Copolymerization systems

The reactivity ratios of a copolymerization system are the fundamental parameters in terms of which the system is described. Since the copolymer composition equation relates the compositions of the product and the feedstock, it is clear that values of r can be evaluated from experimental data in which the corresponding compositions are measured. We shall consider this evaluation procedure in Sec. 7.7, where it will be found that this approach is not as free of ambiguity as might be desired. For now we shall simply assume that we know the desired r values for a system in fact, extensive tabulations of such values exist. An especially convenient source of this information is the Polymer Handbook (Ref. 4). Table 7.1 lists some typical r values at 60°C. 

Several important assumptions are involved in the derivation of the Mayo-Lewis equation and care must be taken when it is applied to ionic copolymerization systems. In ring-opening polymerizations, depolymerization and equilibration of the heterochain copolymers may become important in some cases. In such cases, the copolymer composition is no longer determined by die four propagation reactions. 

Polymerization equilibria frequently observed in the polymerization of cyclic monomers may become important in copolymerization systems. The four propagation reactions assumed to be irreversible in the derivation of the Mayo-Lewis equation must be modified to include reversible processes. Lowry114,11S first derived a copolymer composition equation for the case in which some of the propagation reactions are reversible and it was applied to ring-opening copalymerization systems1 16, m. In the case of equilibrium copolymerization with complete reversibility, the following reactions must be considered. 

In the copolymerization of five- and six-membered oxacyclic monomers, the effective monomer concentration in the propagation reaction decreases because only the monomer in excess of equilibrium is available for copolymerization. However, it is not easy to determine the equilibrium monomer concentration in a copolymerization system. The following equilibrium is expected to exist in the copolymerization of THF. 

In this section of our review, we shall discuss the morphological aspects and structure-property relationships of a few specific copolymeric systems which we think will represent the general features of siloxane containing multiphase copolymers. More detailed discussions about the properties of each copolymer system may be found in the references cited during our review of the copolymer preparation methods. On the other hand an in-depth discussion of the interesting surface morphology and the resultant surface properties of the siloxane containing copolymers and blends will be provided. 

Other reports on the morphology and mechanical behavior of organosiloxane containing copolymeric systems include polyurethanes 201 202), aliphatic 185, 86) and aromatic117,195> polyesters, polycarbonates 233 236>, polyhydroxyethers69,311, siloxane zwitterionomers 294 295) and epoxy networks 115>. All of these systems display two phase morphologies and composition dependent mechanical properties, as expected. 

Surface composition and morphology of copolymeric systems and blends are usually studied by contact angle (wettability) and surface tension measurements and more recently by x-ray photoelectron spectroscopy (XPS or ESCA). Other techniques that are also used include surface sensitive FT-IR (e.g., Attenuated Total Reflectance, ATR, and Diffuse Reflectance, DR) and EDAX. Due to the nature of each of these techniques, they provide information on varying surface thicknesses, ranging from 5 to 50 A (contact angle and ESCA) to 20,000-30,000 A (ATR-IR and EDAX). Therefore, they can be used together to complement each other in studying the depth profiles of polymer surfaces. 

Comparisons with experimental results 147) show that the reliability of this rule increases as the more the copolymerization system increasingly depends on the solvent. This is remarkable because the following crude approximations were used ... 

Indeed, cumyl carbocations are known to be effective initiators of IB polymerization, while the p-substituted benzyl cation is expected to react effectively with IB (p-methylstyrene and IB form a nearly ideal copolymerization system ). Severe disparity between the reactivities of the vinyl and cumyl ether groups of the inimer would result in either linear polymers or branched polymers with much lower MW than predicted for an in/mcr-mediated living polymerization. Styrene was subsequently blocked from the tert-chloride chain ends of high-MW DIB, activated by excess TiCU (Scheme 7.2). 

