Mechanism polycondensation

The participation of metallacyclobutane rings in metathesis polycondensation is the reason why the aforementioned sterically encumbered acyclic diene monomers hardly undergo reaction [7-9] in such cases, steric influences at particular positions in the monomer hinder the formation of the necessary metallacyclobutane ring, thus inhibiting polymer formation [1], 

Note that the acyclic diene metathesis reaction [scheme (22)] and the ringclosing metathesis reaction [scheme (23)] are inter- and intramolecular displays of the same mechanistic event [1]  

In the ring-closing metathesis reaction, intramolecular metathesis closes a ring to form a small cyclic molecule with concurrent loss of a small molecule (ethylene). Conversely, in the case of the acyclic diene metathesis reaction, macromolecules are formed by successive intermolecular condensation of two olefinic molecules [1], 

Using 1,5-hexadiene, it was shown that, depending upon whether molybdenum- or ruthenium-based catalysts are employed, a change in mechanism appears to occur. In the presence of Schrock s molybdenum catalyst, 1,5-hexadiene produces principally linear poly(l-butenylene) [scheme (24)] [33], but with Grubbs s ruthenium catalyst the primary product is the cyclic dimer 1,5-cyclooctadiene [scheme (25)] [25,33]  

This change in mechanism has been attributed to the formation of a n chelate complex via coordination of the second olefinic bond present in the diene at the Ru atom, which then favours the intramolecular ring-closing reaction over intermolecular condensation polymerisation [1], 

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Grafting can also occur in the amide nitrogen, either through an anionic-type mechanism which is beheved to operate when ethylene oxide [75-21 -8] and similar copolymers are grafted to polyamides, or through a polycondensation mechanism when secondary amides are formed as graft copolymers (70). 

The Tafel slopes obtained under concentrations of the chemical components that we suspect act on the initiation reaction (monomer, electrolyte, water contaminant, temperature, etc.) and that correspond to the direct discharge of the monomer on the clean electrode, allow us to obtain knowledge of the empirical kinetics of initiation and nucleation.22-36 These empirical kinetics of initiation were usually interpreted as polymerization kinetics. Monomeric oxidation generates radical cations, which by a polycondensation mechanism give the ideal linear chains ... 

One alternative is to select precursors which form a gas as a reaction product in situ during the network formation of thermosets. However this approach is restricted to a very limited number of precursors reacting via a polycondensation mechanism to split off a gas. For example, flexible polyurethane foams are commercially produced using CO2 that is liberated as a reaction product of the isocyanate monomer with water [5]. Very recently, Macosko and coworkers studied the macroscopic cell opening mechanism in polyurethane foams and unraveled a microphase separation occurring in the cell walls. This leads to nanosized domains, which are considered as hard segments and responsible for a rise in modulus after the cell opening [6]. 

The new polymers 9-28 were synthesized by the Pd-catalyzed reaction of (4,4 - or 5,5 -dibromo-2,2 -bipyridine)-bis(4,4 -terf-butyl-2,2 -bipyridine or 2,2 -bipyridine)ruthenium(II) complexes 5-8 and diethynylarenes (Table 2) in a step-growth polycondensation mechanism (Scheme 4). The typical reaction conditions used for the synthesis of the polymers involved stirring the argon-... 

Nonionic surfactants can be mixed as well, but in practice most of them, at least the ethoxylated ones, are already a mixture because of the polycondensation mechanism in ethylene oxide adduction. Isomerically pure ethoxylates are extremely expensive and are exclusively reserved for research work. Little work has been carried out with mixture of isomerically pure nonionics and the bulk of the work on mixture deals with mixtures of commercial products, i.e., mixture of mixtures, which obey a linear mixing rule on EON, provided that the base mixtures are not too different [8,35]. 

