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| | | Greetings. Allow me begin by telling you the author's title - Phebe. For years he's been operating as a receptionist. What I love doing is to gather badges but I've been taking on new issues recently. Puerto Rico is exactly where he and his wife reside.<br><br>Check out my web blog :: [http://netwk.hannam.ac.kr/xe/data_2/38191 hannam.ac.kr] |
| [[Image:Syndiotactic polypropene.png|thumb|right|A ball-and-stick model of syndiotactic [[polypropylene]].]]
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| '''Tacticity''' (from Greek τακτικός ''taktikos'' "of or relating to arrangement or order") is the relative [[stereochemistry]] of adjacent [[chirality (chemistry)|chiral]] centers within a [[macromolecule]].<ref>''Introduction to polymers'' R.J. Young ISBN 0-412-22170-5</ref> The practical significance of tacticity rests on the effects on the physical properties of the [[polymer]]. The regularity of the macromolecular structure influences the degree to which it has rigid, [[Crystallinity|crystalline]] long range order or flexible, [[amorphous]] long range disorder. Precise knowledge of tacticity of a polymer also helps understanding at what temperature a polymer [[melting|melts]], how [[soluble]] it is in a [[solvent]] and its mechanical properties.
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| A '''tactic macromolecule''' in the [[IUPAC]] definition is a macromolecule in which essentially all the configurational (repeating) units are identical. Tacticity is particularly significant in [[vinyl polymer]]s of the type -H<sub>2</sub>C-CH(R)- where each [[repeating unit]] with a [[substituent]] R on one side of the polymer backbone is followed by the next repeating unit with the substituent on the same side as the previous one, the other side as the previous one or positioned randomly with respect to the previous one. In a hydrocarbon macromolecule with all carbon atoms making up the backbone in a [[tetrahedral molecular geometry]], the zigzag backbone is in the paper plane with the substituents either sticking out of the paper or retreating into the paper. This projection is called the [[Natta projection]] after [[Giulio Natta]]. '''Monotactic''' macromolecules have one stereoisomeric atom per repeat unit, '''ditactic''' to '''n-tactic''' macromolecules have more than one stereoisomeric atom per unit.
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| {{Quote box
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| |title = IUPAC definition
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| |quote = The orderliness of the succession of configurational repeating units in<br/>the main chain of a regular [[macromolecule]], a regular oligomer molecule,<br/>a regular block, or a regular chain. <ref>{{cite journal|title=Glossary of basic terms in polymer science (IUPAC Recommendations 1996)|journal=[[Pure and Applied Chemistry]]|year=1996|volume=68|issue=12|pages=2287-2311|doi=10.1351/pac199668122287|url=http://pac.iupac.org/publications/pac/pdf/1996/pdf/6812x2287.pdf}}</ref>
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| }}
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| ==Describing tacticity==
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| [[Image:Meso diad.PNG|thumb|right|200px|An example of ''meso'' diads in a polypropylene molecule.]]
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| [[Image:Racemo diad.PNG|thumb|right|200px|An example of ''racemo'' diads in a polypropylene molecule.]]
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| [[Image:Mm triad.PNG|thumb|right|200px|An isotactic (''mm'') triad in a polypropylene molecule.]]
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| [[Image:Rr triad.PNG|thumb|right|200px|A syndiotactic (''rr'') triad in a polypropylene molecule.]]
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| [[Image:Rm triad.PNG|thumb|right|200px|A heterotactic (''rm'') triad in a polypropylene molecule.]]
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| ===Diads===
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| Two adjacent structural units in a polymer molecule constitute a '''diad'''. If the diad consists of two identically oriented units, the diad is called a '''meso diad''' reflecting similar features as a [[meso compound]]. If the diad consists of units oriented in opposition, the diad is called a '''racemo diad''' as in a [[racemic]] compound. In the case of vinyl polymer molecules, a meso diad is one in which the book carbon chains are oriented on the same side of the polymer backbone.
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| ===Triads===
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| The stereochemistry of macromolecules can be defined even more precisely with the introduction of triads. An '''isotactic triad''' (mm) is made up of two adjacent meso diads, a '''syndiotactic triad''' (rr) consists of two adjacent racemo diads and a '''heterotactic triad''' (rm) is composed of a meso diad adjacent to a racemo diad. The mass fraction of isotactic (mm) triads is a common quantitative measure of tacticity.
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| When the stereochemistry of a macromolecule is considered to be a [[Bernoulli process]], triad composition can be calculated from the probability of finding meso diads (P<sub>m</sub>). When this probability is 0.25 then the probability of finding:
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| *an isotactic triad is P<sub>m</sub><sup>2</sup> or 0.0625
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| *an heterotactic triad is 2P<sub>m</sub>(1–P<sub>m</sub>) or 0.375
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| *a syndiotactic triad is (1–P<sub>m</sub>)<sup>2</sup> or 0.5625
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| with a total probability of 1. Similar relationships with diads exist for tetrads.
