Binary Polycondensations

Tubular reactors are used for some polycondensations. Para-blocked phenols can be reacted with formalin to form linear oligomers. When the same reactor is used with ordinary phenol, plugging will occur if the tube diameter is above a critical size, even though the reaction stoichiometry is outside the region that causes gelation in a batch reactor. Polymer chains at the wall continue to receive formaldehyde by diffusion from the center of the tube and can crosslink. Local stoichiometry is not preserved when the reactants have different diffusion coefficients. See Section 2.8. 

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Example 2.9 Repeat Example 2.8 assuming that the polymerization is second order in monomer concentration. This assumption is appropriate for a binary polycondensation with good initial stoichiometry, while the pseudo-first-order assumption of Example 2.8 is typical of an addition polymerization. 

Example 13.1 Determine the stoichiometric requirements for achieving various degrees of polymerization for a binary polycondensation. 

Solution Table 13.1 shows results calculated using Equation (13.4). The stoichiometric requirement for a binary polycondensation is very demanding. High-molecular-weight pol5Tner, say li > 100, requires a weighing accuracy that is difficult to achieve in a flow system. 

Example 13.1 shows one reason that binary polycondensations are usually performed in batch vessels with batch-weighing systems. Another reason is... 

TABLE 13 1 Number Average Chain Lengths for Binary Polycondensations Going to Completion... 

Example 13.2 Determine PD as a function of chain length for binary polycondensations that go to completion. 

Table 13.4 tabulates results at the same value of as in Table 13.3. The polydispersities are lower than when the same average chain length is prepared by a binary polycondensation going to completion. The stoichiometry-limited binary polycondensations have a higher polydispersity because the monomer in stoichiometric excess (the B monomer) is included in the calculations. This broadens the molecular weight distribution.

The probability distribution for chain lengths in a binary polycondensation is... 

FIGURE 13.1 Molecular weight distribution by number fraction for a binary polycondensation going to completion with In — 5 and Iw — 9.8. 

A binary polycondensation of AMA and BNB is to be performed in a batch reactor. A number average chain length of at least 100 is required. What minimum accuracy is required for weighing the two components ... 

Find some real chemistry where a binary polycondensation will give a homopolymer i.e., AMA reacts with BMB to form poly-M. 

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. 

Table 13.3 Polydispersities for Binary Polycondensations Reacting to Completion...

Figure 13.2 Molecular weight distribution by weight fraction for binary polycondensation going to completion with Im = 5 and Iw = 9.8.

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