Chain growth polymerization description

Polymerization reactions require stringent operating conditions for continuous production of quality resins. In this paper the chain-growth polymerization of styrene initiated with n-butyllithium in the presence of a solvent is described. A perfectly mixed isothermal, constant volume reactor is employed. Coupled kinetic relationships descriptive of the initiator, monomer, polystyryl anion and polymer mass concentration are simulated. Trommsdorff effects (1) are incorporated. Controlled variables include number average molecular weight and production rate of total polymer. Manipulated variables are flow rate, input monomer concentration, and input initiator concentration. The... 

For an excellent description of the mechanism of chain-growth polymerization, see Odian, G. In Principles of Polymerization, 3rd edn., Wiley-Interscience New York 1991, Ch. 3. 

The description of the variety of chemistries that are used to produce thermosetting polymers can be the subject of a whole book and is beyond the scope of this chapter. A description of chemistries involved in the synthesis of several families of thermosets can be found elsewhere [2]. In this section, we focus on some aspects of the chemistry of epoxy polymers because it provides examples of both step-growth and chain-growth polymerizations employed in the synthesis of polymer networks. 

The more commonly used descriptive name chain-growth polymerization is given to these addition polymerization synthetic methods. 

Another convenient and effective scheme for the approximate solution of a mathematical description of the polymerization reaction replaces the discrete variable of infinite range, polymer chain length, by a continuous variable. The difference-differential equations become partial differential equations. Barn-ford and coworkers [16,27,28] used this procedure in their analysis of vinyl (radical chain growth) polymerization. Zeman and Amundson [18,19] used it extensively to study batch and continuous polymerizations. Recently, Coyle et al. [4] have applied it to analysis of high conversion free radical polymerizations while Taylor et al. [3] used it in their modelling efforts oriented to control of high conversion polymerization of methyl methacrylate. A rather extensive review of the numerical techniques and approximations has been presented by Amundson and Luss [29] and later by Tirrell et al. [30]. 

Regardless of the scenario it must be kept in mind that a ROP plus polycondensation process needs two different definitions of conversion for its proper description. For the ROP part the conversion is defined by the consumption of monomers as usual for a chain-growth polymerization. Yet, for polycondensations the conversion is defined by the consumption of functional (end) groups regardless, if inter- or intramolecular condensation steps take place. 

Recently, we reported detailed descriptions of hydrocarbon chain growth on supported Ru catalysts (7,8) we showed that product distributions do not follow simple polymerization kinetics and proposed a diffusion-enhanced olefin readsorption model in order to account for such deviations (7,8). In this paper, we describe this model and show that it also applies to Co and Fe catalysts. Finally, we use this model to discuss a few examples from the literature where catalyst physical structure and reaction conditions markedly influence hydrocarbon product distributions. 

Although these definitions were perfectly adequate at the time, it soon became obvious that notable exceptions existed and that a fundamentally sounder classification should be based on a description of the chain-growth mechanism. It is preferable to replace the term condensation with step-growth or step-reaction. Reclassification as step-growth polymerization now logically includes polymers such as polyurethanes, which grow by a step-reaction mechanism without elimination of a small molecule. 

Moreover, in chain-growth copolymerizations, which involve the polymerization of two or more comonomers, different comonomer sequences have to be tracked by chain length, as illustrated in Fig. 10.2, for simplicity considering only well-defined monomer sequences (Matyaszewski et al., 2012). This clearly complicates the mathematical description of the polymerization kinetics. In addition, if different CLDs can be obtained by type of active center leading to an observed CLD that is a superposition of individual CLDs, the computational cost increases further (Soares and Hamielec, 1995). 

The description of free-radical chain poljnnerization kinetics must take into account four basic steps initiation, which creates free-radical active centers propagation, which grows the polymer chains termination, which destroys the active centers and ends chain growth and chain transfer, which ends a growing chain and begins another. These classical steps also describe thermal polymerizations however, different descriptions are required for thermal- and photoinitiation. 

The reactions taking place during the synthesis of a polymer are rather complex in nature. The description of the chemistry of a polymerization reaction often involves over 20 different elementary reactions. This means that control of the overall reaction rate that governs the process safety may be rather complicated. Nevertheless the kinetically determining step in polymerization reactions is the chain growth reaction. 

This description is quite similar to that given for the polymerization of ethylene oxide initiated by sodium methoxide in dioxane and methanol, provided by Gee, Higginson and Merrall ( ). If the rate of chain end activation proceeded at a very rapid rate relative to chain growth, a naxrow, symmetrical distribution would be obtained. If this activation process was very much slower than the rate of monomer addition, a broad distribution would result. In the case of the poly(propylene ether) diols... 

It is to be noted that not all polymers made by the condensation method form a condensate during the reaction. Polyurethanes which are formed by a reaction of isocyanates and alcohols are such an example. Also, ring opening polymerization reactions are considered to be of the addition type even though they form polymers which can also be formed by a condensation reaction, e.g., the polymerization of caprolactam to form nylon 6,6 (see Painter and Coleman, (1994)). As a result, most modem texts do not use the polymerization descriptions, condensation and addition. Rather, the terms step growth and chain are used in place of condensation and addition respectively. 

The term acrylic apphes to a family of copolymers of monomers that are polymerized by a chain growth mechanism. Most often, the mechanism of polymerization is by free radical initiation. Other mechanisms of polymerization, such as ionic and group transfer polymerization, are possible but will not be discussed in this publication. For a description of other polymerization mechanisms, polymer textbooks are available (5,6). Technically, acrylic monomers are derivatives of acrylic or methacrylic acid. These derivatives are nonfunctional esters (methyl methacrylate, butyl acrylate, etc.), amides (acrylamide), nitrile (acrylonitrile), and esters that contain functional groups (hydroxyethyl acrylate, glycidyl methacrylate, dimethylaminoethyl acrylate). Other monomers that are not acryhc derivatives are often included as components of acryhc resins because they are readily copolymerized with the acryhc derivatives. Styrene is often used in significant quantities in acryhc copolymers. 

A detailed description of AA, BB, CC step-growth copolymerization with phase separation is an involved task. Generally, the system we are attempting to model is a polymerization which proceeds homogeneously until some critical point when phase separation occurs into what we will call hard and soft domains. Each chemical species present is assumed to distribute itself between the two phases at the instant of phase separation as dictated by equilibrium thermodynamics. The polymerization proceeds now in the separate domains, perhaps at differen-rates. The monomers continue to distribute themselves between the phases, according to thermodynamic dictates, insofar as the time scales of diffusion and reaction will allow. Newly-formed polymer goes to one or the other phase, also dictated by the thermodynamic preference of its built-in chain micro — architecture. 

The fourth dassiiication scheme has been used by Cifetri, differentiating SPs on the basis of the physical stmcture assumed through polymerization. Example classes indude linear chains, helical chains, columnar assemblies, micdlar assemblies, planar assemblies, composite assemblies, and three-dimensional assemblies. Ciferri induded a breakdown and description of the linear and hdical chain assemblies as being controlled by different growth mechanisms however, these types are ultimatdy dassified by resulting stmcture. 

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