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| ===Tetrads, Pentads, etc.===
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| The definition of tetrads and pentads introduce further sophistication and precision to defining tacticity, especially when information on long-range ordering is desirable. Tacticity measurements obtained by [[Carbon-13]] [[Nuclear magnetic resonance spectroscopy|NMR]] are typically expressed in terms of the relative abundance of various pentads within the polymer molecule, e.g. ''mmmm'', ''mrrm''.
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| ===Other conventions for quantifying tacticity===
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| The primary convention for expressing tacticity is in terms of the relative weight fraction of triad or higher-order components, as described above. An alternative expression for tacticity is the average length of ''meso'' and ''racemo'' sequences within the polymer molecule. The average meso sequence length may be approximated from the relative abundance of pentads as follows:<ref>{{cite journal|doi=10.1016/0032-3861(93)90577-W|title=Microstructural analysis of polypropylenes produced with heterogeneous Ziegler-Natta catalysts|year=1993|last1=Paukkeri|first1=R|last2=Vaananen|first2=T|last3=Lehtinen|first3=A|journal=Polymer|volume=34|pages=2488|issue=12}}</ref>
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| <math>MSL = \frac{mmmm + \tfrac{3}{2} mrrr + 2 rmmr + \tfrac{1}{2} rmrm + \tfrac{1}{2} rmrr}{\tfrac{1}{2} mmmr + rmmr + \tfrac{1}{2}rmrm + \tfrac{1}{2}rmrr}</math>
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| {{clear}}
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| ==Polymers==
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| ===Isotactic polymers===
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| Isotactic polymers are composed of isotactic macromolecules (IUPAC definition).<ref>[[IUPAC]] [http://www.iupac.org/reports/1996/6812jenkins/molecules.html macromolecular glossary]</ref> In isotactic macromolecules all the substituents are located on the same side of the macromolecular backbone. An isotactic macromolecule consists of 100% meso diads. [[Polypropylene]] formed by [[Ziegler-Natta catalyst|Ziegler-Natta catalysis]] is an isotactic polymer.<ref>Stevens, P. S. Polymer Chemistry: An Introduction, 3rd ed.; Oxford Press: New York, 1999; pp 234-235</ref> Isotactic polymers are usually [[semicrystalline]] and often form a helix configuration.
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| [[File:Isotactic-A-2D-skeletal.png|center|400px|isotactic polymers]]
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| [[File:Isotactic-polypropylene-3D-balls.png|center|400px|isotactic polypropylene]]
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| ===Syndiotactic polymers===
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| In syndiotactic or '''syntactic''' macromolecules the substituents have alternate positions along the chain. The macromolecule consists 100% of racemo diads. Syndiotactic [[polystyrene]], made by [[metallocene catalysis polymerization]], is crystalline with a [[melting point]] of 161 °C. [[gutta percha]] is also an example for Syndiotactic polymer.<ref>Brandrup, Immergut, Grulke (Editors), Polymer Handbook 4th edition, Wiley-Interscience, New York, 1999. VI/11</ref>
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| [[File:Syndiotactic-2D-skeletal.png|center|400px|syndiotactic polymers]]
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| [[File:Syndiotactic-polypropylene-3D-balls.png|center|400px|syndiotactic polypropylene]]
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| ===Atactic polymers===
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| In atactic macromolecules the substituents are placed randomly along the chain. The percentage of meso diads is between 1 and 99%. With the aid of spectroscopic techniques such as [[Nuclear magnetic resonance|NMR]] it is possible to pinpoint the composition of a polymer in terms of the percentages for each triad.{{Citation needed|date=February 2007}}
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| [[File:Atactic-2D-skeletal.png|center|400px|atactic polymers]]
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| Polymers that are formed by [[free radical polymerization|free-radical mechanisms]] such as [[polyvinyl chloride]] are usually atactic. Due to their random nature atactic polymers are usually [[amorphous]]. In '''hemi isotactic macromolecules''' every other repeat unit has a random substituent.
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| Atactic polymers are technologically very important. A good example is polystyrene (PS). If a special catalyst is used in its synthesis it is possible to obtain the syndiotactic version of this polymer, but most industrial polystyrene produced is atactic. The two materials have very different properties because the irregular structure of the atactic version makes it impossible for the polymer chains to stack in a regular fashion. The result is that, whereas syndiotactic PS is a semicrystalline material, the more common atactic version cannot crystallize and forms a ''glass'' instead. This example is quite general in that many polymers of economic importance are atactic glass formers.
