Polycondensation controlled

Kinetic experiments and rigorous modelling of the mass-transfer controlled polycondensation reaction have shown that even at low melt viscosities the diffusion of EG in the polymer melt and the mass transfer of EG into the gas phase are the rate-determining steps. Therefore, the generation of a large surface area is essential even in the prepolycondensation step. 

Kricheldorf, H. R., Rabenstein, M., Maskos, M. and Schmidt, M., Macrocycles 15. The role of cyclization in kinetically controlled polycondensations. 1. Polyester synthesis, Macromolecules, 34, 713 (2001). 

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. 

FIGURE 3.25 Mechanism of the controlled polycondensation in a biphasic system. Source Yokozawa and Suzuki [24]. Reproduced with permission from American Chemical Society. 

As it follows from the Eq. (29), the value is defined through reaction kinetic characteristics and, as consequence, is the parameter, controlling polycondensation kinetics [44, 45]. 

More recent experiments proving the presence of very large rings in step reactions and a discussion of the role of ring formation in step polymerization are summarized by Kricheldorf HR (2003) The Role of Ring-ring Equilibria in Thermodynamically Controlled Polycondensation, Macromol Symp 199 15-22 see also other papers in the same issue and the introduction What Does Polycondensation Mean Ibid pp 1-13. 

Figure 16 Synthesis of polyimide precursor 35 by the controlled polycondensation in solution of the bis- ortho-acid ester 33 and 4,4 -methylenebisbenzeneamine 34. The reaction initially gives short-chain amide-acid such as 35 that are transformed into polyimide 36 when the adhesive is processed at high temperature.

The synthesis of hydrido-amino phosphazenes can be carried out directly from aminophosphines. Thus, the dialkylamino(amino)phosphine (Me2N)2PNH2 (generated in situ by amminolysis of the corresponding chlorophosphine) underwent a fast kinetically controlled polycondensation process without cross-linking in solution at low temperature to form the low P-hydrido(dimethylamino)polyphosphazene [N P(H)(NMc2)] (186) with absolute = 41.000 (PDI = 1.5) in a planar cis-trans or twisted helical conformation. The proposed mechanism for the polymerization was supported by ab initio calculations on the model (H2N)2PNH2 reported in ref. 1 (see above). 

An attractive approach to controlled polycondensation developed by Yokoyama takes advantage of a biphasie system using phase-transfer catalysis (scheme 1.14) [111, 112]. The authors begin by dispersing the solid monomer, potassium-... 

Kricheldorf, H.R. (2003) Macrocycles. 21. Role of ring-ring equilibria in thermodynamically controlled polycondensations. Macromolecules, 36,2302-2308. 

The bulk polycondensation of (10) is normally carried out in evacuated, sealed vessels such as glass ampules or stainless steel Parr reactors, at temperatures between 160 and 220°C for 2—12 d (67). Two monomers with different substituents on each can be cocondensed to yield random copolymers. The by-product sdyl ether is readily removed under reduced pressure, and the polymer purified by precipitation from appropriate solvents. Catalysis of the polycondensation of (10) by phenoxide ion in particular, as well as by other species, has been reported to bring about complete polymerisation in 24—48 h at 150°C (68). Catalysis of the polycondensation of phosphoranimines that are similar to (10), but which yield P—O-substituted polymers (1), has also been described and appears promising for the synthesis of (1) with controlled stmctures (69,70). 

An unusual method has been used to prepare a hyperbranched polyimide starting from two monomers a difunctional A2 and a trifunctional B3. The gel formation can be avoided with careful control of the polycondensation conditions (molar ratio, order of the monomer addition, and low concentration). The A2 and B3 monomers were respectively 6FDA and tris(4-aminophenyl)... 

Shorter chain dienes have an increased propensity to form stable five-, six-, and seven-membered rings. This thermodynamically controlled phenomenon is known as the Thorpe-Ingold effect.15 Since ADMET polymerization is performed over extended time periods under equilibrium conditions, it is ultimately thermodynamics rather than kinetics that determine the choice between a selected diene monomer undergoing either polycondensation or cyclization. 

