FROM CORRESPONDING PRECURSOR POLYMERS... 

Dioxiranes constitute a new class of organic peroxides that possess great potential as oxidants with a variety of applications in synthetic organic chemistry.5 7 A new convenient route for the synthesis of silanol polymers has been developed by the selective oxidation of =Si—H bonds with dimethyldioxirane. A series of styrene-based silanol polymers and copolymers were synthesized (Scheme l).8 9 The precursor polymers and styrene copolymers containing =Si—H bond were first synthesized by free radical polymerization of the corresponding monomers or copolymerization of the... 

A new convenient polymer modification for the conversion of the Si—H to Si—OH by the selective oxidation of the Si—H bond by dimethyldioxirane has been described. The oxyfunctionalization of the silane precursor polymers proceeded rapidly and quantitatively and can be applied to the synthesis of a wide variety of novel silanol polymers with specific properties from the corresponding precursor polymers containing Si—H functional groups. Control over the properties of these silanol polymers, such as reactivity and self-association of silanols, was realized through the placement of different substitute groups bonded directly to the silicon atom and by the variation of silanol composition in a copolymer. These novel silanol polymers with a... 

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