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| ===Eutactic polymers===
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| In eutactic macromolecules, substituents may occupy any specific (but potentially complex) sequence of positions along the chain. Isotactic and syndiotactic polymers are instances of the more general class of eutactic polymers, which also includes heterogeneous macromolecules in which the sequence consists of substituents of different kinds (for example, the side-chains in proteins and the bases in nucleic acids).
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| ==Head/tail configuration==
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| [[Image:Tail head isomerism.svg|right]]
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| In vinyl polymers the complete configuration can be further described by defining polymer head/tail configuration. In a regular macromolecule all monomer units are normally linked in a head to tail configuration so that all β-substituents are separated by three carbon atoms. In head to head configuration this separation is only by 2 carbon atoms and the separation with tail to tail configuration is by 4 atoms. Head/tail configurations are not part of polymer tacticity but should be taken into account when considering polymer defects.
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| ==Techniques for measuring tacticity==
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| Tacticity may be measured directly using [[proton]] or [[carbon-13]] [[NMR]]. This technique enables quantification of the tacticity distribution by comparison of peak areas or integral ranges corresponding to known diads (r, m), triads (mm, rm+mr, rr) and/or higher order ''n''-ads depending on spectral resolution. In cases of limited resolution stochastic methods such as [[Bernoullian analysis|Bernoullian]] or [[Markovian analysis]] may also be used to fit the distribution and back then forward predict higher ''n''-ads and calculate the isotacticity of the polymer to the desired level.<ref>{{cite journal|doi=10.1021/ma60057a006|title=Carbon-13 NMR Determination of Pentad Tacticity of Poly(vinyl alcohol)|year=1977|last1=Wu|first1=Ting Kai|last2=Sheer|first2=M. Lana|journal=Macromolecules|volume=10|pages=529|issue=3|bibcode = 1977MaMol..10..529W }}</ref>
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| Other techniques sensitive to tacticity include [[x-ray powder diffraction]], [[secondary ion mass spectrometry]] (SIMS),<ref>{{cite journal|doi=10.1002/(SICI)1096-9918(199701)25:1<41::AID-SIA211>3.0.CO;2-T|title=Influence of Tacticity on Polymer Surfaces Studiedby ToF-SIMS|year=1997|last1=Vanden Eynde|first1=X.|last2=Weng|first2=L. T.|last3=Bertrand|first3=P.|journal=Surface and Interface Analysis|volume=25|pages=41}}</ref> vibrational spectroscopy (FTIR) <ref>{{cite journal|doi=10.1021/ma00207a013|title=Normal-mode analysis of infrared and Raman spectra of crystalline isotactic poly(methyl methacrylate)|year=1990|last1=Dybal|first1=J.|last2=Krimm|first2=S.|journal=Macromolecules|volume=23|pages=1301|issue=5|bibcode = 1990MaMol..23.1301D }}</ref> and especially two-dimensional techniques.<ref>{{cite journal|doi=10.1021/ma00149a011|title=Observation of the stereochemical configuration of poly(methyl methacrylate) by proton two-dimensional J-correlated and NOE-correlated NMR spectroscopy|year=1985|last1=Schilling|first1=Frederic C.|last2=Bovey|first2=Frank A.|last3=Bruch|first3=Martha D.|last4=Kozlowski|first4=Sharon A.|journal=Macromolecules|volume=18|pages=1418|issue=7|bibcode = 1985MaMol..18.1418S }}</ref> Tacticity may also be inferred by measuring another physical property, such as melting temperature, when the relationship between tacticity and that property is well-established.<ref>{{cite journal|doi=10.1021/ma8014992|title=Pressure Dependence of the Glass Transition in Atactic and Isotactic Polypropylene|year=2008|last1=Gitsas|first1=A.|last2=Floudas|first2=G.|journal=Macromolecules|volume=41|pages=9423|issue=23|bibcode = 2008MaMol..41.9423G }}</ref>
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| ==References==
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| {{reflist|2}}
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| ==External links==
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| *Tacticity @ [http://web.archive.org/web/20040612172217/http://scgc.epfl.ch/load/cours_chim/cwandrey_part-2-1.pdf Ecole Polytechnique Fédérale de Lausanne]
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| *Application of spectroscopy in polymer charactisation @ [http://web.archive.org/web/20060914123842/http://www.chemeng.ucla.edu/che112/Notes/polymer+spectroscopy.pdf UCLA Los Angeles ]
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| *Polymer Structure @ [http://openlearn.open.ac.uk/mod/resource/view.php?id=196631 openlearn.open.ac.uk]
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| [[Category:Polymer chemistry]]
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| [[Category:Stereochemistry]]
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Greetings. Allow me begin by telling you the author's title - Phebe. For years he's been operating as a receptionist. What I love doing is to gather badges but I've been taking on new issues recently. Puerto Rico is exactly where he and his wife reside.
Check out my web blog :: hannam.ac.kr