Step polymerization reversible

More recent studies, particularly with slower hafnium complexes, have provided more detailed mechanistic insight As a step polymerization, the reaction is "nonideal" in that inequivalent reactivities for different Si-H functional groups in the system are observed. For exaniple, disilanes tend to be more reactive than monosilanes. Beyond disilane formation, the preferred dehydrocoupling reaction appears to involve addition of one silicon at a time to the growing chain, via M-S1H2R intermediates (n = 1 above). The Si-Si bond-forming reactions are also reversible. 

Many, if not most, step polymerizations involve equilibrium reactions, and it becomes important to analyze how the equilibrium affects the extent of conversion and, more importantly, the polymer molecular weight. A polymerization in which the monomer(s) and polymer are in equilibrium is referred to as an equilibrium polymerization or reversible polymerization. A first consideration is whether an equilibrium polymerization will yield high-molecular-weight polymer if carried out in a closed system. By a closed system is meant one where none of the products of the forward reaction are removed. Nothing is done to push or drive the equilibrium point for the reaction system toward the polymer side. Under these conditions the concentrations of products (polymer and usually a small molecule such as water) build up until the rate of the reverse reaction becomes equal to the polymerization rate. The reverse reaction is referred to generally as a depolymerization reaction other terms such as hydrolysis or glycolysis may be used as applicable in specific systems. The polymer molecular weight is determined by the extent to which the forward reaction has proceeded when equilibrium is established. 

Acyclic diene metathesis polymerization (ADMET) is a related polymerization in which an unconjugated diene polymerizes with loss of ethene [Lehman and Wagener, 2002, 2003 Schwendeman et al., 2002], ADMET is carried out using the Schrock and Gmbbs initiators at about 40-80°C. The process is a step polymerization, not a ROP chain reaction. The reaction is reversible, and high polymer MW is achieved by removal of ethene (usually by reduced... 

Consider a long, thin mold being fed at constant temperatme with two bifunctional monomers, AA and BB. The feed has a molecular weight of Mq, and the polymerization reaction, which is assumed to be reversible, proceeds by the reaction of A and B functional groups in an idealized step polymerization reaction (cf. Section 3.3.1.1) ... 

Beginn developed Percec-type dendrimers, which are known to form supramolecu-lar channels, with polymerizable acrylate groups in order to obtain ion-permeable membranes [97-99]. First, the dendron 78 (Scheme 40) was dissolved in a polymerizable acrylate mixture that does not shrink on polymerization. The second step was the thermo-reversible gelation of the acrylate mixture, which was followed by the last step, polymerization to fix the supramolecular channel structure (Scheme 40). In the first experiments, compounds with only one polymerizable group were used but it turned out that the gelating properties were not sufficient [100, 101] so threefold modified 78 had to be developed. 

Many reactions familiar to organic chemists may be utilized to carry out step polymerizations. Some examples are given in Table 2.2 for polycondensation and in Table 2.3 for polyaddition reactions. These reactions can proceed reversibly or irreversibly. Those involving carbonyls are the most commonly employed for the synthesis of a large number of commercial linear polymers. Chemistries used for polymer network synthesis will be presented in a different way, based on the type of polymer formed (Tables 2.2 and 2.3). Several different conditions may be chosen for the polymerization in solution, in a dispersed phase, or in bulk. For thermosetting polymers the last is generally preferred. 

Domine and Gogos (88-90) considered a very long, very wide, and thin mold being fed by a constant temperature mixture of AA, BB molecules. Both types are bifunctional and the feed has a molecular weight Mq. The polymerization, assumed to be reversible, proceeds by the reaction of A-ends with B-ends, and follows idealized step polymerization (condensation) kinetics without the generation of a small molecule (91). Specifically, we have... 

Thus, random copolymerization of cyclic ethers with cyclic amines is not possible. The other limitation, which will be discussed in more detail in a later part of this section, is the reversibility of homo- and/or crosspropagation steps, when one or both comonomers polymerize reversibly. 

Matsumoto, A. and Nakazawa, H. (2004) Two-step and reversible phase transitions of organic polymer crystals produced by topochemical polymerization. Macromolecules, 37, 8538-8547. 

