Types of Copolymerization

The observed monomer reactivity ratios of different monomer pairs vary widely but can be divided into a rather small number of classes. A useful class cation (Rudin, 1982 Odian, 1991) is based on the product of r and T2, such as rir2 0 (with ri 1, f2 1), rir2 - 1, 0 rir2 1, and rir2 1 (with ri , T2 1), representing, respectively, alternating, random (or ideal), random-alternating, and block copolymeiizations. 

Before measurements of reactivity ratios are reviewed, it is useful to consider what the absolute magnitudes of these parameters imply. Depending on the values of ri and V2, and of the riV2 product four types of copolymerizations can be recognized. 

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During the investigation of the principles governing the process of copolymerization of AN with ISP in DMSO at 30 °C in the presence of ammonium persulfate, it was established that the anisotropic type of copolymerization is characteristic for this pair of monomers. The azeotropic point, as it is seen from Fig. 1 corresponds to a content of 60% of monomeric units of ISP in the monomer mixture. 

Butadiene-styrene copolymerization was attempted using the L3Ln-RX-A1R3 system [109], Especially, (CF3COO)3Nd/C5H11Br/AlisoBu3 (1 3 15) was found to be active in this type of copolymerization, with the ds-content of... 

Different types of copolymerization behavior are observed depending on the values of the monomer reactivity ratios. Copolymerizations can be classified into three types based on whether the product of the two monomer reactivity ratios r rx is unity, less than unity, or greater than unity. 

The first case is the copolymerization of monomer A with diene BB where all the double bonds (i.e., the A double bond and both B double bonds) have the same reactivity. Methyl methacrylate-ethylene glycol dimethacrylate (EGDM), vinyl acetate-divinyl adipate (DVA), and styrene-p- or m-divinylbenzene (DVB) are examples of this type of copolymerization system [Landin and Macosko, 1988 Li et al., 1989 Storey, 1965 Ulbrich et al., 1977]. Since r = Yi, Fi = f and the extent of reaction p of A double bonds equals that of B double bonds. There are p[A] reacted A double bonds, p[B] reacted B double bonds, and p2[BB] reacted BB monomer units. [A] and [B] are the concentrations of A and B double bonds,... 

The interesting ability of 5,6-dihydro-4//-l, 3-oxazine to take part in a new type of copolymerization, without catalyst, involves the combination of a cationic and an anionic monomer.250 The formation of the... 

In addition to a-olefins, p-/-butylstyrene can be used as a comonomer for the isotactic specific copolymerization with carbon monoxide with chiral induction [32], Whereas bipyridyl as ligand gives a syndiotactic polymer, the Pd-catalyzed polymerization with ligand 11 leads to a polymer with isotacticity of over 98% and high optical activity ([ ]D -536° (in CH2C12)) [33, 34]. The BINAPHOS catalyst described above is also effective in this type of copolymerization [25],... 

Cyclopolymerization of bifunctional monomers is an effective method of chirality induction. Optically active vinyl homopolymers and copolymers have been synthesized by using optically active distyrenic monomers (41) based on a readily removable chiral template moiety. Free-radical copolymerization of 41a with styrene and removal of the chiral template moiety from the obtained copolymer led to polystyrene, which showed optical activity ([Oc]365 -0.5-3.5°) (Scheme 11.6) [84], The optical activity was explained in terms of chiral (S,S)-diad units generated in the polymer chain through cyclopolymerization of 41a [85], Several different bifunctional monomers have been synthesized and used for this type of copolymerization [86-90]. 

In our opinion, the area of this type of copolymerization is still open to discussion and research, and this suggestion concerns mainly the mechanism of copolymerization, formation of active centres in the initiation by Lewis bases, the influence of proton donors on the course of copolymerization, and tbe effect of the structure of the initiator on the rate of copolymerization. It is necessary, however, to study the copolymerization on model compounds. 

