Copolymers repeat unit sequencing

These equations show that the composition of the copolymer formed from a specific comonomer mixture is controlled by the monomer reactivity ratios for the copolymerization. Additionally, they control the sequence distribution of the different repeat units in the copolymer. If ta > 1 then "> A prefers to add monomer A (i.e., it prefers to homopropagate) and extended sequences of A-type repeat units are introduced, whereas if ta < 1 A prefers to add monomer B, i.e., to cross-propagate. In a similar way, ra describes the behaviour of monomer B. The effects of some specific combinations of ta and re values upon copolymer composition and repeat unit sequence distribution are considered in the next section. 

Effects of Repeat Unit Sequence Distribution and Specific Interactions. Most theories and quantitative structure-property relationships for Tg only consider the case of a random distribution of repeat units along the polymer chains in treating copolymers. They give equations which predict a monotonic change of Tg between the Tg values of the homopolymers of the constituent repeat... 

Variations in Sequencing of Copolymer Repeat Units. Whether it is a homopolymer possessing differences in repeat unit structures, a copolymer or terpolymer, the sequencing of repeat units can cause wide variations in physical properties. The most fundamental structural categories for copoljmaers, terpoly-mers, etc. are block, alternating and random 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. 

Next let us consider the probability of finding a sequence of repeat units in a copolymer which is exactly Ui units of Mi in length. This may be represented as M2(Mi)i jM2. Working from left to right in this sequence, we note the following ... 

Most of the experimental information concerning copolymer microstructure has been obtained by physical methods based on modern instrumental methods. Techniques such as ultraviolet (UV), visible, and infrared (IR) spectroscopy, NMR spectroscopy, and mass spectroscopy have all been used to good advantage in this type of research. Advances in instrumentation and computer interfacing combine to make these physical methods particularly suitable to answer the question we pose With what frequency do particular sequences of repeat units occur in a copolymer. 

The probabilities of the various dyad, triad, and other sequences that we have examined have all been described by a single probability parameter p. When we used the same kind of statistics for copolymers, we called the situation one of terminal control. We are considering similar statistics here, but the idea that the stereochemistry is controlled by the terminal unit is inappropriate. The active center of the chain end governs the chemistry of the addition, but not the stereochemistry. Neither the terminal unit nor any other repeat unit considered alone has any stereochemistry. Equations (7.62) and (7.63) merely state that an addition must be of one kind or another, but that the rates are not necessarily identical. 

Fig. 8. General structures of polymeric dispersants (a) liomopolymer, (b) random copolymer, (c) diblock copolymer, and (d) comb polymer, where A = anchor group, B = soluble repeat unit, and C = repeat unit with solubility different from B. The repeat units may occur in sequences hundreds of...

The final class of polymers are copolymers containing one or more of the repeat units of classes 2 and 3 (15-18). Copolymer effectiveness would presumably be a function of the chemical structures of each comonomer, comonomer sequence distribution, and polymer molecular weight. The comonomer could be a relatively... 

Copolymers are built from the repetition of two (or more) "repeat units". Depending on the spatial arrangement of those units (e.g., A and B) along the chain (sequencing), various types of copolymers can be made alternating, block, random (or statistical), branched, crosslinked (see Figure 12). 

All polymers discussed so far are homopolymers, i.e., they consist of multiple sequences of the same repeating unit. Regular linear homopolymers without bulky pendant groups, such as hdpe, are easily crystallized. However, the tendency for crystallization is reduced in copolymers, since they contain more than one repeating unit in the chain. Copolymers with random arrangements of repeating units in the polymer chain are generally amorphous. 

The properties of block copolymers are dependent on the length of the sequences of repeating units, or domains. The domains in typical commercial block copolymers of styrene and butadiene are sufficiently long such that the products are flexible plastics. They are called thermoplastic elastomers (TPE). It should be noted that although the Ts for random copolymers is between the T/s of the respective homopolymers, the repeating sequences in block copolymers exhibit their own characteristic Ff s. 

Block copolymers of sufficiently large and incompatible sequences of repeating units undergo microphase separation. Because of the geometric restriction caused by the needed alignment of the junctions between the different blocks, the phases... 

Block copolymer—A copolymer with long sequences or blocks of different repeat units (e.g., AxBy). 

Block copolymers are linear copolymers in which the repeat units exist only in long sequences, or blocks, of the same type. Two common block copolymer structures are represented below and usually are termed AB di-block and ABA tri-block copolymers... 

A synthetic copolymer provides additional degrees of freedom in the arrangement of the repeating units. For example, the spectrum of a copolymer of vinylidine chloride and isobutylene, shown in Fig. 13.5, indicates that various tetrad sequences (sequences of four monomer units) display significantly different spectra. Copolymers composed of more than two monomer types, including biopolymers, have much more complex spectra, as we discuss later. 

Among the structural factors that should be controlled in polymer syntheses (Fig. 1, Section I), perhaps the least exploited is the sequence of constitutional repeat units along a polymer main chain. We have already discussed the syntheses of block copolymers, where two or more homopolymer segments are connected, such as AAAAA-BBBBB- -, which is among the most primitive examples of sequence control in synthetic polymers. 

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