Due to the presence of reactive CH2OH groups, resol oligomers may be converted into highly crosslinked products without the addition of hardeners. Heat curing is conducted at T 130 200°C. The polycondensation mechanisms are complex and different bridges are possible CH2-0-CH2-and CH2. The latter is thermodynamically the most stable. Therefore the methylene bridges are the prevalent crosslinks in cured resols. 

Studies conducted at Masonite Corporation and discussed earlier in this paper reveal that defibra-tion is indeed hydrolytic and produces furfural and mononuclear aromatics. Clearly, the elements of the polycondensation mechanism are available as described by Runkel and Wilke (11). Boehm (15), Klauditz and Stegmann (16), and Gupta et all fl7) go as far as to claim that the high lignin contents of Asplund and Masonite pulps are due to condensation of lignin at high temperatures. 

For polymerizing systems, the polymer structure depends on the polymerization and crosslinking reaction kinetics. Metal alkoxides are easily hydrolyzed. Once hydrolyzed, they polymerize by polycondensation mechanisms. The hydrolysis and polycondensation mechanisms may be represented as [42] follows Hydroljreis as... 

The synthesis of a polymer by a free-radicjd or polycondensation mechanism usually leads to a product with a rather wide distribution of molecular weights. These polydisperse samples must be considered as multicomponent mixtures rather than as chemically pure substances. Using analytical methods like gel permeation chromatography (GPC) it is possible to analyze such mixtures. 

Phenol blocked isocyanates also have been used to prepare hyperbranched polyurethanes by a step-growth polycondensation mechanism, using DBTDL as a catalyst. 4... 

It was not until Heise and coworkers [87] also investigated the enzymatic polymerization by both a ring-opening and polycondensation mechanism in three different ionic liquids that improved products were observed. BMIM PF6, BMIM BF4 and BMIM NTf2 were investigated as solvent using Novozym 435 as catalyst. 

This dependence can be explained in terms of the polycondensation mechanism of formation of aluminosilica gel skeleton from Al(OH)4 hydroxoaluminate ions and (0H)4 mSi(0 )m silicate ions with different degrees of hydroxylation. At equal correlations and concentrations of the silicate and aluminate ions in solutions, the amount of silicate ions participating in the reaction of condensation should increase with the growth of their hydroxylation degree. The degree of hydroxylation of the silicate ions in alkaline solutions is determined by Equilibrium 1 and therefore it must increase with decrease in NaOH/Si02 ratio (= m) and with dilution. 

The adduced in Figs. 55 and 56 plots allow one to proceed to F-2 polycondensation mechanism analysis within the framework of irreversible aggregation models [ 129-131]. First of all it should be noted, that val-... 

NMR spectroscopy has also provided some insights into the polycondensation mechanism rf aqueous silicate solutions [40,41, 42, 43, 44]. The first steps involve the polycondensation of the monomer g , leading to the dimer, the linear trimer and the linear tetramer [43] ... 

A series of thermosets harden or cure by the polycondensation mechanism, but without elimination of low-molar-mass components. Epoxides, EP, and the polyurethanes, UR, belong in this group. A prepolycondensation is not necessary here the monomers themselves can be directly cured. 

Miyakoshi, R., A. Yokoyama, and T. Yokozawa. 2005. Catalyst-transfer polycondensation mechanism of Ni-catalyzed chain-growth polymerization leading to well-defined poly(3-hexylthiophene). JAm Chem Soc 127 (49) 17542-17547. 

A peculiar route to a unique furan polyamide was applied to A-hydroxymethylfuramide prepared in situ from 2-furamide and an excess of formaldehyde, which gave rise to polymer 21 by an acid-catalyzed polycondensation mechanism involving the C5 electrophilic substitution of the heterocycle [4e]. 

The furan-maleimide DA reaction is particularly useful because it provides a means to synthesise polymers through the thermo-reversible polycondensation mechanism shown in Scheme 7.2. That is, the forward step (chain growth) can be carried out at <65 °C without the need for a catalyst, whereas >110 °C the reverse step (chain breaking) dominates [7]. 

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