Copolymer 768839 224706862 2008-07-10T01:33:43Z Bdodo1992 2607388 /* See also */ A '''heteropolymer''' or '''copolymer''' is a [[polymer]] derived from two (or more) [[monomer]]ic species, as opposed to a [[homopolymer]] where only one monomer is used.<ref>Odian, G. ''Principles of Polymerization'', 4th Ed., Wiley-Interscience, Hoboken, NJ '''2004''', Ch. 6. ISBN 0-471-27400-3</ref> '''Copolymerization''' refers to methods used to chemically synthesize a copolymer. Commercially relevant copolymers include [[ABS plastic]], [[Styrene/butadiene co-polymer|SBR]], [[Nitrile rubber]], [[styrene-acrylonitrile]], styrene-isoprene-styrene (SIS) and [[ethylene-vinyl acetate]]. == Types of copolymers == [[Image:Copolymers.png|right|400px|Different types of copolymers]] Since a copolymer consists of at least two types of [[repeating unit|constitutional unit]]s (not [[structural unit]]s), copolymers can be classified based on how these units are arranged along the chain.<ref>IUPAC. "Glossary of Basic Terms in Polymer Science". Pure Appl. Chem. 1996, 68, 2287-2311</ref> 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)<sub>n</sub> * '''Random copolymers (or Statistical Copolymers)''' with random sequences of monomer A and B (3) * '''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. Other special types of branched copolymers include '''star copolymers''', '''brush copolymers''', and '''comb copolymers'''. A '''terpolymer''' is a [[copolymer]] consisting of three distinct [[monomer]]s. The term is derived from ''ter'' (Latin), meaning thrice, and [[polymer]]. === Graft copolymers === '''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. For example, suppose we perform a [[radical polymerization |free-radical polymerization]] of styrene in the presence of [[polybutadiene]], a [[synthetic rubber]], which retains one reactive C=C [[covalent bond|double bond]] per [[residue]]. We get [[polystyrene]] chains growing out in either direction from some of the places where there were double bonds, with a one-carbon rearrangement. Or to look at it the other way around, the result is a polystyrene backbone with polybutadiene chains growing out of it in both directions. This is an interesting copolymer variant in that one of the ingredients was a polymer to begin with. As with [[heteropolymer|block copolymers]], the quasi-[[composite material|composite]] product has properties of both "components". In the example cited, the rubbery chains absorb energy when the substance is hit, so it is much less brittle than ordinary polystyrene. The product is called high-impact polystyrene, or HIPS. === 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 end 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 (chemistry)|ATRP]]), reversible addition fragmentation chain transfer ([[RAFT (chemistry)|RAFT]]), [[ring-opening metathesis polymerization]] (ROMP), and living cationic or living anionic [[living polymerization|polymerization]]s. ====Phase separation==== [[Image:Sbs block copolymer.jpg|thumb|right|Sbs block copolymer in [[TEM]]]] Block copolymers are interesting because they can "microphase separate" to form periodic nanostructures, as in the styrene-butadiene-styrene block copolymer shown at right. The polymer is known as [[Kraton]] and is used for shoe soles and [[adhesive]]s. Owing to the microfine structure, the transmission electron microscope or [[TEM]] was needed to examine the structure. The butadiene matrix was stained with [[osmium tetroxide]] to provide contrast in the image. The material was made by [[living polymerization]] so that the blocks are almost [[monodisperse]], so helping to create a very regular microstructure. The [[molecular weight]] of the [[polystyrene]] blocks in the main picture is 102,000; the inset picture has a molecular weight of 91,000, producing slightly smaller domains. [[Image:SBSstructure.jpg|thumb|right|SBS block copolymer schematic microstructure]] Microphase separation is a situation