Polymer reversible dissociation

Observations on the polymerization of readily polymerizable vinyl monomers such as styrene, vinyl chloride, and butadiene date back approximately to the first recorded isolation of the monomer in each case. Simon 2 reported in 1839 the conversion of styrene to a gelatinous mass, and Berthelot applied the term polymerization to the process in 1866. Bouchardat polymerized isoprene to a rubberlike substance. Depolymerization of a vinyl polymer to its monomer (and other products as well) by heating at elevated temperatures was frequently noted. Lemoine thought that these transformations of styrene could be likened to a reversible dissociation, a commonly held view. While the terms polymerization and depolymerization were quite generally applied in this sense, the constitution of the polymers was almost completely unknown. 

Photo-crosslinking and the reverse process of photodissociation of pre-existing crosslinks relies on a cycloaddition reaction (and on the reverse dissociation of the cyclic adduct). For example, derivatives of vinyl cinnamic acid can form crosslinks which are dissociated by irradiation with short wavelength light (e.g. 254 nm produced by low-pressure mercury arcs). In this process the polymer chains become separated, and the polymer itself is then soluble in organic solvents. 

The transformations themselves involved reactions of ketoacids with a pyridoxamine unit, either covalently attached to the polymer or reversibly bound to the hydrophobic core (29), which converted the ketoacids to amino acids, and the pyridoxamine was converted to a pyridoxal unit either covalently attached to the polymer or reversibly dissociated from the polymer. This reaction was modeled directly on the transamination process observed in natural enzymes. However, the second part of a full transamination in nature is the reaction of the pyridoxal with a different amino acid, which runs the transamination backward to form the pyridoxamine again while converting the new amino acid into its corresponding ketoacid. We found that such a process was too slow in our biomimetic system and could not compete with the rapid aldol condensation of the ketoacids with the pyridoxal. 

One further obtains a controlled growth of the polymer if the characteristic time for the reversible dissociation is sufficiently short in comparison with monomer conversion, that is, if all chains start to grow practically together.2... 

In 1985, Rizzardo et al.27 filed a patent for the use of alkoxyamines (Scheme 12) as regulating initiators for the living radical polymerization and block copolymerization of vinyl monomers. R is a group that upon dissociation (Scheme 10) forms a radical that adds to the monomer. The mechanism was disclosed shortly thereafter and involves the reversible dissociations shown in Scheme 11, with the nitroxide radical taking the role of X.28 In a later simulation, the group also revealed the reason for the remarkable absence of the usual terminations and rediscovered the principles of the persistent radical effect 29 As chains undergo termination transient radicals are removed from the system and the concentration of persistent species builds . Further, the authors noted correctly that, in contrast to normal radical polymer-... 

The results presented above may be compared with previous studies on these polymers. It has been suggested that more than one spectrally distinct excimer species may exist in some naphthalene-containing polymers (6,22). Analyses of the spectral surfaces for the polymers studied in this work clearly indicate that only one spectrally distinct monomer and one excimer species contribute to emission. Phillips and co-workers (3,23) also found that the monomer fluorescence decay from PACE requires a minimum of three exponential terms to provide adequate fitting. They explained their results on the basis of a kinetic model involving two temporally distinct excited monomers (M3, M2 ) which can form the excimer (D ) as outlined in Scheme 1. Reverse dissociation of the excimer was also considered important. 

Since 1-propanol is monofunctional and reacts with "matched dissociation isocyanate" (at internal urethane chain positions) as well as "unmatched isocyanate" (at polymer chain terminal positions) its effects include cleavage of some polyurethane chains in the process of generating more urethane groups. As a result, polymer DP, melt viscosity, and torque drop until the shortstop is consumed, or escapes the mixture by volatilizaton. Figure 9 shovjs that the more 1-propanol used to shortstop the polymerizations, the more pronounced the polymer reversion was. 

Metal loss may occur if the ligands by which the complexes are bound to the polymer undergo reversible dissociation during the reaction. For example, the well known Wilkinson catalyst, RhCKPPhj), ig known to dissociate a tertiary phosphine ligand in order to become catalytically active. If the phosphine bound catalyst depicted in Equation 1 underwent such... 

