Polymer reaction cyclization

Difficulties due to side reactions (cyclization) and a broad molecular weight distribution accompanying the polycondensation of active esters led to the application of methods wherein the polymers are built up stepwise. In 1968, Sakakibara et al.31) introduced the solid-phase technique using Merrifield s resin. By stepwise addition of tert-pentyloxycar-bonyl tripeptides, they have synthesized (Pro-Pro-Gly)n with n = 5, 10, 15 and 20. 

The polymers are crystalline, with a melting temperature of 187°C and a glass transition temperature of 87°C. In contrast, H-T PIB is a rubbery polymer, with a glass transition temperature of -61°C. Further, the polymer contains bromine end groups. As a side reaction, cyclization is observed. 

In order to ensue a clear presentation of the results the authors decided to segregate both synthetic principles All synthetic strategies developed from the multifunctional condensations of Stille and Marvel were assigned to this general type of reaction. At the same time the first multistep sequences (polymer-analogous cyclization of poly(methyl vinyl ketone) and polyacrylonitrile) are used as point of reference for the classification of the other type of synthesis (stepwise procedures). 

Aromatization experiments show some evidence for relatively small aromatic substructures (naphthalene, anthracene, tetracene) and this independently demonstrates the success of the polymer-analogous cyclization process. The occurence of up to 20% non-cyclized vinylic side-groups is an indication of the statistical nature of the ring-closure reaction [58]. 

The above examples of a stepwise synthesis of a ladder polymer involve the formation of single-stranded polymers via polymerization of suitable monomers to functionalized precursors. These consist of substituted poly(ethylene)- or (polyacetelyne)-type macromolecules, from which attempts are made to carry out a defined polymer-analogous cyclization reaction. 

The synthetic sequence to methylene-bridged poly(phenylene)s 71 represents the first successful employment of the stepwise process to ladder-type macromolecules involving backbone formation and subsequent polymer-analogous cyclization. As shown, however, such a procedure needs carefully tailored monomers and reaction conditions in order to obtain structurally defined materials. The following examples demonstrate that the synthesis of structurally defined double-stranded poly(phenylene)s 71 (LPPP) via a non-concerted process is not just a single achievement, but a versatile new synthetic route to ladder polymers. By replacing the dialkyl-phenylenediboronic acid monomer 68 by an iV-protected diamino-phenylenediboronic acid 83, the open-chain intermediates 84 formed after the initial aryl-aryl cross-coupling can te cyclized to an almost planar ladder-type polymer of structure 85, as shown recently by Tour and coworkers [107]. 

Primary ureas 1365 are good substrates in rhodium-catalyzed N-H insertion reactions with an array of 2-diazo-l,3-keto esters 1364. The products from the insertion reaction cyclize with the aid of acid to yield imidazolones 1366. This chemistry has been translated onto insoluble polymer resins and utilized to prepare a small array of imidazolones (Scheme 351) <20030L511, 2004JOC8829>. 

The molecular size distributions for multifunctional polymerization leading to three-dimensional polymers are derived in a manner analogous to those for linear polymers, but with much more difficulty. The derivations have been discussed elsewhere [1,15,19], and only the results will be considered here. The results given below are based on three simplifying assumptions of ideal network formation (1) all functional groups of the same type are equally reactive (2) all groups react independently of one another and (3) no intramolecular reactions (cyclization) occur. 

Under the conditions of a kinetically controlled polycondensation reaction, cyclization reactions compete with propagation steps. The extent of cyclization depends on the flexibility of the polymer chain and on the concentration of the active species. Blends of PES with its homologous macrocyclic oligomers show greatly lowered melt viscosities in comparison to the corresponding original PES. 

Moore and MitchelH prepared soluble poly(enaminoester) (X) from a.a -bis(carbomethoxy) diacetyl benzene and aromatic diamines. The polymers were subsequently thermally cyclized (Conrad-Limpach reaction) to poly(quinolines) (XI), which is analogous to the conversion of poly(amic acids) to polyimides in terms of converting a soluble prepolymer to a stable polymer of cyclized, rigid ring structures with the formation of volatile by-products. The cyclized product however, was infusible and insoluble. 

