A-B step-growth polymerization

Step-growth polymerizations generally involve either one or more types of monomers. In either ease, each monomer has at least two reactive (functional) groups. In cases where only one type of monomer is involved, which is known as A-B step-growth polymerization, the functional groups on the monomer are different and capable of intramolecular reactions. An example is the formation of an aliphatic polyester by the self-condensation of co-hydroxycaproic acid (Equation 2.24). 

A-B step-growth polymerization. One recalls that the simplest kinetic representation for A-B step-growth polymerization (as given in Chapter 16) is ... 

Scheme 5. Structural and chronological overview of the macromonomers used for synthesis of dendronized polymers chain growth (a) and step growth polymerizations (b)...

One of the most important properties in condensation polymers is the polymer s molecular weight. Consider a linear step-growth polymerization of AB-type monomers. For illustrative purpose, let A refer to a hydroxyl group and B to a carboxyhc add group, so that a polyester is formed. The polymerization reaction is ... 

A-A/B-B Step Growth If the two functional groups are on different monomer units, the kinetics of step-growth polymerization are more complex. If two monomers are used, one with two A functional groups and one with two B functional groups, the polymerization is described as A-A/B-B step-growth polymerization. An example is nylon 66. The polymerization may be represented as ... 

Fig. 2.1 (a) Mole fraction distribution and (b) weight fraction distribution of chain lengths for various extents of reaction in a linear step-growth polymerization (both sets of curves taken from Flory). 

In summary, two conclusions may be drawn. First, non-stoichiometric polycondensations of class ni may give much higher molar masses than polycondensations of a-b monomers. Second, the kinetic course of the polymerizations deviate largely from that of a normal step-growth polymerization, and the term non-stoichiometric polycondensation is rather a formal label than a correct terminology. In the extreme case, when cyclization is totally suppressed, class III polymerization exactly obeys the definition of an aa + condensative chain polymerizaion . This term was proposed by lUPAC, and numerous condensative chain polymerizations are known from a-b monomers as discussed in Chap. 16. 

Step-growth polymerizations can be schematically represented by one of the individual reaction steps VA + B V —> Vab V with the realization that the species so connected can be any molecules containing A and B groups. Chain-growth polymerization, by contrast, requires at least three distinctly different kinds of reactions to describe the mechanism. These three types of reactions will be discussed in the following sections in considerable detail. For now our purpose is to introduce some vocabulary rather than develop any of these beyond mere definitions. The principal steps in the chain growth mechanism are the following ... 

Monomers of die type Aa B. are used in step-growth polymerization to produce a variety of polymer architectures, including stars, dendrimers, and hyperbranched polymers.26 28 The unique architecture imparts properties distinctly different from linear polymers of similar compositions. These materials are finding applications in areas such as resin modification, micelles and encapsulation, liquid crystals, pharmaceuticals, catalysis, electroluminescent devices, and analytical chemistry. 

Hyperbranched polyesters are prepared by the step-growth polymerization of A13,-type monomers where A and B are —OH and —COOH groups or derivatives such as CH3COO-, HO-CH2CH2-O-, (CH3)3SiO—, -COOCH3, or -COC1... 

In general, there are two distinctively different classes of polymerization (a) addition or chain growth polymerization and (b) condensation or step growth polymerization. In the former, the polymers are synthesized by the addition of one unsaturated unit to another, resulting in the loss of multiple bonds. Some examples of addition polymers are (a) poly(ethylene), (b) poly(vinyl chloride), (c) poly(methyl methacrylate), and (d) poly(butadiene). The polymerization is initiated by a free radical, which is generated from one of several easily decomposed compounds. Examples of free radical initiators include (a) benzoyl peroxide, (b) di-tert-butyl peroxide, and (c) azobiisobutyronitrile. 

When the concentrations of A and B may be varied independently (Eq. 2.2), the stoichiometric ratio of functionalities is defined by r = A0/B0, where A0 and B0 are the initial concentrations of functional groups A and B. As will be shown in Chapter 3, this ratio is very important in designing and controlling a step-growth polymerization. Statistical parameters at any... 

As we mentioned in the introduction to this chapter, we will start by considering the statistics of linear step-growth polymerization. Remember that there are two types of such reactions in the first, each bifunctional monomer has different but complementary functional groups, an acid, A, and an alcohol, B, for example (i.e., A-B) in the second type, each monomer only has one type of functional group (i.e., A-A and B-B). In each case an A can only react with a B, ,in this example to give an ester, which we ve labeled either AB or BA in Figure 5-3 (think about it— they are equivalent and only differ in direction along the chain). 

The relationship between number average degree of polymerization and p (Equation 5-3) only applies to polymerizations where there are exactly equal numbers of A and B functional groups. Equivalence is obtained directly in step-growth polymerizations of type I (A-B), but is more difficult to achieve for polymerizations of type II (A-A and B-B). However, if the number of functional groups is not exactly the same, we still... 

Figure 5.21. Reaction schemes for the most common types of step-growth polymerization. Shown are (a/c) polyester formation, (b/d) polyamide formation, (e) polyamide formation through reaction of an acid chloride with a diamine, (f) transesterification involving a carboxylic acid ester and an alcohol, (g) polybenzimidazole formation through condensation of a dicarboxyhc add and aromatic tetramines, and (h) polyimide formation from the reaction of dianhydrides and diamines.

Many important step-growth polymerizations are run under conditions in which the reverse reaction between the polymer and condensation products is significant [as in reactions (a) and (b) in Fig. 5-2]. In these cases, the stoichiometry of the reactants must be carefully controlled, and a close balance is needed between the concentrations of functional groups of opposite kinds. 

Fig. 5-3. Some step-growth polymerizations of n-functional monomers (where ri > 1 and any number of different monomers may be present), (a) (b) and (c) represent different possible combinations of monomers.

Fig. 5-5. (a) Mole fraction disiribiition of reaction mixture in linear step-growth polyinerizalion for several extents of reaction, (b) Weight fraction distribution of reaction mixture in linear step-growth polymerization for several extents of leaction [2],... 

When an equilibrium step-growth polymerization is 99% complete, what fraction of the reaction mixture is still monomer (a) on mole basis and (b) on weight basis ... 

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