Active polycondensation method

A general step ahead in polycondensation was achieved by the application of the active ester method by DeTar et al.19) and Kovacs et al.291 Very soon, the nitrophenyl ester, the pentachlorophenyl ester, or the hydroxysucdnimido ester were used exclusively. The esters of the protected tripeptides could be purified by crystallization, then the N-protecting group was split off and the free peptide esters were purified again. Addition of base starts the polycondensation, resulting quickly in the formation of a viscous solution at low temperature. 

Polyamides containing thymine photodimer units in the main chain (17a,b) were prepared by polycondensation of thymine photodimer derivatives (15a,b), which were obtained by the photochemical reaction of the monomeric compound, and various diamines by the activated ester method (Figures 4 and 5) (17, 19). 

This review summarizes the recent results in the preparation of well-defined chiral polymers from optically inactive monomers. To date, optically active polycondensates based on non-natural monomers are still a curiosity in polymer chemistry. Expanding the catalytic toolbox in polymer chemistry by adopting methods from chemo-enzymatic synthesis may enable easy access to chiral polymers and allow the exploration of the added value of chirality in materials. Moreover, chemo-enzymatic approaches have the potential to further enhance macromolecular complexity and hence allow to access new materials with applications envisaged in nanomaterials and biomedical materials. 

Polyamide 136 was first synthesized by polycondensation of active esters96 and then by polymerization of the /3-lactam of a-isobutyl-i.-asparate (isobutyl 4-oxo-2-azetidinecarboxylate) (135).97,98 In this case, the polymerization was conducted cither in solution or thermally. Anionic polymerization in solution, using potassium /ert-butoxidc as catalyst, yielded a polymer, of better quality having a higher molecular weight (intrinsic viscosity 3.0) than that obtained by the active ester method, although it showed lower optical activity, as it was partially racemizcd (Scheme 35). 

Recently, Ocotlan-Flores et al. [51, 52] presented an alternative ultrasonic-activated synthetic method for the preparation of Si02 with TEOS and neutral water in an inert atmosphere. Several days after the end of the sonication, the liquid condensed at room temperature to form Si02 gel. A hydrolysis-like reaction (which they call sonolysis) is produced time separated from the polycondensation reaction, facilitating independent control of each one. 

Strong interaction between monomers and the template is a prerequisite for the template process, while in simple polycondensation high temperature and low pressure is applied. However, in mild conditions using the direct polycondensation method it was demonstrated that terephthalic acid connected with poly(4-vinylpyridine) (79), poly(ethylene oxide) (80), poly(vinylp5UTolidinone) (81) and activated by triphenyl phosphite reacts with hexamethylenediamine, giving polyamides with high molecular weight. 

With regards to new polymeric biomaterial, Gomurashvilli and co-workers (2) have developed new biodegradable and tissue-resorbable co-poly(ester amides) (PEAs) using a versatile Active PolyCondensation (APC) method which involves di-p-toluenesulfonic acid salts of bis-(L-a-amino acid)-a,(o-alkylene diesters and active diesters of dicarboxylic acids as monomers. A wide range of... 

The PEAs reported in this work were prepared in a simple way by solution or interfacial polycondensation, where di-p-toluenesulfonic acid salts of bis-(a-amino acid)-a,co-alkylene diesters react with chlorides of dicarboxylic acids (interfacial polycondensation) or their active diesters (Active Polycondensation, APC). The APC method involves the condensation of two partners (I) bis-electrophilic, activated dicarboxylic acids, and (II) bis-nucleophilic, acid salts of bis-(a-amino acid)-a,(0-alkylene diesters in combination with di-p-toluenesulfonic acid salts of L-lysine benzyl ester. This reaction proceeds under mild conditions in common organic solvents and leads to polymer of high molecular weight (up to 300 KDa). A detailed review of the APC method has been recently summarized by Katsarava (7). 

