Linear step-growth polymerization

The simplest step-growth Unear polymer is of the AB type. Typical examples of AB polymerization are the step polymerization of HORCOOH and H2NRCOOH. For example, the polymer from HORCOOHis 

To derive the molecular weight averages of the polymer, we shall use here the method described by Lopez-Serrano et al. (1980), which is identical in concept to the recursive method of Macosko and Miller (1976) cited above. Selecting an A group (marked by ) at random, the in direction will be de ned as the direction from the chosen A toward the B group of the same mer unit Out  

Let us now ask what will be the expected weight of the polymer chain attached to a randomly chosen A group (indicated by ) looking out , i.e., Since the A group is chosen at 

Here Aa represents the initial moles of A type groups and Aa equals the moles after some reaction time. 

The expected weight attached to a B looking in is equal to the weight of an AB mer plus the expected weight attached to an A looking out . Thus, 

Let us now ask what will be the expected weight of the polymer chain attached to a randomly chosen A group (indicatedby ) looking out , i.e., E (W ). Since the A group is chosen at random, is a random variable. equals 0 if A has not reacted. If A has reacted (with B of the next mer unit) then equals Wil , the weight attached to B looking into B s parent molecule (Macosko and Miller, 1976)  

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Linear step-growth polymerizations require exceptionally pure monomers in order to ensure 1 1 stoichiometry for mutually reactive functional groups. For example, the synthesis of high-molecular-weight polyamides requires a 1 1 molar ratio of a dicarboxylic acid and a diamine. In many commercial processes, the polymerization process is designed to ensure perfect functional group stoichiometry. For example, commercial polyesterification processes often utilize dimethyl terephthalate (DMT) in the presence of excess ethylene glycol (EG) to form the stoichiometric precursor bis(hydroxyethyl)terephthalate (BHET) in situ. 

Linear polyurethanes, 26 Linear step-growth polymerizations, 13 Lipase-catalyzed polyesterifications, 83 Lipases, 82, 84 catalytic site of, 84 Liquefied MDIs, 211, 226-227 Liquid carbon dioxide, 206 Liquid-castable systems, 201 Liquid crystal devices (LCDs), alignment coating for, 269-270 Liquid crystalline aromatic polyesters, 35 Liquid crystalline polyesters, 25, 26, 48-53... 

Linear step-growth polymerization, 20 405 Linear superelastic effect (LSE),... 

Otto Bayer was aware of the work of Staudinger and Carothers. He was particularly impressed with the latter s discovery of polyamides and its implications for the fiber and textile industries. He knew that in the condensation (linear step-growth) polymerization used to prepare nylon and polyesters a small molecule, usually water, is formed and has to be removed. 

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). 

When dwj fdi = 0, the curve has a maximum and / = I / In p. A series expansion of — 1 / In p gives p/(p — I) as the first term and this fraction approaches I /(p — I) as p approaches 1. This is the value of in linear step-growth polymerizations (Eq. 5-20). Thus, as a first approximation, the peak in the weight distribution of high conversion linear step-growth polymers is located at of the polymer if the synthesis was carried out under conditions where interchange reactions and molecular weight equilibration could occur. 

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],... 

A consideration of Eqs. (6.175) and (6.176) indicates that high-molecular-weight polymer (i.e., large DP and DP ) will only be produced if P is close to unity, i.e., if Rp (Rtr+Rt). Equation (6.177) indicates that the size distribution DPwlDPn (also referred to as PDI, the polydispersity index) has a limiting value of 2 as P approaches unity. The situation is thus analogous to that for linear step-growth polymerization considered in Chapter 5 [cf. Eq. (5.54)]. 

FIGURE 2.4 (a) Number fraction distribution curves for linear step-growth polymerizations curve 1, p = 0.9600 curve 2, p = 0.9875 curve 3, p = 0.9950. (b) Corresponding weight fraction distribution for the same system. 

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 ... 

Molecular weight development in non-linear step-growth polymerization... 

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). 

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