Fig. 16 Mechanochemical evolution of metallosupramolecular polymers generated by (a) combination of an Eu(III) salt and a telechelic poly(ethylene-co-butylene) with 2,6-bis(l- -methylbenzimidazolyl)pyridine ligands at the termini counterions are omitted for clarity, (b) Reversible dissociation upon ultrasonication (i) irreversible metal exchange with Fe(II) ions as a result of (ii) ultrasonication or (Hi) other mechanical forces, (c) Dipicolinic acid ligands bind strongly to Eu(III) and the supramolecular network cannot easily be disassembled under mechanical stress. Reproduced with permission from [88]. Copyright 2014 American Chemical Society...

The polymer-nitroxyl adduct P-X reversibly dissociates thermally, in process 1 into the polymer radical P and the nitroxyl radical X. The rate constants of dissociation and combination are and kc, respectively. The, so-called, degenerative transfer takes place in process 11. The second-order rate constant for active species in either direction is k y. Here all the rate constants are assumed to be independent of chain length. Since the frequency of cleavage of the P-X bond is proportional to [P-X] in process 1 and to [P l [P -X]] in process 11, the overall frequency,/ per unit time and per unit volume, of the bond-cleaving or activation reactions, may be expressed by [277] ... 

Upon heating, the active chain end group (the tritylic end) undergoes reversible dissociation. The molecular weight of the polymers obtained increased with conversion, claimed as evidence for the living nature. The trityl end-capped and isolated polymer can serve as a macroiniter for the subsequent polymerization which leads to chain extension and block copolymer formation ... 

Polymer growth J(c) showed nonlinear monomer concentration dependence in the presence of ATP (Carrier et al., 1984), while in the presence of ADP, the plot of J(c) versus monomer concentration for actin was a straight line, as expected for reversible polymerization. The data imply that newly incorporated subunits dissociate from the filament at a slower rate than internal ADP-subunits in other words, (a) the effect of nucleotide hydrolysis is to decrease the stability of the polymer by increasing k and (b) nucleotide hydrolysis is uncoupled from polymerization and occurs in a step that follows incorporation of a ATP-subunit in the polymer. Newly incorporated, slowly dissociating, terminal ATP-subunits form a stable cap at the ends of F-actin filaments. 

In reversible polymerization, the critical concentration is equal to the equilibrium dissociation constant for polymer formation. This parameter is therefore independent of the number of polymers in solution. Confirmation comes from smdying reversible polymerization of ADP-actin when sonic vibration is applied to a solution of F-ADP-actin filaments at equilibrium with G-ADP monomers, no change is observed in the proportion of G- and F-actin (Carlier et al., 1985). Therefore, the only effect of sonic vibration is to increase the number of filaments without affecting the rates of monomer association to and dissociation from filament ends. 

Thermal initiation and ordinary bimolecular termination also occur during polymerization in addition to initiation by the dissociation of the adduct or the active polymer chain-end dissociation and reversible temination (formation of the dormant species). Therefore, the degree of the control of the molecular weight and the molecular weight distribution is determined by the ratio of the polymer chains produced under control and uncontrol. If the contribution of the thermal initiation and bimolecular termination is very small, the molecular weight distribution is close to the Poisson distribution, i.e., Mw/Mn=1 + 1/Pn, where Pn is the degree of polymerization. It was shown that when the number of... 

Isocyanates are capable of co-reacting to form dimers, oligomers and polymers. For example, aromatic isocyanates will readily dimerize when heated, although the presence of a substituent ortho to the -NCO group reduces this tendency. For example, toluene diisocyanate (TDI) is less susceptible to dimer formation than diphenylmethane diisocyanate (MDI). The dimerization reaction is reversible, with dissociation being complete above 200 °C. It is unusual for aliphatic isocyanates to form dimers, but they will readily form trimers, as do aromatic isocyanates. The polymerization of aromatic isocyanates is known, but requires the use of metallic sodium in DMF. 

For a fully dissociated but non-ideal polymer electrolyte (i.e. long range ion interactions are present but not ion association) the following expressions for the steady state potential AV, and current may be derived, again assuming reversible electrode behaviour ... 

Consider a cell consisting of a polymer electrolyte, in which the salt, MX, dissociates fully into M and X " ions, the two electrodes, M, are reversible to M. The cell constant is unity, and the electrolyte is ideal. 

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