Random-alternating copolymerization

A special situation for 0 rir2 1 relates ton 1 andrz 1 or vice versa. In this case, the product composition will tend toward that of the homopolymer of the more reactive monomer and copolymerization cannot occur. For example, when ri 1 and r2 1, both M and M will preferentially add monomer Mi till this monomer is consumed, which will then be followed by homopolymerization of M2. This is therefore a case of consecutive homopolymerizations (Odian, 1991). 

M2 preferentially add monomer Mi till the monomer is consumed, which is then followed by homopolymerization of M2- This is therefore a case of consecutive horaopolymerizations. 

A special situation for 0 riZ2 1 relates to ri 1 and Z2 1 or vice versa. In this case, the product composition will tend toward that of the homopolymer of the more reactive monomer 

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It is apparent from items (l)-(3) above that linear copolymers-even those with the same proportions of different kinds of repeat units-can be very different in structure and properties. In classifying a copolymer as random, alternating, or block, it should be realized that we are describing the average character of the molecule accidental variations from the basic patterns may be present. In Chap. 7 we shall see how an experimental investigation of the sequence of repeat units in a copolymer is a valuable tool for understanding copolymerization reactions. This type of information along with other details of structure are collectively known as the microstructure of a polymer. 

We have already seen that, depending on the values of the reactivity ratios, there is a tendency to get random, alternating, blocky, etc., types of copolymers. Probability theory allows us to quantify this in terms of the frequency of occurrence of various sequences, like the triads AAA or ABA in a copolymerization of A and B monomers. The value of this information is that such sequence distributions can be measured directly by NMR spectroscopy, thus allowing a direct probe of copolymer structure and an alternative method for measuring reactivity ratios. As mentioned above, there are problems, as some spectra can be too complex and rich for easy analysis, as we will see in Chapter 7. 

Alternating copolymerization is dependent upon the presence of a charge transfer complex which serves as the polymerizable species (16,17). Since the stability of the chain transfer complex decreases as the temperature is increased, the tendency for alteration decreases at elevated temperatures. Thus, random copolymers are produced when the polymerization occurs at temperatures above the threshold limit for the existence of the charge transfer complex (18). 

The observed monomer reactivity ratios of different monomer pairs vary widely but can be divided into a rather small number of classes. A useful classification (Rudin, 1982 Odian, 1991) is based on the product of ri and T2, such as rir2 0 (with n 1, T2 1), rir2 1,0 < r r2 < 1, and > 1 (with n > 1, T2 > 1), representing, respectively, alternating, random (or ideal), random-alternating, and block copolymerizations. 

Random, graft, and alternating copolymerization Monomer polymer composition—copolymers Monomer polymer composition—terpolymers Monomer polymer composition—multi-component terpolymers Free radical concentration Reactivity ratios... 

How can the theory of heat of copolymerization for random copolymers be extended to alternating copolymerization ... 

In the international nomenclature, -co-, -alt-, -b-, -g- are often inserted between two monomers to represent random copolymerization, alternating copolymerization, block copolymerization, and graft copolymerization, respectively. In random copolymer names, the former is the main monomer, and the latter is the secondary monomer. In block copolymer names, the order of monomers represents the order of polymerization, whereas in graft copolymer names, the former is the main chain and the latter is the branched chain. 

Radiation-initiated copolymerizations, either bulk or in solution, exhibit no induction period for the a-methylstyrene-MA monomer pair. Standard free-radical inhibitors prevent or stopped the copolymerizations. Apparent activation energies of 6.7 and 5.4kcal/mol were obtained for the bulk (3.6 mol % MA in a-methylstyrene) and (8.0 wt. % chloroform solution of equimolar amounts of the monomer pair) copolymerizations, respectively. Composition and IR spectra studies indicated that homopolymerization of either of the two monomers did not occur, under conditions explored, and that alternating copolymerization occurred over a wide range of monomer concentration of one component. As pressures are raised from 1 bar to 3 bars, considerably enhanced rates of copolymerization are observed. Above 80 C random copolymer is obtained. 

As shown in the table in the appendix to this chapter, the stability of a CTC decreases as polymerization temperatures are increased. Recognizing this, Seymour and coworkers " showed that for the vinyl acetate-MA, styrene-MA, and a-methylstyrene-MA pairs the charge-transfer complexes were nonexistent above 90, 130, and 80°C, respectively. Above these threshold temperatures for the three charge-transfer complexes only random copolymers are produced. Fritze confirmed this discovery for the styrene-MA pair. Seymour and Garner recently authored a review on thermal aspects of alternating copolymerization, covering this subject. 

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