Chain copolymerization

As for step copolymerization, differences in monomer reactivity in chain copolymerization affect the sequence distribution of the different repeat units in the copolymer molecules formed. The most reactive monomer again is incorporated preferentially into the copolymer chains but, because of the different nature of chain polymerization, high molar mass copolymer molecules are formed early in the reaction. Thus, at low overall conversions of the comonomers, the high molar mass copolymer molecules formed can have compositions which differ significantly from the composition of the initial comonomer mixture. Also in contrast to step copolymerization, theoretical prediction of the relative rates at which the different monomers add to a growing chain is more firmly established. In the next section a general theoretical treatment of chain copolymerization of two monomers is presented and introduces an approach which can be applied to derive equations for more complex chain copolymerizations involving three or more monomers. 

In order to predict the composition of the copolymer formed at a particular instant in time during a chain copolymerization, it is necessary to construct a kinetics model of the reaction. The simplest model will be analysed here and is the terminal model which assumes that the reactivity of an active centre depends only upon the terminal monomer unit on which it is located. It is further assumed that the amount of monomer consumed in reactions other than propagation is negligible and that copolymer molecules of high molar mass are formed. Thus for copolymerization of monomer A with monomer B, only two types of active centre need be considered 

The reactions with rate constants /taa and A bb are known as homopropagation reactions and those with rate constants A ab and / ba are called cross-propagation reactions. Hence the rate of consumption of monomer A is given by 

At any instant in time during the reaction, the ratio of the amount of monomer A to monomer B being incorporated into the copolymer chains is obtained by dividing Equation (2.82) into Equation (2.81) 

In terms of the creation and loss of active centres of a particular type, the contribution of initiation and termination reactions is negligible compared to that of the cross-propagation reactions. Thus 

We have considered so far free-radical polymerizations where only one monomer is used and the product is a homopdlymer. The same type of polymerization can also be carried out with a mixture of two or more monomers to produce a polymer product that contains two or more different mer units in the same polymer chain. The polymerization is then termed a copolymerization and the product is termed a copolymer. Monomers taking part in copolymerization are referred to as comonomers. The simultaneous polymerization of two monomers is known as binary copolymerization and that of three monomers as ternary copolymerization, and so on. The term multicomponent copolymerization embraces all such cases. The relative proportions of the different mer units in the copolymer chain depend on the relative concentrations of the comonomers in the feed mixture and on their relative reactivities. This will be the main subject of our discussion in this chapter. 

It should be noted that the chain copolymerization may be initiated by any of the chain initiation mechanisms, namely, free-radical chain initiations considered in the preceding chapter, or ionic chain initiations, which will be described in a later chapter. While polymerization of a single monomer is relatively hmited as regards the number of different products that are possible, copolymerization enables the polymer engineer to synthesize an almost unlimited number of products with different properties by variations in the nature and relative amounts of the two monomers in the feed mixture and to tailormake polymers with specific properties. Copolymerization is thus very important from a technological viewpoint. 

The classification of copolymers according to structural types and the nomenclature for copolymers have been described previously in Chapter 1. The present chapter is primarily concerned with the simultaneous polymerization of two mono- 

In the preceding chapter we have considered free-radical polymerizations where only one monomer is used to produce a homopolymer. However, chain polymerizations can be carried out with mixtures of two or more monomers to form polymeric products that contain two or more different structures in the polymer chain. This type of chain polymerization process in which two or more monomers are simultaneously polymerized is termed a copofymerization and the product is a copolymer. It is important to note that the copolymer is not an alloy of two or more homopolymers but contains units of all the different monomers incorporated into each copolymer molecule. The process can be depicted, for copolymerization of two monomers, as 

Chain copolymerization is important both from academic and technological viewpoints. Thus much of our knowledge of the reactivities of monomers, free radicals, carbocations, and carboanions in chain polymerization comes from copolymerization studies. The behavior of monomers in copolymerization reactions is especially useful for studying the relation between chemical structure and reactivity of monomers. From the technological viewpoint. 

The classi cation of copolymers according to structural types and the nomenclature for copolymers have been described previously in Chapter 1. The present chapter is primarily concerned with the simultaneous polymerization of two monomers by free-radical mechanism to produce random, statistical, and alternating eopolymers. Copolymers having completely random distribution of the different monomer units along the copolymer chain are referred to as random copolymers. Statistical copolymers are those in which the distribution of the two monomers in the chain is essentially random but in uenced by the individual monomer reactivities. The other types of copolymers, namely, graft and block copolymers, are not synthesized by the simultaneous polymerization of two monomers. These are generally obtained by other types of reactions (see Section 7.6). 

