Step Reaction Copolymers

The live molecules may randomize by, for example, a transesterification or transamidation 

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Step reaction copolymers tend to be more or less random. Even with different monomer reactivities that lead initially to non-random copolymers, randomization is ultimately helped by interchange reactions that can proceed after the initial polymerization. Figure 3.47 illustrates the interchange of chain segments by transesterification. One molecule has the chain-ends R, and R2, the other R3 and R4. After the reaction, the ends are exchanged. 

If the chains are long, the composition of the copolymer and the arrangement oi units along the chain are determined almost entirely by the relative rates of the various chain propagation reactions. On the other hand, the rate of polymerization depends not only on the rates of these propagation steps but also on the rates of the termination reactions. Copolymer composition has received far more attention than has the rate of copolymerization. The present section will be confined to consideration of the composition of copolymers formed by a free radical mechanism. 

Although the mechanism of copolymerization is similar to that discussed for the polymerization of one reactant (homopolymerization), the reactivities of monomers may differ when more than one is present in the feed, i.e., reaction mixture. Copolymers may be produced by step-reaction or by chain reaction polymerization. It is important to note that if the reactant species are Mi and M2, then the composition of the copolymer is not a physical mixture or blend, though the topic of blends will be dealt with in this chapter. 

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... 

Copolymers may be produced by step reaction or by chain reaction polymerization in similar mechanisms to those of homopolymerization. The most widely used synthetic rubber (SBR) is a copolymer of styrene (S) and butadiene (B). Also, ABS, a widely used plastic, is a copolymer or blend of polymers of acrylonitrile, butadiene, and styrene. A special... 

Copolymers can be made not just from two different monomers but from three, four, or even more. They can be made not only by free-radical chain reactions, but by any of the polymerization methods we shall take up ionic, coordination, or step-reaction. The monomer units may be distributed in various ways, depending on the technique used. As we have seen, they may alternate along a chain, either randomly or with varying degrees of regularity. In block copolymers sections made up of one monomer alternate with sections of another ... 

A/f-copolymers have a unique situation among macromolecular compounds. They have an ordered structure of the type -[A-B-]n, which can be viewed as the structure of a homopolymer. The fact that a/f-copolymers can be formed from two starting monomers is not their unique property, and many homopolymers formed in step reactions have an -[A-B-]n formula. For example. Nylon 66, being formed from adipic acid and 1,6-hexandiamine, can be considered an a/f-copolymer and named poly(hexamethylene-diamine-a/f-adipic acid), or it can have the name poly(hexamethylene adipamide) or poly(iminohexa-methylene iminoadipoyl) and be viewed as a homopolymer with the structure -[NH-(CH2)6-NHC(0)-(CH2)4-C(O)-]n. Many other examples of the same type can be listed. 

In Table 10 we have gathered different 1,2-disubstituted tetraphenylethanes reported in the literature to get telechelic polymers. We can remark that few studies were undertaken in the area of telechelic polymers hence, despite a one-step reaction to get a telechelic structure, the main interest attributed to initer systems concerns the ability to restart a block copolymerization. The number of publications concerning the synthesis of diblock copolymers may prove this assumption. Under certain polymerization conditions, the chain ends, comprising the last monomer unit and the primary radical formed from the intiator, may split up into new radicals able to reinitiate further polymerization of a second monomer, leading to block copolymers. This is certainly the reason why 1,2-disubstituted tetraphenylethane does not present such interesting condensable functions (X in Scheme 10) for polycondensation reactions (Table 10). 

Pumyani and Singh [28] described the synthesis of iodine-containing quaternary amine methacrylate (QAMA) copolymers. The monomers were prepared via a two-step reaction i) the reaction of ethylene glycol dimethacrylate with piperazine in methanol at 35 °C and ii) the quaternisation of the synthesised monomer with 1-iodooctane. The antimicrobial activities of the QAMA-containing copolymers were evaluated against Escherichia coli and Staphylococcus aureus. 

The condensation grafting of polyethylenimine (PEI) onto poly(acrylamide-co-acryhc acid) (PAM-co-AA) was studied by FTIR spectroscopy. The reaction mechanism, which proceeded by a two-step reaction, involved the conversion of AA to acid chloride (AC) using thionyl chloride, followed by the condensation of AC onto PAM and with amine onto PEI to form the graft copolymer (125). 

Copolymers are made to produce unique or functional properties in the polymeric product. The properties of step copolymers can be understood and, in some cases, predicted from an analysis of the chain length and functional groups in the monomers. The composition and composition-dependent properties of a free radical, chain reaction copolymer can be predicted from monomer reactivity ratios, a property first correctly quantified in 1944 (11-14). These ratios have been extensively measured and tabulated (15). They allow, by use of differential equations, the calculation of the monomer content in a copolymer as a function of time during the reaction. Reactivity ratios have also been measured for cationic chain reactions (16). Anionic chain reactions in monomer mixtures are generally so fast and indiscriminate that reactivity ratios are meaningless. 

Introduction. We have, so far, considered ionic propagation, coordination catalysis, and the step reactions of a pol37mer terminus as techniques for the preparation of block copol3nners. Free radical polymerization may also be utilized by application of one of several chemical manipulations. For example, block copolymers may be prepared by coupling macroradicals, or by generating new radicals in the presence of a second monomer by photolytic or mechanical degradation. As an alternate, difunctional initiators may be employed. 

It has been observed that in carbocationic copolymerization the molecular weight of the resulting copolymer is always lower than that of the individual homopolymers prepared imder identical conditions (179). This was explained by the high tr/ p ratio of the favored cross-propagation step (reaction of the most reactive chain end with the monomer giving the most stable carbenium ion), compared to the respective homopolymerization tr/ p ratios (173). 

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