Reversibility of polymerization

The reversibility of propagation, or more specifically, the position of the equilibrium as determined by the ratio of the rate constants of propagation and depropagation is also independent of the mechanism. The equilibrium monomer concentration of monosubstituted alkenes such as styrenes and vinyl ethers are so low ([M] < 10-6 mol/L) at temperatures used for carbocationic polymerizations that the reversibility of polymerization can be neglected. 

Before discussing in more detail the factors influencing the enthalpy and entropy of polymerization of heterocyclic monomers, it is worth reviewing some practical consequences of the reversibility of polymerization ... 

The other consequence of the reversibility of polymerization is the broadening of the molecular weight distribution. Even if initiation is fast and all the chains grow simultaneously, M,JM is equal to 2 under equilibrium conditions. Polymers with narrower MWD, however, may be isolated at early stages of polymerization (at low conversion) when the contribution of depropagation is not very significant, thus the preparation of polytetrahydrofuran with MJM 1.1 at <10% conversion has been described [56]. 

Whitfield, J. Morelli, A. Warner, J. C. Enzymatic Reversal of Polymeric Thymine Photocrosslinking with E. coli DNA Photolyase. J. Macromol. Sci. 2005,442, 1541-1546. 

Reversibility Reversals of polymerization, such as pyrophosphorolysis of a DNA chain and exchange between pyrophosphate and the p,y-groups of a dNTP, requires a primer,... 

The reversal of polymerization, however, is not the norm for polymer decomposition. Most polymers show decomposition to multiple products. Especially in the presence of oxygen is there always a chance of oxidation, or even self-sustaining burning. Such reactions are better called pyrolysis than decomposition. The thermogravimetry is for this reason usually done in an inert atmosphere, such as nitrogen, unless oxidation is to be studied. 

Unzipping n. The fast reversal of polymerization, with release of monomer that can occur in addition to homopolymers once a stable end group has been removed. Copolymerization helps to minimize unzipping. Odian GC (2004) Principles of polymerization. John Wiley and Sons Inc., New York. 

Bolig, A. D. Chen, E. Y.-X. Reversal of polymerization stereoregulation in anionic polymerization of MMA by chiral metallocene and non-metaUocene initiators A new reaction pathway for metallocene-initiated MMA polymerization. J. Am. Chem. Soc. 2001,123, 7943-7944. 

In the condensation polymerization of two-component silicones, byproducts are released. These systems are less likely to be inhibited and can be used on a greater variety of substrates. However, reversion of polymerization is a potential problem. 

In a manner similar to DNA polymerases, RTase requires Mg (or Mn ), dNTPs, an RNA template, and a primer with a 3 -OH terminus for polymerization reactions. The nucleophilic attack on the a phosphate of the dNTP by the oxygen atom of the ribose 3 -OH of the primer strand is thought to be metal-mediated. As a reversal of polymerization, RTases carry out pyrophosphorolysis and pyrophosphate exchange reactions (10,11). AMV RTase has been shown to carry out strand displacement synthesis in the RNA DNA heteroduplex and in the DNA DNA homoduplex as well (12,13). 

Flammouda A, Gulik T and Piieni M-P 1995 Synthesis of nanosize latexes by reverse micelle polymerization Langmuir 3656-9... 

In the last three chapters we have examined the mechanical properties of bulk polymers. Although the structure of individual molecules has not been our primary concern, we have sought to understand the influence of molecular properties on the mechanical behavior of polymeric materials. We have seen, for example, how the viscosity of a liquid polymer depends on the substituents along the chain backbone, how the elasticity depends on crosslinking, and how the crystallinity depends on the stereoregularity of the polymer. In the preceding chapters we took the existence of these polymers for granted and focused attention on their bulk behavior. In the next three chapters these priorities are reversed Our main concern is some of the reactions which produce polymers and the structures of the products formed. 

Lactam polymerization represented by reaction 5 in Table 5.4 is another example of a ring-opening reaction, the reverse of which is a possible competitor with polymer for reactants. We shall discuss this situation in Sec. 5.10. 

