Step copolymerization

Copolymers are formed by simultaneous polymerization of two or more different monomers. Thus the simplest step copolymerizations are of the general type ARB + AR B or RA2 + R B2 + R B2. For example, reaction of hexamethylene diamine with a mixture of adipic and sebacic acids yields a copolyamide containing both nylon 6.6 and nylon 6.10 repeat units. 

In the following two sections some of the more simple aspects and uses of linear step copolymerization will be described. The principles which will be introduced are equally applicable to non-linear step copolymerization which, therefore, will not be considered separately. 

Most step copolymerizations are taken to high extents of reaction in order to produce copolymers with suitably high molar masses (Sections 2.2.4 and 2.2.5). A consequence of this is that the overall compositions of the copolymers obtained correspond to those of the comonomer mixtures used to prepare them. However, it must be borne in mind that the sequence distribution of the different repeat units along the copolymer chains is an important factor controlling the properties of a copolymer and that the distribution is affected by differences in monomer reactivity. 

When the mutually-reactive functional groups (i.e. A and B) have reactivities which are the same for each monomer, the probability of reaction of a particular monomer depends only upon the mole fraction of functional groups that it provides. Most commonly this situation obtains when the different monomers containing the same functional groups are of similar structure, as for adipic acid and sebacic acid in the example given above. Under these conditions random copolymers which have the most probable distribution of molar mass are formed and the reaction kinetics described in Section 2.2.6 apply. 

By using as comonomers low molar mass prepolymers with terminal functional groups, step copolymerization can be used to prepare alternating block copolymers. For example, the ester interchange reaction of dimethylterephthalate, CH3OOC— o)—COOCH3, with poly(oxy- 

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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... 

Most of the methods for synthesizing block copolymers were described previously. Block copolymers are obtained by step copolymerization of polymers with functional end groups capable of reacting with each other (Sec. 2-13c-2). Sequential polymerization methods by living radical, anionic, cationic, and group transfer propagation were described in Secs. 3-15b-4, 5-4a, and 7-12e. The use of telechelic polymers, coupling and transformations reactions were described in Secs. 5-4b, 5-4c, and 5-4d. A few methods not previously described are considered here. 

The structure of precursors, the number of functional groups per precursor molecule, and the reaction path leading to the final network all play important roles in the final structure of the polymer network. Some thermosets can be considered homogeneous ideal networks relative to a reference state. It is usually the case when networks are prepared by step copolymerization of two monomers (epoxy-diamine or triol-diisocyanate reactions) at the stoichiometric ratio and at full conversion. 

This syndiotactic model resembles the isotactic one in that it also is based on an octahedral complex that has an alkyl ligand as the growth site, and a vacant octahedral position through which propylene is complexed. In this homogeneous model for polymerization, methyl-methyl interactions force the propylene to be complexed in opposite configuration at each consecutive growth step. Copolymerization studies (85) with soluble syndiotactic-specific catalysts support this view. 

In this method, the ion exchange membrane or precursor film can be prepared from vinyl monomers in one step (copolymerization) and the fact that the obtained membrane has excellent and stable electrochemical properties is attractive. 

So far, our discussion has been restricted to chain block and graft copolymerization. This is largely because the practical utility of copolymerization is more elaborate in chain polymerization than step polymerization. Also, in step copolymerization, block copolymers are generally preferred to the other types of copolymers. Therefore only block step-polymerization copolymers are discussed here and only in a very limited scope to illustrate the principles involved in their preparation. 

Monomers and one of the oligomer species generated during the DGEBA-EDA step copolymerization... 

A second aspect is that some adhesives/sealants cure by the diffusion of water vapour. This causes a sequence of chemical reactions, which leads to chain extension and possibly cross-linking by step copolymerization. Examples are some silicone and isocyanate materials, where the depth of cure is often proportional to the square root of RH, and to the square root of time" (Moisture cure of adhesives, Silicone adhesion). 

Co- and graft polymerization with vinyl and acryl monomers in the form of a two-step copolymerization process [146]. 

This chapter presents a modified two-step copolymerization route by employing urethane chains made of short polyols and 4,4 -diphenylmethane diisocyanate (MDI) as soft segments, which is anticipated to expand the flexibility for molecular design of T -SMPUs. First, the PPG400 diols were used to alternatively copolymerize with MDI to produce N-C-O group terminated urethane prepolymers. Subsequently the prepolymers reacted with extra MDI and chain extenders to produce the T -SMPUs. The Tone MDI between PPG400 diols are considered miscible with the polyols (Garrett et al., 2000). 

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