Step polymerization processes

Step polymerization [40,41] occurs by reaction between two functionalized coreactants either with or without the elimination of an unwanted small molecule or condensation product which usually has to be removed from the reaction mixture. 

In order to produce a polymer by step addition and step condensation reactions it is necessary to start with precursors which each have at least two functional groups. Monofunctional reactants stop the formation of polymer chains and can be used to moderate polymer molecular weight. 

The kinetic laws which apply to both of these reactions are the same and are relatively simple to formulate if the reactions are described in terms of the concentration or number of functional groups of any given type present in the reaction mixture. If equimolar amounts of two coreactants are used in a step polymerization reaction then the concentration of each type of functional group involved in the polymerization process is the same. If the concentration of a given functional group is given by c then the rate of polymerization (/ p) is given by 

This has been explained by suggesting that the reaction is catalysed by the acid groups in one of the coreactants, i.e. 

If a perfect molar ratio of reactants is achieved and there is no thermodynamic barrier to the reaction going to completion, then a series of large ring molecules could be formed. In essence, an infinitely high molecular weight product is formed. Practically, it is impossible to achieve perfect 

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Figure 24.3 Two-step polymerization process for the manufacture of polyethylene terephthalate ...

Chemical vapor deposition (CVD) is a process whereby a thin solid film is synthesized from the gaseous phase by a chemical reaction. It is this reactive process that distinguishes CVD from physical deposition processes, such as evaporation, sputtering, and sublimation.8 This process is well known and is used to generate inorganic thin films of high purity and quality as well as form polyimides by a step-polymerization process.9-11 Vapor deposition polymerization (VDP) is the method in which the chemical reaction in question is the polymerization of a reactive species generated in the gas phase by thermal (or radiative) activation. 

On the other hand, it is apparent that there are different reactant systems that can yield the exact same polymer. Thus the use of the diacid chloride or anhydride instead of the diacid in Eq. 2-120 would give exactly the same polymer product. The organic chemical aspects of the synthesis of various different polymers by different step polymerization processes have been discussed [Elias, 1984 Lenz, 1967 Morgan, 1965]. Whether one particular reaction or another is employed to produce a specific polymer depends on several factors. These include the availability, ease of purification, and properties (both chemical and physical) of the different reactants and whether one or another reaction is more devoid of destructive side reactions. 

In this application bimodal pressure-sensitive adhesives consisting of C i C i x polymethacrylates were prepared that exhibited high adhesion to low solubility threshold (LSE) substrates at ambient temperature and at elevated temperature. These materials were free radically prepared in a two-stage single-step polymerization process. 

This review deals with the syntheses of various hyperbranched polyamines that are prepared through a one-step polymerization process. Furthermore, we present the current status of polyamines as gene carriers and describe their versatility, and their properties such as structure-property dependency, gene transfection efficiency, and cytotoxicity profiles of hyperbranched polyamines. 

It can be stated that networks based on a simple formulation (one monomer reacting with a comonomer), obtained from the step-polymerization process will exhibit a homogeneous structure. This is the case for epoxy-amine networks (the most studied) and polyurethane networks that have been used very often as ideal networks for structure-property correlations. 

But the Step-Polymerization Process Can Also Induce Inhomogeneities... 

This polymerization process is a polycondensation in which the molecular weight builds up slowly as the small molecules of water are eliminated. Most step polymerization processes are polycondensations thus the terms step polymerization and condensation polymerization are often used synonymously. The stepwise reaction leads successively from monomers to dimers, trimers, and so on, until finally polymer molecules are formed. The polymers obtained are classified by taking into account the functional group of the repeating unit, for example, polyesters (— CO —O—), polyamides ( — CO— NH —), polyurethanes (—O — CO — NH —), polyethers ( — O —), and polycarbonates ( — O — CO —O —). 

A multi-step polymerization process and a simultaneous use of a proper catalytic system can give rise to a very broad MWD. This technique with many operative modifications, offers the possibility of regulating polyethylene MWD in a wide range. In fact by producing fixed amounts of polymers, according to determined molecular weights of each reactor and to the polymer final molecular weight, is possible to get the product with the desired MWD. This could be theoretically calculated by a simple equation but one must also consider the residence time distributions in the various reactors. 

A very broad MWD is generally obtained by operating with multi-step polymerization processes each of which yielding a polymer with different average molecular weight. The success of such an industrial arrangement however, always seems to be linked to the availability of suitable catalytic systems which supply intrinsically broad MWD (the patent literature often teaches the use of catalysts containing compounds of more than one transition metal). 

Most (but not all) of the step polymerization processes involve polycondensation (repeated condensation) reactions. Consequently, the terms "step polymerization" and "condensation polymerization" are often used synonymously. In a condensation reaction between two molecules, each molecule loses one atom or a group of atoms at the reacting end, which leads to the formation of a covalent bond between the two, while the eliminated atoms bond with each other to form small molecules such as water—hence the term condensation reactions. Consider, for example, the synthesis of a polyamide, i.e., a polymer with amide -(-CONH-)- as the characteristic... 

While the lower-molecular-weight polysiloxanes can be synthesized by the hydrolytic step polymerization process, the higher-molecular-weight polymers are synthesized by ring-opening polymerization using ionic initiators ... 

Alternative ways of representing the reaction-induced phase separation in the case of a polycondensation (step-polymerization) process are plotted in Figs. 7 and 8 [38]. 

Figure 1.19 Number (a) and weight (b) distribution of the products of a step polymerization process which gives rise to linear products with the extent of reaction (p) of the functional groups ...

Pyo and Reynolds (52,53) reported the selective releasing of ions, such as adenosine 5 -triphosphate and heparin respectively, from conducting polymer bilayer structures. Piro and co-workers (54) successfully incorporated oligonucleotides into conducting polymer films using a two-step polymerization process. The release of these trapped oligonucleotides was also investigated, where the... 

In its most restricted sense, the term photopolymerization should be reserved for step polymerization processes in which each reaction step requires the absorption... 

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