Random copolymers structures

It is apparent from items (l)-(3) above that linear copolymers-even those with the same proportions of different kinds of repeat units-can be very different in structure and properties. In classifying a copolymer as random, alternating, or block, it should be realized that we are describing the average character of the molecule accidental variations from the basic patterns may be present. In Chap. 7 we shall see how an experimental investigation of the sequence of repeat units in a copolymer is a valuable tool for understanding copolymerization reactions. This type of information along with other details of structure are collectively known as the microstructure of a polymer. 

Figure 4. Contrasting synthesis of homopolymers and copolymers showing possible copolymer structures which are randomly distributed along the polymer backbone.

Fig. 5.5 (A) Alginate block copolymer structure with random sequences (B) divalent cations induced gelation of alginate (formation of egg-box structure).

The product of the reactivity ratios can be used to estimate the copolymer structure. When the product of the reactivity ratios is near 1, the copolymer arrangement is random when the product is near zero, the arrangement is alternating when one of the reactivity ratios is large, blocks corresponding to that monomer addition will occur. 

Draw representative structures for (a) homopolymers, (b) alternation copolymers, (c) random copolymers, (d) AB block copolymers, and (e) graft copolymers of styrene and acrylonitrile. 

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]. 

The copolymer described by Eq. 6-1, referred to as a statistical copolymer, has a distribution of the two monomer units along the copolymer chain that follows some statistical law, for example, Bemoullian (zero-order Markov) or first- or second-order Markov. Copolymers formed via Bemoullian processes have the two monomer units distributed randomly and are referred to as random copolymers. The reader is cautioned that the distinction between the terms statistical and random, recommended by IUPAC [IUPAC, 1991, in press], has often not been followed in the literature. Most references use the term random copolymer independent of the type of statistical process involved in synthesizing the copolymer. There are three other types of copolymer structures—alternating, block, and graft. The alternating copolymer contains the two monomer units in equimolar amounts in a regular alternating distribution ... 

Copolymerization Initiators. The copolyineri/ution of styrene and dienes in hydrocarbon solution wilh alkyllithium initiators produces a tapered block copolymer structure because of the large differences in monomer reactivity ratios lor styrene (r, < 0.11 and dienes (rj > 10). In urder to obtain random copolymers of styrene and dienes, it is necessary to either add small amounts of a Lewis base such us tetrahydrofuran or an alkali metal alkoxide (MtOR. where Ml = Na, K. Rb, or Cs>. 

Polymeric polypeptides have long been the focus of many research groups and are, therefore, covered in Section 14.2. Early on, these polymers were considered to be protein model structures. Discussions of homopolypeptides prepared by a-amino acid TV-carboxy-anhydride (NCA) polymerizations are an important part of the volume (Sections 14.2,14.2.1, and 14.2.1.1). 19,20 Copolypeptides are prepared by routes which lead to random or sequential copolymer structures (Sections 14.2.2 and 14.2.3).[21,22 ... 

Other copolymers that may show micellization similarto that of BAB triblock copolymers include random copolymers if the hydrophobic units are located near both ends of the polymeric structure (Schild and Tirrell, 1991 Ringsdorf et al., 1992 Chung et al., 1999 Jones and Leroux, 1999), and graft copolymers if the hydrophobic blocks are grafted near both ends of a hydrophilic backbone. 

Aiba S-i (1991) Studies on chitosan 3. Evidence for the presence of random and block copolymer structures in partially N-acetylated chitosans. Int J Biol Macromol 13(l) 40-44... 

By introducing DXO units into the backbone of PLA, the Tg of the lactides may be lowered to or below body temperature, and the crystallinity may be lowered. Thus, more flexible materials are obtained where the properties and degradation rate can be varied over a broad range by means of the composition. This is interesting for biomedical applications. Blends of PLA and PDXO provide additional opportunities to vary the properties. There is a large difference in reactivity ratio of LA and DXO [154]. As a result, a more block-like copolymer structure than expected for a totally random copolymer is obtained, even though the structure is somewhat randomized by transesterification reactions. In contrast, DXO and e-CL form a truly random copolymer [156]. Tri-block copolymers of LA and DXO have recently been described [303]. 