As a variation on the base-catalyzed nucleopbilic displacement chemistry described, polysulfones and other polyarylethers have been prepared by cuprous chloride-catalyzed polycondensation of aromatic dihydroxy compounds with aromatic dibromo compounds. The advantage of this route is that it does not require that the aromatic dibromo compound be activated by an electron-withdrawing group such as the sulfone group. Details of this polymerization method, known as the Ullmaim synthesis, have been described (8). 

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. 

Vacuum was applied to shift the equilibrium forward by removal of the activated alcohol formed [30, 31, 37, 38]. In the enzymatic polycondensation of bis(2,2,2-trifluoroethyl) sebacate and aliphatic diols, the polymer with Mw of more than 1 x 104 was obtained using lipases CC, MM, PPL, and Pseudomonas cepacia lipase (lipase PC) as catalyst and lipase MM showed the highest catalytic activity [37]. Solvent screening indicated that diphenyl ether and veratrole were suitable for the production of the high molecular weight polyesters under vacuum. In the PPL-catalyzed reaction of bis(2,2,2-trifluoroethyl) glutarate with 1,4-butanediol in veratrole or 1,3-dimethoxybenzene, periodical vacuum method improved the molecular weight (Mw 4 x 104) [38]. 

Initially PDPs were synthesized by stepwise polycondensation of linear activated depsipeptide [93]. In 1985, Helder, Feijen and coworkers reported the synthesis of PDPs by ROP of a morpholine-2,5-dione derivative (cyclic dimer of ot-hydroxy- and a-amino acid cyclodepsipeptide, cDP) [94, 95]. The ROP method gives an alternative type of PDP by homopolymerization and also allows the copolymerization with other monomers (lactones and cyclic diesters) including LA, GA, and CL to give a wide variety of functional biodegradable materials. The synthesis of PDPs as functional biomaterials has been recently reviewed [17]. 

Mamyama et al.25 have obtained high-molecular-weight poly(benzoxazole)s by the low-temperature solution polycondensation of A,A 0,0 -tetrais(trimethyl-silyl)-substituted 2,2-bis(3-amino-4-hydroxyphenyl)-l,l,l,3,3,3-hexafluoro-propane (25) with aromatic diacids and subsequent thermal cyclodehydration of the resulting poly(o-hydroxy amide)s in vacuo. In this method, aromatic diamines with low nucleophilicity are activated more positively through the conversion to the /V-silylated diamines, and the nucleophilicity of the fluorine-containing bis(o-aminophenol) can be improved by silylation. 

Chemical activation is indeed the weak point of the prebiotic chemistry of polycondensation. In principle, this should be a prebiotic activation, namely a kind of spontaneous reaction under prebiotic conditions. Several more or less friendly activation methods have been used in the field, and most of them cannot, reasonably, be called prebiotic. On the other hand, the chemist must start working with some tool at hand. Let us now take a few examples from the literature on the prebiotic synthesis of biopolymers. 

Johnson et al. (15) reported the first attempt to synthesize PEEK by polycondensation of bisphenolate with activated dihalides using DMSO as a solvent and NaOH as a base. High molecular weight polymers were difficult to obtain due to the crystallinity and the resulting insolubility of polymers in DMSO. To circumvent the solubility problem, Attwood and Rose (16) used diphenyl sulfone as a solvent, and the polymerization was carried out close to the melting point. Victrex PEEK was commercialized by the British company ICI in 1982 using this method. Since its commercialization, this thermoplastic polymer has been used in a wide range of applications, from medicine to the electronic, telecommunications and transport industries (automobile, aeronautic and aerospace) (17,18). 

In addition to the repeat unit sequence, another area of current interest in polymer structural control (Fig. 1) may be the spatial or three-dimensional shapes of macromolecules. In fact, the recent development of star [181-184] and graft [185] polymers, as well as starburst dendrimers [126], arborols [186,187], and related multibranched or multiarmed polymers of unique and controlled topology, has been eliciting active interest among polymer scientists. In this section, let us consider the following macromolecules of unique topology for which living cationic polymerizations offers convenient synthetic methods that differ from the stepwise syntheses (polycondensation and polyaddition) [126,186,187]. 

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