similar to that of [[oil]] and [[water]]. Oil and water are immiscible - they phase separate. Due to incompatibility between the blocks, block copolymers undergo a similar phase separation. Because the blocks are covalently bonded to each other, they cannot demix macroscopically as water and oil. In "microphase separation" the blocks form [[nanometer]]-sized structures. Depending on the relative lengths of each block, several morphologies can be obtained. In diblock copolymers, sufficiently different block lengths lead to nanometer-sized spheres of one block in a matrix of the second (for example [[Polymethyl methacrylate|PMMA]] in [[polystyrene]]). By using less different block lengths, a hexagonally-packed-cylinder geometry can be obtained. Blocks of similar length form layers (often called [[lamellae]] in the technical literature). Between the cylindrical and lamellar phase is the [[gyroid]] phase. The nanoscale structures created from block copolymers could potentially be used for creating devices for use in computer [[memory]], nanoscale-templating and nanoscale separations. Polymer scientists use [[thermodynamics]] to describe how the different blocks interact. The product of the degree of polymerization, <math>N</math>, and the Flory-Huggins [[interaction parameter]], <math>\chi</math>, gives an indication of how incompatible the two blocks are and whether or not they will microphase separate. For example, a diblock copolymer of symmetric composition will microphase separate if the product <math>\chi N</math> is greater than 10.5. If <math>\chi N</math> is less than 10.5, the blocks will mix and microphase separation is not observed. ==Copolymer equation== An alternating copolymer has the formula: -A-B-A-B-A-B-A-B-A-B-, or -(-A-B-)<sub>n</sub>-. The molar ratios of the monomer in the polymer is close to one, which happens when the reactivity ratios r<sub>1</sub> & r<sub>2</sub> are close to zero, as given by the [[Mayo-Lewis equation]] also called the '''copolymerization equation''':<ref>''Copolymerization. I. A Basis for Comparing the Behavior of Monomers in Copolymerization; The Copolymerization of Styrene and Methyl Methacrylate''Frank R. Mayo and Frederick M. Lewis [[J. Am. Chem. Soc.]]; '''1944'''; 66(9) pp 1594 - 1601; {{DOI|10.1021/ja01237a052}}</ref> <math>\frac {d\left [M_1 \right]}{d\left [M_2\right]}=\frac{\left [M_1\right]\left (r_1\left[M_1\right]+\left [M_2\right]\right)}{\left [M_2\right]\left (\left [M_1\right]+r_2\left [M_2\right]\right)}</math> where r<sub>1</sub> = k<sub>11</sub>/k<sub>12</sub> & r<sub>2</sub> = k<sub>22</sub>/k<sub>21</sub> ==Copolymer engineering== Copolymerization is used to modify the properties of man-made plastics to specific needs, for example to reduce crystallinity, modify [[glass transition temperature]] or to improve solubility. It is a way of improving mechanical properties, in a technique known as [[rubber toughening]]. Elastomeric phases within a rigid matrix act as crack initiators, and so increase the energy absorption when the material is impacted for example. [[Acrylonitrile butadiene styrene]] is a common example. ==External links== * http://www.chem.rochester.edu/~chem421/copoly.htm * [http://www-dick.chemie.uni-regensburg.de/group/stephan_baeurle/31,0,block-copolymers,index,0.html Block Copolymers: Institute of Physical & Theoretical Chemistry, University of Regensburg, Regensburg, Germany] * [http://www3.open.ac.uk/courses/bin/p12.dll?C01T838 Distance learning course in polymers] * [http://openlearn.open.ac.uk/mod/resource/view.php?id=196631 Polymer structures] == See also == * [[Polymer#Monomer_arrangement_in_copolymers|Copolymers section of Polymer article]] * [[Tholin]] ==References== <div class="references-small"><references/></div> [[Category:polymer chemistry]] [[de:Copolymer]] [[es:Copolímero]] [[it:Copolimero]] [[nl:Copolymeer]] [[ja:共重合]] [[pl:Kopolimer]] [[pl:Kopolimeryzacja]] [[pt:Copolímero]] [[sv:Sampolymer]] [[ru:Сополимеры]]