We have already seen that, depending on the values of the reactivity ratios, there is a tendency to get random, alternating, blocky, etc., types of copolymers. Probability theory allows us to quantify this in terms of the frequency of occurrence of various sequences, like the triads AAA or ABA in a copolymerization of A and B monomers. The value of this information is that such sequence distributions can be measured directly by NMR spectroscopy, thus allowing a direct probe of copolymer structure and an alternative method for measuring reactivity ratios. As mentioned above, there are problems, as some spectra can be too complex and rich for easy analysis, as we will see in Chapter 7. 

Homopolymers of isobutylene and 0-pinene are, respectively, nonvulcanizable rubbers and brittle plastics. The projected structure of a random poly(isobutylene-co- -pinene) is of interest, since depending on the relative composition of the copolymer structure e.g.. 

The considerable importance of copolymers for practical purposes generated a considerable number of studies dedicated to thermal stability and pyrolysis of copolymers (see e.g. [10-15]). The presence of two or more monomeric structures in a macromolecule can influence significantly the thermal behavior and the composition of its pyrolysate. Depending on the ratio of the comonomers, as well as on the structure of the polymer (random, alt, block, graft, etc ), the pyrolysis output can be very different. Based on this, pyrolysis results are frequently used for the analysis of copolymer structures. 

The radical nature of nitroxide-mediated processes also allows novel types of block copolymers to be prepared in which copolymers, not homopolymer, are employed as one of the blocks. One of the simplest examples incorporate random copolymers124 and the novelty of these structures is based on the inability to prepare random copolymers by living anionic or cationic procedures. This is in direct contrast to the facile synthesis of well-defined random copolymers by nitroxide-mediated systems. While similar in concept, random block copolymers are more like traditional block copolymers than random copolymers in that there are two discrete blocks, the main difference being one or more of these blocks is composed of a random copolymer segment. For example, homopolystyrene starting blocks can be used to initiate the copolymerization of styrene and 4-vi-nylpyridine to give a block copolymer consisting of a polystyrene block and a random copolymer of styrene and 4-vinylpyridine as the second block.166... 

Polymers that consist of repeated long chain structures of the same monomer units are called homopolymers, whereas polymers consisting of different monomers are referred to as copolymers. Polymerization to make copolymers is called copolymerization. There are several types of copolymers, such as random copolymers with random sequences of... 

The effect of the copolymer structure has not been sufficiently investigated. Nevertheless, a comparison of the diblock copolymers with the previously presented random copolymers showed that smaller amounts of the stabilizer were usually needed in the first case to achieve similar results. As a consequence, with the same amount of stabilizer, the diblock copolymers lead to smaller particles than the corresponding random copolymers. For... 

A novel well-defined macromonomer of epoxy end-functionalized poly(V -capro-lactone) (PCL) was synthesized and its reactivity in photoinitiated cationic polymerization was examined [28]. PCL macromonomer as the comonomer allowed a rather simple incorporation of PCL side chains into poly(cyclohexene oxide) (PCHO) backbone. This way PCHO-g-PCL copolymer with random sequences of the structure shown in Scheme 13.16 is formed. 

Extensive work with condensation polymers and copolymers fully confirms the importance of structural regularity on crystallization tendency, and consequently on associated properties. Thus, copolymers containing regular alteration of each copolymer unit, either ABABAB type or block type, show a distinct tendency to crystallize, and corresponding copolymers with random distributions of the two are intrinsically amorphous, less rigid, lower melting, and more soluble. 

Thus by the use of Wood s approximation we were able to construct a complete 7g/composition curve for the poly(isobutylene-co -/3-pinene) system. In view of the experimental difficulties in obtaining the Tg for poly()3-pinene), the accuracy of the plot is questionable beyond V2 = 63 volume% ie., above 63 volume%0-pinene in the copolymer. However, in the range from 0 to 63 volume% /3-pinene, the curve is considered to be accurate and, importantly, reveals that useful rubbery poly(isobutylene-co-p-pinene) could be made with up to about 28 vohime% /3-pinene, ie, to a Tg of —40 °C. In agreement with our spectroscopic and reactivity ratio studies, the (admittedly somewhat limited) applicability of Eq. (1) developed for random copolymers also suggests a statistically random copolymer structure for our poly(isobutylene-co-/3-pinene) products. 

Since polymer properties are derived from finite length segments of a particular structure, random copolymers do not compatibilize homopolymers and, in many cases, copolymers based on one ratio of comonomers are not compatible with copolymers based on a different ratio of the same comonomers. 

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