The mechanism of ring-opening polymerization has received much attention in recent years. The studies of it has made control of the polymerization reactions possible resulting in desirable products. However, many problems still remain unsolved in this field. In fact, the situation is far less satisfactory than the fields of vinyl polymerization and polycondensation. Extensive studies of copolymerization should be useful for the establishment of the chemistry of ring-opening polymerization. 

Interest in anionic polymerizations arises in part from the reactivity of the living carbanionic sites4 7) Access can be provided to polymers with a functional chain end. Such species are difficult to obtain by other methods. Polycondensations yield ro-functional polymers but they provide neither accurate molecular weight control nor low polydispersity. Recently Kennedy51) developed the inifer technique which is based upon selective transfer to fit vinylic polymers obtained cationically with functions at chain end. Also some cationic ring-opening polymerizations52) without spontaneous termination can yield re-functional polymers upon induced deactivation. Anionic polymerization remains however the most versatile and widely used method to synthesize tailor made re-functional macromolecules. 

The first step in sol-gel processing is the catalytic hydrolysis of TEOS and the second step is the polycondensation of SiOH moieties framing into silica (Scheme 3.1). In the first step of the reaction, water is present as a reactant while it is the by-product in the second step. It is likely that the molar ratio of TEOS/H2O would influence the sol-gel chemistry and hence the end properties of the resultant hybrids. The most interesting part of the sol-gel chemistry is that the catalytic hydrolysis of TEOS is an ion-controlled reaction, while polymerization of silica is not. Usually, the ionic reactions are much faster than the condensation reactions. The stoichiometric equation showing the silica formation from TEOS is presented in Scheme 3.3. 

The fact that this polycondensation process takes place at room temperature, with careful control of both molecular weight and molecular weight distribution of the final polymers or copolymers produced are definitive advantages over the corresponding ROP reaction of (NPCl2)3 in molten state. 

Horhold et al. and Lenz et al. [94,95]. The polycondensation provides the cyano-PPVs as insoluble, intractable powders. Holmes et al. [96], and later on Rikken et al. [97], described a new family of soluble, well-characterized 2,5-dialkyl- and 2,5-dialkoxy-substituted poly(pflrfl-phenylene-cyanovinylene)s (74b) synthesized by Knoevenagel condensation-polymerization of the corresponding alkyl-or alkoxy-substituted aromatic monomers. Careful control of the reaction conditions (tetra-n-butyl ammonium hydroxide as base) is required to avoid Michael-type addition. 

Noteworthy that all the above formulated results can be applied to calculate the statistical characteristics of the products of polycondensation of an arbitrary mixture of monomers with kinetically independent groups under any regime of this process. To determine the values of the elements of the probability transition matrix of corresponding Markov chains it will suffice to calculate only the concentrations Q()- of chemical bonds (ij) at different conversions of functional groups. In the case of equilibrium polycondensation the concentrations Qy are controlled by the thermodynamic parameters, whereas under the nonequilibrium regime of this process they depend on kinetic parameters. 

As mentioned previously, the main drawbacks of the thermal route to poly-borylborazine are (1) the presence of both direct intercyclic bonds and three-atom bridges between the rings, and (2) a difficulty in controlling the polycondensation rate. One solution we investigated to address these drawbacks is a route based on the room temperature reaction of /i-chloroborazine with trialkylaminoborane.31 32 We used 2-methylamino-4,6-dichloroborazine instead of 2,4,6-trichloroborazine to prepare a two-point polymer (scheme 4), which is theoretically less cross-linked. 

What is the importance of this enzyme family for the biogenesis problem These enzymes form the link between the protein world and the nucleic acid world . They catalyse the reaction between amino acids and transfer RNA molecules, which includes an activation step involving ATR The formation of the peptide bond, i.e., the actual polycondensation reaction, takes place at the ribosome and involves mRNA participation and process control via codon-anticodon interaction. 

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