As a natural consequence of the crosslinking reaction process, the density of the primary polymer differs depending on the time of this primary polymer formation. That is, in the case of the copolymerization of vinyl and divinyl monomers, the generally formed inhomogeneous crosslink formation can be regarded as a natural consequence of the mechanism of crosslink formation. This is true except for die special reaction conditions by favorable timing of the incorporation of divinyl monomer in the polymer chain (formation of pendant double bonds) and consumption of pendant double bonds (formation of crosslinks). These special reaction conditions are used by Flory as simplified conditions when the Flory-Stockmayer theory is applied to the copolymerization of vinyl and divinyl monomers. Flory s simplified conditions include die following three assumptions (1) the reactivities of the monomer and die double bonds in the polymer are all equal (2) any double bond reacts independently and (3) there will be no intramolecular reactions (cyclization) within the finite size molecules (sols). 

Table 14.2 The Influence of the Substitution at the Methylene Bridge on the Course of the Polymer-Analogous Cyclization Reaction...

For the preparation of ladder polymers and copolymers of the PPP type, as described in detail, the classical synthetic methods were adopted as the polymer-analogous cyclization of a suitably substituted single-stranded precursor polymer. In spite of frequently expressed doubts regarding the power of this method [351, it was possible to attain the impressive proof that it really does lead to defect-free band structures after careful selection of reaction centers and reaction conditions. The effect of this was a renaissance for the classical synthetic routes for ladder polymers [36-391 (see Section II.C). 

Table 3.3 gives the general mechanism of nylon 6 polymerization. Let us assume that reaction with mono functional polymer and cyclization (i.e., steps 5, 6, and 7) do not occur. Derive the following mole balance relation... 

The synthesis described met some difficulties. D-Valyl-L-prolyl resin was found to undergo intramolecular aminoiysis during the coupling step with DCC. 70< o of the dipeptide was cleaved from the polymer, and the diketopiperazine of D-valyl-L-proline was excreted into solution. The reaction was catalyzed by small amounts of acetic acid and inhibited by a higher concentration (protonation of amine). This side-reaction can be suppressed by adding the DCC prior to the carboxyl component. In this way, the carboxyl component is "consumed immediately to form the DCC adduct and cannot catalyze the cyclization. 

First the protected oligopeptide is coupled with polymer-bound nitrophenol by DCC. N"-Deblocking leads then to simultaneous cycliiation and detachment of the product from the polymer (M. Fridkin, 1965). Recent work indicates that high dilution in liquid-phase cycli-zation is only necessary, if the cyclization reaction is sterically hindered. Working at low temperatures and moderate dilution with moderately activated acid derivatives is the method of choice for the formation of macrocyclic lactams (R.F. Nutt, 1980). 

This first-stage polymer is then introduced into the application environment, where the final cyclization reaction occurs. 

Diazo Coupling Reactions. Alkylphenols undergo a coupling reaction with dia2onium salts which is the basis for the preparation of a class of uv light stabilizers for polymers. The interaction of orxv i -nitrobenzenediazonium chloride with 2,4-di-/ r2 -butylphenol results in an azo-coupled product (30). Reduction of the nitro group followed by m situ cyclization affords the benzottiazole (31) (19). 

Catalysts. Silver and silver compounds are widely used in research and industry as catalysts for oxidation, reduction, and polymerization reactions. Silver nitrate has been reported as a catalyst for the preparation of propylene oxide (qv) from propylene (qv) (58), and silver acetate has been reported as being a suitable catalyst for the production of ethylene oxide (qv) from ethylene (qv) (59). The solubiUty of silver perchlorate in organic solvents makes it a possible catalyst for polymerization reactions, such as the production of butyl acrylate polymers in dimethylformamide (60) or the polymerization of methacrylamide (61). Similarly, the solubiUty of silver tetrafiuoroborate in organic solvents has enhanced its use in the synthesis of 3-pyrrolines by the cyclization of aHenic amines (62). 

Cross-linked PVP can also be obtained by cross-linking the preformed polymer chemically (with persulfates, hydrazine, or peroxides) or with actinic radiation (63). This approach requires a source of free radicals capable of hydrogen abstraction from one or another of the labile hydrogens attached alpha to the pyrrohdone carbonyl or lactam nitrogen. The subsequently formed PVP radical can combine with another such radical to form a cross-link or undergo side reactions such as scission or cyclization (64,65), thus ... 

Tabushi and Fujiyoshi have prepared alkylated cyclam derivatives for the purpose of suspending these systems from a polymer backbone. They have utilized a malonic ester alkylation reaction followed by cyclization and reduction to accomplish this end. ... 

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