Condensation polymers branching

PoIya.mines are condensation polymers containing nitrogen they are made by a variety of synthetic routes. Most of the commercial polyamines are made by reaction of epichlorohydrin with amines such as methylamine [25988-97-0] or dimethylamine [39660-17-8] (18,19). Branching can be increased by a dding small amounts of diamines such as ethylenediamine [42751-79-1]. A typical stmcture of this type of polyamine is stmcture (9). 

Water. Latices should be made with deionized water or condensate water. The resistivity of the water should be at least lO Q. Long-term storage of water should be avoided to prevent bacteria growth. If the ionic nature of the water is poor, problems of poor latex stabiUty and failed redox systems can occur. Antifreeze additives are added to the water when polymerization below 0°C is required (37). Low temperature polymerization is used to limit polymer branching, thereby increasing crystallinity. 

Thermolysis. Lower tetraalkyl titanates are reasonably stable and can be distilled quickly at atmospheric pressure. Protracted heating forms condensation polymers plus, usually, alcohol and alkene. Longer or more branched chains are less stable. Thus, tetra- -pentyl titanate [10585-24-7] can be distilled at 314°C and 101.3 kPa (1 atm), whereas tetra- -hexyl titanate [7360-52-3] must be distilled at below 18.7 kPa (140 mm Hg) and tetra- -hexadecyl... 

To examine the significance of this approximation further, it should be noted that a highly branched condensation polymer molecule, such as the one shown in Fig. 61, retains many unreacted functional groups which offer a number of opportunities for reaction between pairs on the same molecule. That intramolecular reaction between them proceeds to an appreciable degree in competition with intermolecular condensa-... 

If more than one type of branching unit is present, (/—I) must be replaced by the appropriate average, weighted according to the numbers of functional groups attached to the various branched units and the molar amount of each present. The critical condition can be expressed in various ways Eq. (7) is a particularly convenient form for application to condensation polymers. 

The topological structure of condensation polymers is predetermined by the functionality of the initial monomers. If all of them are bifunctional then linear polymers are known to form. Branched and crosslinked molecules are prepared only when at least one of the monomers involves three or more functional groups. 

A general theory of the equilibrium polycondensation of an arbitrary mixture of monomers, described by the FSSE model, has been developed [75]. Proceeding from rigorous thermodynamic considerations a branching process has been indicated which describes the chemical structure of condensation polymers and expressions have been derived which relate the probability parameters of this stochastic process to the thermodynamic parameters of the FSSE model. 

Branched and crossiinked condensation polymers are produced when the reaction mixture includes tri-functional monomers as well as bi-functional ones. The incorporation of a single tri-functional monomer into a chain generates a branch point. As we increase the fraction of tri-functional monomers, branching becomes more prevalent and the resulting molecules more complex. When sufficient tri-functional monomers are present we create a three-dimensional crossiinked network. Figure 1.12 shows the general outline of the effects of tri-functional monomers on condensation polymers. 

When Paul Flory wrote his famous book Principles of Polymer Chemistry in 1952, he indicated an alternative scheme for polymer synthesis [1]. He theorized about synthesizing condensation polymers from multifunctional monomers. These polymers were predicted to have a broad molecular weight distribution and to be non-entangled and non-crystalline due to their highly branched structure. However, they were considered to be less interesting since they would provide materials with poor mechanical strength, and at that time Flory did not feel it was worthwhile pursuing this line of research. 

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 ... 

Size exclusion chromatography is the premier polymer characterization method for determining molar mass distributions. In SEC, the separation mechanism is based on molecular hydrodynamic volume. For homopolymers, condensation polymers and strictly alternating copolymers, there is a correspondence between elution volume and molar mass. Thus, chemically similar polymer standards of known molar mass can be used for calibration. However, for SEC of random and block copolymers and branched polymers, no simple correspondence exists between elution volume and molar mass because of the possible compositional heterogeneity of these materials. As a result, molar mass calibration with polymer standards can introduce a considerable amount of error. To address this problem, selective detection techniques have to be combined with SEC separation. 

Very highly branched polymers, like polyethylene made by free-radical, high-pressure processes, will have Mw/Mn ratios of 20 and more. Most polymers made by free-radical or coordination polymerization of vinyl monomers have ratios of from 2 to about 10. The M /M ratios of condensation polymers like nylons and thermoplastic polyesters tend to be about 2, and this is generally about the narrowest distribution found in commercial thermoplastics. 

Because of these factors, molecular weight calculations are used mainly for systematic modifications of formulations which have unsatisfactory property balances rather than for accurate predictions of gel points under practical operating conditions. The design of branched condensation polymers still relies heavily on Xn estimates, since these are less complicated than the theoretically more accurate Xw method described in the next section of this chapter. 

Typical condensation polymers, such as polyester and nylon, often exhibit these properties. If the fiber is to be ironed, its Tg should be above 200 °C if it is to be drawn from the melt, its Tg should be below 300 °C. Branching and cross-linking are undesirable because they disrupt crystalline formation even though a small amount of cross-linking may increase some physical properties if effected after the material is suitably drawn and processed. 

If the fiber is to be ironed, its Tg should be above 200°C. Branching and crosslinking are nndesirable since they inhibit crystalline formation. Even so, some crossfinkmg may be present to maintain a given orientation, snch as desired in permanent press clothing. While most fibers are made from condensation polymers, new treatments allow some fibers to be made from olefinic materials snch as polypropylene (Table 3). 

While strictly linear polymers are important conceptually and, as is the case with many condensation polymers, important industrially, many polymers, especially those made by addition polymerization, contain branches. Often such branching arises due to side reactions, though it may be introduced deliberately. Two types of branching may be distinguished ... 

There are a vast number of monomers that may be polymerized. The resulting polymers fall broadly into two categories, addition polymers and condensation polymers, depending on the mechanism of pofymerization. These two methods of polymerization will be illustrated in the following section, using as examples polymers of technological interest. It is important to keep in mind that the synthesized polymer—whether produced an addition or a condensation reaction—may be linear (or linear but with small side-branches) or, on the other hand, cross-linked. 

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