Monomer reactivity ratio INDEX

For polymerizations carried out to high conversions where the concentrations of propagating centers, monomer, and transfer agent as well as rate constants change, the polydispersity index increases considerably. Relatively broad molecular weight distributions are generally encountered in cationic polymerizations. Table 8.5 Representative Monomer Reactivity Ratios in Cationic Polymerization ... 

In copolymerization reactions, the monomer reactivity ratio is often referred to in order to discuss how the reaction proceeds. The monomer reactivity ratios and F2i are defined as the ratios of the rate constant of the reaction of a given radical with its own monomer (Afj) to the rate constant of its addition to the other monomer (M2). Thus, rj2 > 1 means that the radicalMj prefers the addition ofMj, whereas r 2 < 1 means that it prefers the addition ofMj. In these copolymerization methods, comonomers satisfying the following requirements are used as the base material r 2 > 1, T2i < 1, and the refractive index oftheMj homopolymer is lower than that of the M2 homopolymer. More details about monomer reactivity ratios are described later in this chapter. 

In this method, nonreactive compounds are employed as the high-refractive-index component [11]. For example, MM A and bromobenzene (BB), which have higher refractive indices than PMMA, can be utilized as the monomer and the nonreactive compound, respectively. The fabrication procedure is the same as in the photo-copolymerization and interfacial-gel polymerization methods. However, the principle of formation the GI profile is different. In contrast to the previous methods that use the difference in the monomer reactivity ratios, in this method the difference in the molecular size is important. Because the molecular size of MM A is smaller than that of BB, MM A more easily diffuses into the gel phase. Thus, BB molecules are concentrated into the middle region to form the GI profile as the polymerization progresses. The mechanism is schematically described in Figure 5.11. 

Pentafluorophenyl sulfone is highly reactive even at room temperature. The reaction is catalyzed by potassium fluoride. It activates the phenol group and acts as a base to absorb the hydrogen fluoride, which is a byproduct of the polycondensation. By adjusting the feed ratio of monomers, the refractive index and crosslinking density of the polymers can be readily controlled. ... 

Monomers which produce polymers with the desired polymer interaction parameter are now selected for possible testing as lens materials. Formulations of those monomers with the appropriate amount of ctosslinker and ctosslinking are identified by estimating modulus of elasticity that a reaction product would possess using a Monte Carlo method developed by Xu and Mark (28). Potential formulations to make the lens are further refined by comparison of reactivity ratios between prospective monomers. The potential for a mixture of monomers to form a desired copolymer is estimated fiom the AUrey-Price, Q-e scheme (29) for radical-monomer reactivity. If the reactivity ratios ofthe monomers in a formulation are close in numerical value, the polymer is projected to be truly random and it may form a functional lens. These formulations are accepted. If the reactivity ratios are very different the monomers will react in block fashion. A block copolymer is prone to be hazy because the blocks in the copolymer can aggregate and create sharp changes in index of refraction in a lens. These formulations are rejected. 

In ordinary batch copolymerization there is usually a considerable drift in monomer composition because of different reactivities of the two monomers (based on the values of the reactivity ratios). This leads to a copolymer with a broad chemical composition distribution (CCD). In many cases (depending on the specific final product application) a composition drift as low as 3-5% cannot be tolerated, for example, copolymers for optical applications on the other hand, during production of GRIN (gradient index) lenses, a controlled traj ectory of copolymer composition is required. This is partly circumvented in semibatch operation where the composition drift can be minimized (i.e., copolymer composition can be kept constant ) by feeding a mixture of the monomers to the reactor with the same rate by which each of them is consumed in the reactor. 

In order to calculate a copolymerization reactivity ratio, it is first necessary to determine the composition of the copolymer or of the unconverted monomer mixture (or both). Elemental analysis, spectroscopic methods (IR, UV, NMR), refractive index determination, or turbidimetric titration can be suitable for determining the copolymer composition. 

Perfluoro-2-methylene-l,3-dioxolane monomers can be copolymerized with each other to modify the physical properties of the polymers. The refractive index and Tg depend on the copolymer composition. The copolymers are readily prepared in solution and in bulk. For example, the copolymerization reactivity ratios of monomers A and C (Figure 4.10) are = 0.97 and - 0.85 [35]. The data show that this copolymerization yields nearly ideal random copolymers. Figure 4.11 shows the change in Tg as a function of the copolymer composition. The copolymers have only one T, which increases from 110 to 165 C as the mole fraction of monomer A increases. The copolymer films prepared by casting are flexible and tough and have a high optical transparency. 

The product of the polymerization constants, riT2, is very frequently used as an index for evaluating the alternating tendency in binary copolymeriz-ation/" In fact, the reciprocal of the product rir2 is often called the alternation tendency index. The ideal (random) copolymerization condition exists for the case / i/ 2 = Where ri and r2 are very low and the rir2 product tends to zero the alternating tendency increases.The product r r2 can be zero in two cases—where one or both reactivity ratios are zero. Where r — 0, the copolymer chain is built of isolated Mi units separated by sequences of M2 monomer units. Strictly alternating copolymer would be obtained when both / i and V2 are zero. The second condition is often found for MA copolymerizations, as described in Chapter 10. Where ri > 1, the polymer is richer in Ml monomers than the monomer feed. For ri < 1, the opposite holds. 

The product of reactivity ratios rjTj can be related to other quantities for which more direa physical evidence is available, e.p. the parameter introduced by Chuj6 (48) to describe deviations from randomness in terms of the number and weight averages of isotactic (or syndiotactic) diads. The analogy with the true copolymerization is remarkable here a similar parameter, the index of sequential homogeneity (49), characterizes the distribution of sequence length of the two monomers. 

A variety of copolymers of vinyl esters have been prepared in solution. For example, vinyl 2-ethylhexanoate and 2-ethylhexyl acrylate have been copolymerized in feed ratios ranging from 1 to 1 to 1 to 3 (on a molar basis). The acrylic monomer was said to have a greater reactivity than the vinyl ester. The product was studied as a possible viscosity index improver [112]. 

The product of this polymerization is applied in specialty areas, such as the manufacture of recording tapes and toners for photocopiers. When styrene (St) and butylmethacrylate (BMA) are mixed in the molar ratio 1 1, the polymer formed is transparent and has a glass transition temperature of about 30°C. Figure 8.3 shows the two monomers. The conversion in this case is measured gravimetrically. The product was dissolved in THF, and a small amount of hydroquinone was added to inhibit further reaetion. The molecular weight distribution was determined by gel permeation ehromato-graphy (GPC) with two mixed columns of polymer laboratories. The detection method used was by reactive index. 

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