Copolymer

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A heteropolymer or copolymer is a polymer derived from two (or more) monomeric species, as opposed to a homopolymer where only one monomer is used.[1] Copolymerization refers to methods used to chemically synthesize a copolymer.

Commercially relevant copolymers include ABS plastic, SBR, styrene-isoprene-styrene (SIS) and ethylene-vinyl acetate.

Types of copolymers

Since a copolymer consists of at least two types of constitutional units (not structural units), copolymers can be classified based on how these units are arranged along the chain.[2] These include:

  • Alternating copolymers with regular alternating A and B units (2)
  • Periodic copolymers with A and B units arranged in a repeating sequence (e.g. (A-B-A-B-B-A-A-A-A-B-B-B)n
  • Random copolymers with random sequences of monomer A and B (3)
  • Statistical copolymers in which the ordering of the distinct monomers within the polymer sequence obeys known statistical rules
  • Block copolymers comprised of two or more homopolymer subunits linked by covalent bonds (4). The union of the homopolymer subunits may require an intermediate non-repeating subunit, known as a junction block. Block copolymers with two or three distinct blocks are called diblock copolymers and triblock copolymers, respectively.

Copolymers may also be described in terms of the existence of or arrangement of branches in the polymer structure. Linear copolymers consist of a single main chain whereas branched copolymers consist of a single main chain with one or more polymeric side chains.

Graft copolymers are a special type of branched copolymer in which the side chains are structurally distinct from the main chain. The illustration (5) depicts a special case where the main chain and side chains are composed of distinct homopolymers. However, the individual chains of a graft copolymer may be homopolymers or copolymers. Note that different copolymer sequencing is sufficient to define a structural difference, thus an A-B diblock copolymer with A-B alternating copolymer side chains is properly called a graft copolymer.

Other special types of branched copolymers include star copolymers, brush copolymers, and comb copolymers.

Block Copolymers

A special kind of copolymer is called a "block copolymer". Block copolymers are made up of blocks of different polymerized monomers. For example, PS-b-PMMA is short for polystyrene-b-poly(methyl methacrylate) and is made by first polymerizing styrene, and then subsequently polymerizing MMA from the reactive ends of the polystyrene chains. This polymer is a "diblock copolymer" because it contains two different chemical blocks. You can also make triblocks, tetrablocks, multiblocks, etc. Diblock copolymers are made using living polymerization techniques, such as atom transfer free radical polymerization (ATRP), reversible addition fragmentation chain transfer (RAFT), ring-opening metathesis polymerization (ROMP), and living cationic or living anionic polymerizations.

Block copolymers are interesting because they can "microphase separate" to form periodic nanostructures.

Microphase separation is a situation similar to that of oil and water. Oil and water don't mix together- they macrophase separate. If you have an "oil-like" first block and a "water-like" second block, the block copolymers undergo microphase separation. The blocks want to get as far from each other as possible, but they are covalently bonded, so they're not going to get very far. In "microphase separation" the "oil" and "water" blocks form nanometer-sized structures. These structures can look like spheres of PMMA in a matrix of PS or vice versa, or they could be stripes (often called lamellae in the technical literature) or cylinders. The nanoscale structures created from block copolymers could potentially be used for creating devices for use in computer memory, nanoscale-templating and nanoscale separations.

In official terms, polymer scientists wouldn't use "oil-like" and "water-like" to describe the polymer blocks' interactions. Polymer scientists use thermodynamics to describe how the different blocks interact. The interaction parameter, also called "chi" gives an indication of how different, chemically, the two blocks are and whether or not they will microphase separate. If the product of chi and the molecular weight is large (greater than 10.5), the blocks will microphase separate. If the product of chi and the molecular weight is too small (less than 10.5), the different blocks are able to mix.

Copolymer equation

An alternating copolymer has the formula: -A-B-A-B-A-B-A-B-A-B-, or -(-A-B-)n-. The molar ratios of the monomer in the polymer is close to one, which happens when the reactivity ratios r1 & r2 are close to zero, as given by the Mayo-Lewis equation also called the copolymerization equation:[3]

where r1 = k11/k12 & r2 = k22/k21

Copolymer engineering

In man-made plastics copolymerization is used to modify the properties of the material to a specific needs, for example to reduce crystallinity, modify glass transition temperature or to improve solubility.

External links

See also

References

  1. Odian, G. Principles of Polymerization, 4th Ed., Wiley-Interscience, Hoboken, NJ 2004, Ch. 6. ISBN 0-471-27400-3
  2. IUPAC. "Glossary of Basic Terms in Polymer Science". Pure Appl. Chem. 1996, 68, 2287-2311
  3. Copolymerization. I. A Basis for Comparing the Behavior of Monomers in Copolymerization; The Copolymerization of Styrene and Methyl MethacrylateFrank R. Mayo and Frederick M. Lewis J. Am. Chem. Soc.; 1944; 66(9) pp 1594 - 1601; doi:10.1021/ja01237a052

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