Copolymers, monomer arrangement

Thermoplastic polymers can be homopolymers or copolymers where two or more different monomers are used. With copolymers, different arrangements of the monomers are possible (see also Chapter 2). 

Polymer molecules may be linear or branched, and separate linear or branched chains may be joined by crosslinks. Extensive crosslinking leads to a three-dimensional and often insoluble polymer network. Polymers in which all the monomeric units are identical are referred to as homopolymers those formed from more than one monomer type are called copolymers. Various arrangements of the monomers A and B in the copolymer molecules (Fig. 8.1) can be produced with consequent effects on the physical properties of the resulting polymer. Synthetic polymers may have their main chains substituted in different ways, depending on the conditions of the reaction, such that atactic (random), isotactic or syndiotactic forms are produced, as diagrammatically represented in Fig. 8.1. 

Block copolymers are polymers constimted of at least two different monomers arranged in a specific manner — they could be di-block, tri-block,... 

Block copolymers are polymers constituted of at least two different monomers arranged in a specific manner - they could be diblock, triblock, multi-block, linear, star shaped, etc. Those based on styrene and butadiene, SB or SBS, are the earliest to be applied and studied, as well as the largest as far as the volume of production is concerned (Holden et al. 1967). 

A polymer with monomers arranged in regular sequence, viz., alternating or block copolymers. 

Polymer chains may be linear, or possibly with additional chemical groups forming branches along the primary chain. If a polymer is made up of the same repeating units of monomers, it is called a homopolymer otherwise, if it is made up of different types of monomers arranged in some sequence, then it is called a copolymer. 

Wang and Smith [101] used PGC to carry out compositional and structural studies of these copolymers. Composition and number average sequence lengths (which reflects monomer arrangements) in the copolymers were calculated using formulae that incorporate pure trimer and hybrid trimer peak intensities. 

In general, there are four kinds of copolymers random, alternating, block and graft. In the random copolymer the arrangement of monomer units is in a statistically random placement along a chain, e.g. 

Wang and co-workers [90] applied pyrolysis - gas chromatography to the determination of the structnre of styrene-w-bntyl acrylate copolymers. The nnmber average sequence length which reflects monomer arrangement was calcnlated using formulae that incorporate pure trimer peak intensities and hybrid trimer peak intensities. 

Monomer arrangement in copolymers In a copolymer, the different types of monomers are distributed along the backbone in a number of ways. In a random copolymer the monomers distribute irregularly, while an alternating copolymer possesses regularly alternating monomer residues. Block copolymers have two or more homopolymer subunits linked by covalent bonds, while graft copolymers contain side chains attached to the main polymer chains. 

The desired form in homopolymers is the isotactic arrangement (at least 93% is required to give the desired properties). Copolymers have a random arrangement. In block copolymers a secondary reactor is used where active polymer chains can further polymerize to produce segments that use ethylene monomer. 

In the Type II case, the copolymerization tends toward an alternating arrangement of monomer units. Curve II of Figure 1 shows an example of an alternating copolymer that has an azeotropic copolymer composition, ie, a copolymer composition equal to the monomer feed at a single monomer feed composition. This case is analogous to a constant Foiling mixture ia vapor—Hquid equihbria.T) III... 

The commercial poly-(4-methypent-1-ene) (P4MP1) is an essentially isotactic material which shows 65% crystallinity when annealed but under more normal conditions about 40%. For reasons given later the material is believed to be a copolymer. In the crystalline state P4MP1 molecules take up a helical disposition and in order to accommodate the side chains require seven monomer units per two turns of the helix (c.f. three monomers per turn with polypropylene and polybut-I-ene). Because of the space required for this arrangement the density of the crystalline zone is slightly less than that of the amorphous zone at room temperature. 

Alternating copolymer—In these copolymers, the monomers are arranged in an alternate sequence. Alternating copolymers are produced by special copolymerization processes. 

The arrangement of monomer units in copolymer chains is determined by the monomer reactivity ratios which can be influenced by the reaction medium and various additives. The average sequence distribution to the triad level can often be measured by NMR (Section 7.3.3.2) and in special cases by other techniques.100 101 Longer sequences are usually difficult to determine experimentally, however, by assuming a model (terminal, penultimate, etc.) they can be predicted.7 102 Where sequence distributions can be accurately determined Lhey provide, in principle, a powerful method for determining monomer reactivity ratios. 

For the case of copolymers, it is also possible to control the arrangement of monomer units in the chain. 

Vinyl monomers are often copolymerized, usually with free-radical or coordination metal catalysis, but occasionally by other mechanisms. Random copolymers are important items of commerce. The two monomers are present together in the reaction mixture and copolymerize to give more-or-less random arrangements of the monomers along the pol5aner chain. 

Mixtures of two or more monomers can polymerize to form copolymers. Many copolymers have been developed to combine the best features of each monomer. For example, poly(vinyl chloride) (called a homopolymer because it is made from a single monomers) is brittle. By copolymerizing vinyl chloride with vinyl acetate, a copolymer is obtained that is flexible. Arrangement of the monomer units in a copolymer depends on the rates at which the monomers react with each other. Graft copolymers are formed when a monomer is initiated by free radical sites created on an already-formed polymer chain. 

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