Generally condensation reactions, such as (5.7)-(5.10), are reversible, so that the eliminated water must be removed if a high polymeric product is to be formed. The rate of a step polymerization is the sum of the rates of reaction between molecules of various sizes, that is, the sura of the rates for reactions such as (5.7)-(5.10). To describe the course of these reactions in terms of reaction kinetics would seem at first sight to be a very complicated task. However, fortunately it is possible to introduce simplifying approximations that make the kinetic problem tractable. 

The second key element of DCC-reversibility-is a feature that is intrinsic to many polymers, specifically those that are formed by step polymerizations. Although the conditions under which bond formation takes place for the most common functional groups (esters, amides) are quite harsh, a large variety of bonding forming reactions that are reversible under mild conditions have been studied recently and have become known under the name of dynamic covalent chemistry. Reversibility is an even more prominent feature of supramolecular polymers - polymers in which the monomeric units are held together by noncova-lent bonds. 

The first two steps are reversible and involve deprotonation by the base, followed by 1,6-elimination of a sulfide group. This equilibrium can be shifted to the right by the use of an organic imis-cible solvent, which removes the sulfide from the aqueous phase as it is formed. Whether step 3 is an ionic or a free-radical polymerization process has been under discussion for several years. In 1985, Wessling [21] stated that it had a free-radical character, and, in 1988, Lahti et al. [23] reported a number of experiments from which they concluded that it was ionic. Nevertheless, in a more recent paper [24], some of these authors returned to the free-radical mechanism. 

These modifications include variations on the length of the spacer which connects the azobenzene and the polymeric chain in side-chain azopolymers, or the electronic nature of substituents at the azobenzene moiety (Ruhmann, 1997). This requires the synthesis of appropriate monomers. Conventional techniques can be used for the polymerization of azobenzene monomers. For instance, step polymerization has been used by Hvilsted and co-workers to synthesize different series of liquid crystalline polyesters with potential applications in reversible optical data storage. These polyesters have a side-chain architecture, which synthesis is represented in Fig. 16.6 (Hvilsted et al, 1995). Being a modular synthetic approach, the influence of different structural parameters on the photoinduced optical properties can be evaluated in relatively simple manner. 

It is useful to start the kinetic analysis with an idealized case, which avoids complications that arise due to unequal stoichiometry, chain length-dependent reactivity, monofunctional impurities, cyclization, and reversible polymerization. The model addressed here is a linear AB step polymerization. 

Equations (2.19) and (2.21) have been derived assuming that the reverse reaction (i.e. depolymerization) is negligible. This is satisfactory for many polyadditions, but for reversible polycondensations requires the elimination product to be removed continuously as it is formed. The equations have been verified experimentally using step polymerizations that satisfy this requirement, as is shown by the polyesterification data plotted in Fig. 2.2. These results further substantiate the validity of the principle of equal reactivity of functional groups. 

The concept of a ceiling temperature has been outlined by several research groups (39-42). In order to obtain quantitative information about a depolymerization-polymerization equilibrium, the following steps of reversible addition... 

Polymer surface properties control wettability, adhesion, and fnction, and, in some cases, electronic properties. Gas-phase chlorination of polyethylene surfaces is done just for this purpose, and the reaction can be followed using x-ray photoelectron spectroscopy (XPS). The XPS technique can identify various chemical species within 10-70 pm of the surface. In the chlorination of polyethylene, the species are —CH2, —CHC—, —CCI2—, —CH—CH—, and —CH—CX—. Observe that the chlorination proceeds through a radical mechanism. The mechanism of polymerization, assuming that aU reaction steps are reversible, can be represented by... 

The propagation step is reversible due to a back-biting reaction of the active center with its own chain, and this leads to the formation of a series of cyclic monomers of various ring sizes. The silanolate may attack a Si—O bond of another chain, leading to the chain transfer (Equation 3), and this results in chain randomization. In the absence of any protonic impurities, the reaction proceeds without termination, while the polymerization can be quenched to deactivate the silanolate center. 

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