According to Eq. (25), a cyclic phosphite monomer (MN) 38 is oxidized to a phosphate unit yielding copolymer 40 whereas the a-keto acid monomer (ME) 39 is reduced to the corresponding a-hydroxy acid ester. Thus, the term redox copolymerization has been proposed to designate this type of copolymerization in which one monomer is reduced and the other monomer oxidized. The redox copolymerization clearly differs from the so-called redox polymerization in classical polymer chemistry where the redox reaction between the two catalyst components (oxidant and reductant) is responsible for the production of free radicals. 

Now it s your turn Having taken you through one example in detail, we now leave it to you to do three more, each chosen because they illustrate a particular type of copolymerization. Using the reactivity ratios and initial molar concentrations given... 

The miscibility of PMI-containing polymers with SAN has been studied by different researchers and is of particular interest to understand the possibilities of blending approaches to increase the heat resistance of ABS. There is a surprisingly high degree of miscibility between styrene-co-A-phenylmaleimide (SPMI) and SAN copolymers, that increases as the PMI volume fraction increases [41]. Various types of copolymerizations of PMI and styrene are described in the literature [42,43] and various grades of SPMI copolymers are commercially available and used to increase the heat resistance of ABS [44,45]. 

Inspection of the copolymer equation shows the importance of the reactivity ratios in determining the type of copolymerization reaction that will occur. Thus, we consider the following possibilities. 

The relative amounts of the two monomer units along the copolymer chain are thus determined by the relative concentrations of the monomer units in the feed and the relative reactivities of the two monomers. Thus a very important practical consequence of ideal copolymerizations is that it becomes progressively more difficult to produce copolymers containing appreciable amounts of both monomers as the difference in reactivities of the two monomers increases. It should be noted that the term ideal copofymerization does not in any sense connote a desirable type of copolymerization. 

Equation (7.18) may be used to calculate the instantaneous composition of copolymer as a function of feed composition for various monomer reactivity ratios. A series of such curves are shown in Fig. 7.1 for ideal copolymerization, i.e., r r2 = 1. The term ideal copolymerization is used to show the analogy between the curves in Fig. 7.1 and Aose for vapor-liquid equilibria in ideal liquid mixtures. The vapor-liquid composition curves of ideal binary mixtures have no inflection points and neither do the polymer-composition curves for random copolymerization in which riV2 = 1. Such monomer systems are therefore called ideal. It does not in any sense imply an ideal type of copolymerization. 

Problem 7.11 Predict the type of copolymerization behavior that would be expected for the following monomer pairs ... 

In a series of papers58,66,675, it was established that this process represents a special type of copolymerization in which a statistic copolymer is formed in spite of the fact that formaldehyde and dioxolane are polymerized on different types of active centers. 

Recently two spedal types of copolymerization have been investigated intensely and have shown promise for use in large-scale industrial production. They are called block polymerization and graft polymerization, respectively. In block polymerization, sequences of one type of monomer are prepolymerized and then joined to the prepolymerized sequences of another monomer ... 

If both ii and i2 are greater than unity or if both are less than unity, then (fi) lies within the acceptable range 0 < fj < 1. If rj > 1 and rj < 1 or rj < 1 and rj > 1, then there will be no critical feed composition, (fi)c. As we said earlier, cases where both r, and rj are greater than unity are not known whereas, there are numerous cases where r, and rj are less than unity. When r, 1 and rj 1 or r, 1 and rj l, the two monomers have a tendency to polymerize consecutively the first polymer is composed predominantly of the more reactive monomer the other monomer polymerizes only after almost all of the more reactive monomer has been exhausted. The case of styrene-vinyl acetate discussed above exemplifies this type of copolymerization. 

In many cases, all that is required in the first instance is a feeling for the type of copolymerization behaviour to be expected. If values of r and r for a particular copolymerization are not available, then the simple Q-e scheme developed by Alfrey and Price [12] is useful for estimating their values. The basis of the scheme is that the rate coefficient, fcpm, for reaction of a polymeric radical, p, with a monomer, m, is given by... 

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