When more than one monomer is polymerized at the same time, a variety of structures can result. In a step-growth polymerization with more than one monomer but the same pair of functional groups, the reaction rate can be assumed to be the same for each monomer. Thus, they react in a statistical manner that leads to a random distribution of monomers in the final polymer. 

In the simple case of two monomers A and B in a chain polymerization, the rate of polymerization of each monomer is rarely the same. One can derive relationships that will help answer some questions about the degree of heterogeneity of the products made under various conditions  

What is the composition of a polymer made from the limited conversion of a mixture of two monomers  

How will two monomers that never have copolymerized before interact  

The first is answered by introducing relative reactivities. The second is approached by the Alfrey-Price Q-e scheme. To express the relationship of polymer composition to the monomer composition from which it is being formed, we go back to the kinetic scheme of Section 4.4. If we can assume that we achieve a steady-state population of chain radicals that grow to high molecular weight, the material balances for various species are few and simple for a binary system. 

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Copolymerization involves the reaction of at least two different monomers A and B. In the case of chain copolymerization, the reactivity ratios and are important, aiid rg = / bb BA di re /cy die... 

For polymerization reactors, the main concern is the characteristics of the product that relate to the mechanical properties. The distribution of molar masses in the polymer product, orientation of groups along the chain, cross-linking of the polymer chains, copolymerization with a mixture of monomers, and so on, are the main considerations. Ultimately, the main concern is the mechanical properties of the polymer product. 

The most common and most versatile technique for the production of synthetic hydrogels is the free radical (chain) copolymerization of monofunctional and multifunctional vinyl monomers. Gels can also be created by the step (e.g., con-... 

A similar mechanism of chain oxidation of olefinic hydrocarbons was observed experimentally by Bolland and Gee [53] in 1946 after a detailed study of the kinetics of the oxidation of nonsaturated compounds. Miller and Mayo [54] studied the oxidation of styrene and found that this reaction is in essence the chain copolymerization of styrene and dioxygen with production of polymeric peroxide. Rust [55] observed dihydroperoxide formation in his study of the oxidation of branched aliphatic hydrocarbons and treated this fact as the result of intramolecular isomerization of peroxyl radicals. 

As in the case of the oxidation of saturated esters, the rate of chain copolymerization monomer and dioxygen obeys the equation similar to that for aliphatic ester oxidation. 

The PMMA-Phe synthesis, characterization, film preparation, apparatus and experimental scheme are described elsewhere (H) Briefly, the PMMA chains, copolymerized from MMA and Phe-labelled monomers, were characterized via gel permeation chromatography (GPC) M = 411,000, M = 197,000 and M M - 2.08. UV-absoifption measurementsnindicated that ca. I % of all monomer units were Phe-labelled. The sample was dissolved in toluene and was spin-coated onto 1-inch diameter quartz disks. Then, the films (ca. 1 /zm thick) were annealed at 160 C for 60 minutes under vacuum. 

Figure 11. Scheme for crosslinking chain copolymerization x free radical site, a inter-and a intramolecular reaction. 

Typical Free Radical Chain Copolymerization Reactivity Ratios at 60°C... 

It is highly unlikely that the reactivities of the various monomers would be such as to yield either block or alternating copolymes. The quantitative dependence of copolymer composition on monomer reactivities has been described [Korshak et al., 1976 Mackey et al., 1978 Russell et al., 1981]. The treatment is the same as that described in Chap. 6 for chain copolymerization (Secs. 6-2 and 6-5). The overall composition of the copolymer obtained in a step polymerization will almost always be the same as the composition of the monomer mixture since these reactions are carried out to essentially 100% conversion (a necessity for obtaining high-molecular-weight polymer). Further, for step copolymerizations of monomer mixtures such as in Eq. 2-192 one often observes the formation of random copolymers. This occurs either because there are no differences in the reactivities of the various monomers or the polymerization proceeds under reaction conditions where there is extensive interchange (Sec. 2-7c). The use of only one diacid or one diamine would produce a variation on the copolymer structure with either R = R" or R = R " [Jackson and Morris, 1988]. 

Copolymerization is also important in step polymerization. Relatively few studies on step copolymerization have been carried out, although there are considerable commercial applications. Unlike the situation in chain copolymerization, the overall composition of the copolymer obtained in a step copolymerization is usually the same as the feed composition since step reactions must be carried out to close to 100% conversion for the synthesis of... 

The copolymerization equation has been experimentally verified in innumerable comonomer systems. The copolymerization equation is equally applicable to radical, cationic, and anionic chain copolymerizations, although the r and r2 values for any particular comonomer pair can be drastically different depending on the mode of initiation. 

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