Condensation polymerization characteristics

Among the physical characteristics of these nonlinear condensation polymerizations, the occurrence of a sharp gel point is of foremost significance. At the gel point, which occurs at a well-defined stage in the course of the polymerization, the condensate transforms suddenly from a viscous liquid to an elastic gel. Prior to the gel point, all of the polymer is soluble in suitable solvents, and it is fusible also. Beyond the gel point, it is no longer fusible to a liquid nor is it entirely soluble in solvents. Linear polymers, on the other hand, remain soluble in suitable solvents and fusible to liquids as well (unless the melting point is above the temperature of thermal decomposition), regardless of the extent of condensation. 

Another characteristic is that most involve multiple phases, the explicit subject of the next chapter. The polymer formed is a Hquid or solid, while the monomer is a gas or Hquid. In condensation polymerization water is frequently a byproduct so in many polymerization processes there will be an aqueous phase as well as an organic phase. 

There are two characteristic classes of polymerization process, those of step growth and of chain growth.73 In the former, monomers combine to form dimers, and dimers to form tetramers and all oligomers of intermediate size combine to form larger molecules at random. In this kind of process, typical of classical, equilibrium-condensation polymerization, products of high molecular weight are obtained at very high conversion only. 

Phenol-formaldehyde adhesives are produced by a condensation polymerization reaction between phenol and formaldehyde. The phenolics used for exterior particleboard are made at a formalde-hyde/phenol ratio greater than 1.0 i.e., they are classified as resoles and additional formaldehyde is not required to complete the curing reaction to a highly cross-linked network structure. Many characteristics can be incorporated into the adhesives by changes in the F/P ratio, condensation pH, and condensation time. The reactive solids content is normally between kO and 50 percent since the stability and viscosity are adversely affected at higher solids. 

Condensation polymerizations are equilibrium reactions, which means they eventually stop reacting when small molecular weight reaction products like water are no longer removed from the system. These characteristics of the condensation polymerization reaction also have an effect on the chemical properties of such plastics. In the presence of water, particularly at high temperatures, polyethylene terephthalate begins to hydrolyze and low molecular weight oligomers are produced which can be transferred into a food in contact with the plastic. 

Although Diels-Alder polymerizations involve a reaction of an unsaturated molecule, and the polymer does not have a structure typical of a condensation polymer, the characteristics of the polymerization reaction are those of a condensation polymerization. The kinetics of a Diels-Alder polymerization should be those of a typical condensation polymerization and involve a series of individual reactions, and not a chain type mechanism. A typical second-order rate expression, -dC/dt = kC2 should hold for a good portion of the polymerization reaction, where C is the concentration of both diene and dienophile ends. Typically, this rate would break down only when the molecular mobility is extremely low or when shielding of functional groups occurs in dilute solutions. 

The structure of final product prepared by sol-gel method is in shape preliminarily in solution, and following process is relevant to the characteristic of sol directly. So the characteristic of sol is very important. The essential reason for transformation of homogeneous solution into sol is the hydrolysis of alcoholate and the condensation polymerization. Controlling conditions for hydrolysis of alcoholate is the key to the preparation of sol with promising quality. 

Substitution at the vinylic carbons can also be used to alter the electronic properties. For example, the substitution of electronegative -CN groups has been found to increase the electron affinity and ionization potential of PPV derivatives, thus improving the electron-injection characteristics.35,36 The synthesis is performed by the Knovenagel condensation polymerization of a terephthaldehyde and a benzene-1,4-diacetonitrile derivative, as shown in Fig. 5.10. 

Representative features of chain reaction polymerization are listed in Table 22.3 for comparison with the characteristics of typical condensation polymerizations. In both cases, the degree of polymerization is determined by dividing the molecular weight of the polymer by the molecular weight of the... 

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... 

Thus, the polyesterification reaction mixture at any instance consists of various-sized diol, diacid, and hydroxyacid molecules. Any OH-containing molecule can react with any COOH-containing molecule. This is a general characteristic of step polymerization. A comparative account of the differences between step polymerization or condensation polymerization on the one hand and chain polymerization or addition polymerization (see Chapter 6) on the other hand is given in Table 5.1. 

A characteristic behavior of the conductance of polyelectrolytes is shown in Figure 2. These strong polyelectrolytes were made by quaternization with methyl bromide of the polyester obtained from condensation polymerization... 

Problem 5.7 Polyurethanes having the characteristic linkage -NHCOO- are formed when a diol undergoes condensation polymerization with a diisocyanate (see Table 1.2). The polymerization kinetics is found to be of third order at very low concentrations but of second order at higher concentrations. This led to the proposition (Baker et al., 1949) of the following reaction mechanism ... 

Methyl Vinyl Ketone 3-Buten-2-one A3-2 -but-enone methylene acetone acetyl ethylene 3-oxo -a -butyl-ene. C4H(0 mol wt 70.09. C 68.55%, H 8.63%, O 22.83%. CHJCOCH=CHi. Prepn by condensation of acetone and formaldehyde to 3 -keto butanol and dehydration to methyl vinyl ketone Merling Kohler, J. Soc. Chem. Ind 29> 1037 (1910) White, Haward, X Chem. Soc. 1943, 25. Prepn from vinylaoetylene Conaway, U S. pat. 1,967,225 (1934 to du Pont). For review and polymerization characteristics see 4<Vinyl Ketone Polymers in Encyclopedia of Polymer Science and Technology vol. 14 (Interscience, New York, 197]) pp 617-636. 

The structural formulae are COOH (CH 2) if COOH (adipic acid) and H2N(CH2) 6NH2 This polymer was larought about by condensation polymerization with the loss of a small molecule (H2O), which is a characteristic of all condensation polymerizations. 

The polymerization of acrylic adhesives via free radicals (chain reaction) allows acrylic adhesives to be dispensed and cured using unique techniques. Other types of adhesive products often cure by condensation polymerization reactions which dictate certain cure characteristics and handling techniques. 

The second general pattern of polymerization is step-growth or condensation polymerization. While terminal alkenes are the most common monomers in chain-growth polymerization, bifunctional molecules are the characteristic monomers for step-growth polymerization. The polyester-, polyamide-, and polyurethane-forming reactions shown below are examples of step-growth polymerizations ... 

Several aspects of this research are particularly exciting. The condensation polymerization method allows for preparation of a broad spectrum of polymers with different primary structures based simply on using different diacid and diamine monomers. As a result, by choice of the acid and diamine monomers, one can alter, in somewhat predictive fashion, such polymer properties as melting point, solvent solubility, crystallinity, film and adhesive characteristics and undoubtedly other polymer properties that remain to be studied. Furthermore, the condensations require no aldaric acid hydroxyl group protection/deprotection, a very important cost consideration in the potential production of any of these materials. 

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