High molecular weight polymers or gums are made from cyclotrisdoxane monomer and base catalyst. In order to achieve a good peroxide-curable gum, vinyl groups are added at 0.1 to 0.6% by copolymerization with methylvinylcyclosiloxanes. Gum polymers have a degree of polymerization (DP) of about 5000 and are useful for manufacture of fluorosiUcone mbber. In order to achieve the gum state, the polymerization must be conducted in a kineticaHy controlled manner because of the rapid depolymerization rate of fluorosiUcone. The expected thermodynamic end point of such a process is the conversion of cyclotrisdoxane to polymer and then rapid reversion of the polymer to cyclotetrasdoxane [429-67 ]. Careful control of the monomer purity, reaction time, reaction temperature, and method for quenching the base catalyst are essential for rehable gum production. 

The nitro alcohols available in commercial quantities are manufactured by the condensation of nitroparaffins with formaldehyde [50-00-0]. These condensations are equiUbrium reactions, and potential exists for the formation of polymeric materials. Therefore, reaction conditions, eg, reaction time, temperature, mole ratio of the reactants, catalyst level, and catalyst removal, must be carefully controlled in order to obtain the desired nitro alcohol in good yield (6). Paraformaldehyde can be used in place of aqueous formaldehyde. A wide variety of basic catalysts, including amines, quaternary ammonium hydroxides, and inorganic hydroxides and carbonates, can be used. After completion of the reaction, the reaction mixture must be made acidic, either by addition of mineral acid or by removal of base by an ion-exchange resin in order to prevent reversal of the reaction during the isolation of the nitro alcohol (see Ion exchange). 

The resulting amino acid then condenses in a stepwise manner to form the growing polymer chain. As in direct polymerization, cycHc oligomers are also formed hence, caprolactam (qv) can be formed in the reverse of the reaction just shown above. 

The polymerizations of tetrahydrofuran [1693-74-9] (THF) and of oxetane [503-30-0] (OX) are classic examples of cationic ring-opening polymerizations. Under ideal conditions, the polymerization of the five-membered tetrahydrofuran ring is a reversible equiUbtium polymerization, whereas the polymerization of the strained four-membered oxetane ring is irreversible (1,2). 

Commercial grades of socbum aluminate contain both waters of hycbation and excess socbum hycboxide. In solution, a high pH retards the reversion of socbum aluminate to insoluble aluminum hycboxide. The chemical identity of the soluble species in socbum aluminate solutions has been the focus of much work (1). Solutions of sodium aluminate appear to be totaby ionic. The aluminate ion is monovalent and the predominant species present is deterrnined by the Na20 concentration. The tetrahydroxyaluminate ion [14485-39-3], Al(OH) 4, exists in lower concentrations of caustic dehydration of Al(OH) 4, to the aluminate ion [20653-98-9], A10 2) is postulated at concentrations of Na20 above 25%. The formation of polymeric aluminate ions similar to the positively charged polymeric ions formed by hydrolysis of aluminum at low pH does not seem to occur. Al(OH) 4 has been identified as the predominant ion in dilute aluminate solutions (2). 

Since amines initiate cyanoacrylate polymerization, the monomer cannot be isolated directly, because a polymer is generated immediately after formation of the monomer. An acid is then added to the polymer, and heat (140-180°C) is applied to the reaction mixture. Because of the relatively low ceiling temperature of the polymer, the pure monomer can be isolated, in greater than 80% yield, by the thermal reversion of the polymer back to the free monomer [4,5]. 

Adsorption of macromolecules has been widely investigated both theoretically [9—12] and experimentally [13 -17]. In this paper our purpose was to analyze the probable structures of polymeric stationary phases, so we shall not go into complicated mathematical models but instead consider the main features of the phenomenon. The current state of the art was comprehensively summarized by Fleer and Lyklema [18]. According to them, the reversible adsorption of macromolecules and the structure of adsorbed layers is governed by a subtle balance between energetic and entropic factors. For neutral polymers, the simplest situation, already four contributor factors must be distinguished ... 

Membranes used for the pressure driven separation processes, microfiltration (MF), ultrafiltration (UF) and reverse osmosis (RO), as well as those used for dialysis, are most commonly made of polymeric materials. Initially most such membranes were cellulosic in nature. These ate now being replaced by polyamide, polysulphone, polycarbonate and several other advanced polymers. These synthetic polymers have improved chemical stability and better resistance to microbial degradation. Membranes have most commonly been produced by a form of phase inversion known as immersion precipitation.11 This process has four main steps ... 

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