Polyfunctional Condensation Polymerization

The formation of multichain condensation polymers using an excess of A-B with a small amount of the multifunctional monomer R-A/, under conditions where no A can react with another A was described above in Sect. 1.2.1. Polyfunctional condensation polymerization using multifunctional monomers in other contexts can produce infinite... 

Modelization of the System. Theoretical treatment of polyfunctional monomers condensation polymerization has been firstly proposed by Flory and Stockmayer (22.23 and later by Gordon, Bruneau, Macosko and others (24-26. These theories lay out the basic relation between extent of reaction and average molecular weight of the resulting non linear polymers. 

Condensation polymerizations (polycondensations) are stepwise reactions between bifunctional or polyfunctional components, with elimination of small molecules such as water, alcohol, or hydrogen and the formation of macromo-lecular substances. For the preparation of linear condensation polymers from bifunctional compounds (the same considerations apply to polyfunctional compounds which then lead to branched, hyperbranched, or crosslinked condensation polymers) there are basically two possibilities. One either starts from a monomer which has two unlike groups suitable for polycondensation (AB type), or one starts from two different monomers, each possessing a pair of identical reactive groups that can react with each other (AABB type). An example of the AB type is the polycondensation of hydroxycarboxylic acids ... 

Cyclotrimerization of polyfunctional aryl acetylenes offers a unique route to a class of highly aromatic polymers of potential value to the micro-electronics industry. These polymers have high thermal stability and improved melt planarization as well as decreased water absorption and dielectric constant, relative to polyimides. Copolymerization of two or more monomers is often necessary to achieve the proper combination of polymer properties. Use of this type of condensation polymerization reaction with monomers of different reactivity can lead to a heterogeneous polymer. Accordingly, the relative rates of cyclotrimerization of six para-substituted aryl acetylenes were determined. These relative rates were found to closely follow both the Hammett values and the spectroscopic constants A h and AfiCp for the para substituents. With this information, production of such heterogeneous materials can be either avoided or controlled. 

Condensation Polymerization. Networks can form as a result of non-linear condensation polymerization. For example, if bifunctional units. A, are present in solution with Af, polyfunctional units of functionality/, then the following structure may result ... 

In condensation polymerization, the reaction takes place between two polyfunctional molecules to produce one Icirger polyfunctional molecule with the possible elimination of a small molecule such as water. Long reaction times are essential for forming high molecular weight polymers by this step reaction. 

There are two primary polymerization approaches step-reaction polymerization and chain-reaction polymerization. In step-reaction (also referred to as condensation polymerization), reaction occurs between two polyfunctional monomers, often liberating a small molecule such as water. As the reaction proceeds, higher-molecular-weight species... 

In condensation polymerization, the reaction takes place between two polyfunctional molecules to produce one larger polyfunctional molecule with the possible... 

Polymerization reactions are divided into two groups known as step reactions (also called condensation reactions) and chain reactions, also known as addition reactions. Step reactions require bifunctional or polyfunctional monomers, while chain reactions require the presence of an initiator. 

Thus many heterocycles of various sizes can be formed and the reductive cyclization of polyfunctional nitro compounds offers a useful synthetic tool for the synthesis of complex molecules. When the interacting functions are not located at positions conducive to ring formation, bimolecular condensation or even polymerization may occur. 

Example 13.1 shows one reason why binary polycondensations are usually performed in batch vessels with batch-weighing systems. Another reason is that some polycondensation reactions involve polyfunctional molecules that will crosslink and plug a continuous flow reactor. An example is phenol, which is trifunctional when condensed with formaldehyde. It can react at two ortho locations and one para location to build an infinite, three-dimensional network. This may occur even when the stoichiometry is less than perfect. See Problem 13.3 for a specific example. In a batch polymerization, any crosslinked polymer is removed after each batch, while it can slowly accumulate and eventually plug a flow reactor. 

The distribution functions of the degrees of polymerization are very complicated in polyfunctional polycondensations. If a monomer with three A groups is condensed with a bifunctional monomer with two B groups. 

Carothers, in 1929, classified synthetic polymers into two classes, according to the method of their preparation, i.e., condensation polymers and addition polymers. In polycondensation, or step-growth polymerization, polymers are obtained by reaction between two polyfunctional molecules and elimination of a small molecule, for example water. Typical condensation polymers are shown in Figure 2. Addition (or chain reaction) polymers are formed from unsaturated monomers in a chain reaction. Examples of addition polymers are shown in Figure 2. 

Chemists usually synthesize polymers by condensation (or step-reaction polymerization) or addition (also known as chain-reaction polymerization). A good example of chain polymerization is the free-radical mechanism in which free radicals are created (initiation), fecilitating the addition of monomers (propagation), and ending when two free radicals react with each other (termination). A general example of step-reaction polymerization is the reaction of two or more polyfunctional molecules to produce... 

The condensation route to wall polymers is the best method for pesticide encapsulation. In this process the two reactive monomers, one dissolved in each of the two phases (of the emulsified oil/pesticide in water), polymerize at the interface and generate the wall material. Typically, the oil-phase monomers are polyfunctional isocyanates (A) or acid chlorides (B) and the water-phase reactants are polyalcohols or amines. Compounds sufficiently reactive are chosen such that when they meet at the interface the condensation polymer forms the capsule wall (Fig. 4). Alternatively, the two reactants (a diol and a diisocyanate) and a low boiling solvent make up the oil phase of the emulsion along with the pesticide. When heated the solvent evaporates bringing the monomers together at the droplet surface to form the capsule wall (27). 

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