By virtue of the conditions xi+X2 = 1>Xi+X2 = 1, only one of two equations (Eq. 98) (e.g. the first one) is independent. Analytical integration of this equation results in explicit expression connecting monomer composition jc with conversion p. This expression in conjunction with formula (Eq. 99) describes the dependence of the instantaneous copolymer composition X on conversion. The analysis of the results achieved revealed [74] that the mode of the drift with conversion of compositions x and X differs from that occurring in the processes of homophase copolymerization. It was found that at any values of parameters p, p2 and initial monomer composition x° both vectors, x and X, will tend with the growth of p to common limit x = X. In traditional copolymerization, systems also exist in which the instantaneous composition of a copolymer coincides with that of the monomer mixture. Such a composition, x =X, is known as the azeotrop . Its values, controlled by parameters of the model, are defined for homophase (a) [1,86] and interphase (b) copolymerization as follows... 

Equation 6-34 or its equivalent has been used to correlate the drift in the feed and copolymer compositions with conversion for a number of different copolymerization systems [Capek et al., 1983 O Driscoll et al., 1984 Stejskal et al., 1986 Teramachi et al., 1985], The larger the difference in the t and r2 values of a comonomer pair, the greater is the variation in copolymer composition with conversion [Dadmun, 2001]. 

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,... 

General mechanism of template radical copolymerization was described in Chapter 2. From general consideration, it is clear that two monomers in copolymerizing system can interact with the template in a different manner. Generally, we have two groups in the systems ... 

To obtain the concentration of crosslinked units in the copolymerizing system one may proceed as follows (124,45). Denoting by Xs, mx and m2 the number of moles of doubly reacted divinyl molecules, reacted monovinyl molecules, and divinyl molecules reacted with one vinyl respectively, the crosslinking density can be expressed as... 

From the preceding sections it is clear that isomorphism of monomeric units in synthetic copolymeric systems is a quite general phenomenon. We wish to recall here that the requirements for the isomorphous substitution in the macromolecular field are similar to those holding for solid solutions in ionic or metallic crystals however, the degree of... 

The behavior of copolymerizing systems can be deduced from the r values. For example, when Mx = styrene and M2 = vinyl acetate, r1 = 50 and r2 is less than 0.02 156 then any chain, whether it ends in — or —M2-, will... 

In a copolymerization system in which the product rxrz would exceed unity, the copolymer would contain sequences of like units in greater abundance than in a random copolymer of the same composition, and this tendency should be greater the larger the product rxrz (79). However to our knowledge no example of this case is known therefore the synthesis of copolymers with long uninterrupted stretches of a same monomer must be carried out by other methods than by direct copolymerization. In feet such copolymers have been synthesized their properties are more similar to those of a mechanical mixture of both homopolymers and differ markedly from those of a random copolymer. 

If the initiator is consumed rapidly and irreversibly in the first stages of copolymerization, the anions formed, irrespective of whether alkoxide or carboxylate, are the same for the given copolymerization system. Taking into account the dissociation effect of the initiator and the growing chain, the copolymerizing... 

In contrast to the copolymerization of cyclic carbonates, the molecular weights are lower in the epoxide copolymerization 6,41 43). We assume that this is due to the presence of proton donors in the reaction mixture. They occur as impurities in epoxides 19,20) or anhydrides, moisture in all components of the copolymerization system, or their presence is a consequence of the high rate of hydrolysis of cyclic anhydrides 21). Proton donors added to the monomer feed remarkably decrease the molecular weight42 even in the copolymerization of ethylene glycol carbonate at 200 °C. Under these conditions, when recyclization of phthalic acid takes place 64) and the released C02 can tear off moisture to the gas phase, the molecular weight Mv decreases without proton donors from 45200 to 7100 in the presence of 5% phthalic acid or ethylene glycol or to 9300 in the presence of 15% water42,54. ... 

No clear evidence was obtained about the alteration of the main reaction locus for the copolymerization of St with MA. This would be attributed to the difference in the partition coefficient between MA and AA (see above) and the difference in the number of latex particles formed in their copolymerization systems with St. (The number of particles in St-AA copolymer latex was about a third of that in St-AA latex. The small number for St-MA system is considered to result from the lower particle-stabilizing ability of MA due to its lower hydrophilicity.) These factors would alter the balance of the polymerization in two reaction loci, that is, the aqueous phase and the particles, and consequently serve to change the reaction mode. 

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