tacticity in polymers

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  • 8/10/2019 Tacticity in Polymers

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    Lec25 (Cl 27)NMR for Sequence Isomerism

    NMR can also provide clear evidence of sequence isomerism inpolymers. For example, poly(vinylidene fluoride), or PVDFpolymer which containsH-H and T-T sequences.Because the 19F nucleus has a spin number of 1/2 and itsnatural abundance is 100% so 19F NMR spectroscopy is apowerful tool for studying these fluoropolymers.

    NMR for Sequence Isomerism

    The F atoms (small yellow balls) with one of the units in backward position,giving a H-H and T-T sequence. The peaks marked A, B, C and D in thecorresponding 19 F NMRspectrum of PVDF correspond to the fluorine atomslabeled in the Figure below.

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    NMR for Sequence Isomerism

    NMR for Sequence Isomerism

    To know the number fraction of VDF- monomers that areincorporated backwards into the chain, the relative intensitiesof the bands in the spectra is used and we can use probabilitytheory for this.If we consider Vinyl monomer CH2=CXY then we define theprobability of "normal" addition as P1{TH} and the probabilityof "backward addition as P1{HT}. It can only be one or theother, so it follows that P1{TH}+P1{HT}=1

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    NMR for Sequence Isomerism

    If we want to calculate the number or fraction of "backward"unitsin a sample, we need to determine P1{HT} = 1 P1{TH}.This determination will depend upon what information isavailable in the NMR spectrum.For Diads=4 possible sequences,Traids= 8 possible sequencesTetrads =16, and so on.Not all of these are distinguishable. For example, NMR cannotdistinguish between {TH-TH-TH}and {HT-HT-HT}- this isequivalent to reading the chain in one direction or the other.

    NMR for Sequence Isomerism

    We should see 3 NMR linesif we are observing diadinformation and 4 for triads (there would be 5 lines for tetradsand 6 for pentads).Even more interesting, n of the observed NMR lines (assumingthey are resolved in the spectrum) will have the same intensityif n-ad information is available. This just means that if we havetriad information available, 3 of the 4 observed lines will havethe same intensity as we saw in PMMA example, where 3bands have the same intensity.

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    IR & NMR for Structural Isomerism

    Both NMR and vibrational spectroscopy can be used to detectand study structuctural isomerism in dienepolymers. Forexample, the infrared transmission spectrum of poly (2,3dimethylbutadiene).No band near 1660 cm -1 from C=C double bonds, because ofthe symmetry in the repeat unit of this polymer. Just likesymmetric stretch in CO2 , which is IR inactive.But, there is a strong Raman line at 1665 cm -1, therefore weneed both IR and Raman spectra to obtain information aboutthis polymer.

    IR & NMR for Structural Isomerism

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    IR & NMR for Structural Isomerism

    Polychloroprene, for example, withlargely trans-1,4- units. but howmany cis- 1,4, -1,2 and -3,4 unitsare there?Consider the IR spectra marked Aand B. Sample A was polymerizedat -40C, while sample B waspolymerized at -20C . This resultsin a difference in microstructure,but it can be seen that the spectraof the two samples are very similar,simply because there is only asmall difference in the content ofstructural irregularities.

    IR & NMR for Structural Isomerism

    By modem computer-based instruments it is possible to subtractone spectrum from the other (A - B)to give a differencespectrum C revealing the presence of bands that can beassigned to cis-1,4-, 1,2-, and 3,4- units.For quantitative analysis, we have to use 13C NMR.In addition,1H NMRis less effective in this case as there is only minimalchemical shift differences between cis and trans isomers in the1H NMRspectra of polymers like polybutadiene andpolyisoprene.

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    IR & NMR for Structural Isomerism

    13C NMRhowever, is extremely effective in characterizingthese types of polymers. These spectra not only display well-separated lines for the various structural isomers that may bepresent, but also provide sequence distribution information andreadily allow quantitative analysis .For example, the proton decoupled 13C NMRspectrum ofpolybutadiene will show band that can be assigned to specifictriad sequences. We have marked with arrows those that havea central cis unit.

    IR & NMR for Structural Isomerism

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    Tacticity in Vinyl Polymers

    IRmay be used though for tacticity of apolymer but NMR gives in depth details ofthe tacticity.Inpolypropylene , for example, the ratio ofthe intensity of the 988 cm -1 band to thatof the 973 cm-1 band varies from about 1 in a completely isotactic to 0 in atacticsample. But what the intensity ratio of thisband actually represents is the degree ofchain order in the sample, not the tacticityas such.That is where NMR can unleash the details.

    Tacticity in Vinyl Polymers

    Normally plexiglass R is atactic, although both the isotatctic(iso-PMMA) and syndiotactic (syn-PMMA)can be polymerised.Proton NMR spectroscopy has been used to characterize thetacticity of various vinyl polymers in

    solution. In the case of isotactic polymers,there are two magnetically nonequivalentprotons in PMMA and appearance of04 bands in NMR spectra.

    On the other hand, in syndiotacticpolymers the two methylene protons areequivalent and we observe only 01 line .

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    Tacticity in PMMA

    The two hydrogen atoms of the CH2 groups in isotactic PMMAare notmagnetically equivalent. One is alwaysin the environment of-two methylgroups, while the other lies betweentwo ester groups .In contrast, for a syndiotactic polymer,the CH2, protons are magneticallyequivalent, each being on one side bya methyl group and the other by anester. Naturally, atactic polymers haveboth types of CH2 units.

    Tacticity in PMMA

    Because the two protons of the methylene group of iso-PMMA,are not magnetically equivalent, resulting in the four-linepattern of CH2.On the other hand, the CH2, protons of the syndiotactic PMMA are equivalent, there is no spin-spin interaction so only 01 linein the NMR. An atactic sample should resemble a combinationof these two.But, we would not be able to tell if the sample was a mixtureof isotactic and syndiotactic chains, or was actually atacticbecause the methylene protons are only sensitive to the diadstructures (the stereochemical arrangements of two adjacentunits).

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    Tacticity in PMMA

    Tacticity in PMMA

    The additional splitting observed here is due to

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    NMR Spectroscopy:Molecular mass13C NMRis the most important technique for structuralcharacterization of polymers.NMR spectra of linear polymers of low molecular mass oftenshow unique absorptions due to the end groups . By referencingthese absorptions with the nuclei of the repeat units, it ispossible to obtain the ratio of the no. of end groups to the no.of repeat units, thereby evaluate the no. average molecularmasses of such polymers.

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    NMR Spectroscopy:Molecular mass

    13C NMRis the most important technique for structuralcharacterization of polymers.For branched polymers, NMR absorptions due to the branchpoints can be identified and reveal the chemical structure ofthose branch points, thus leading to a better understanding ofthe mechanisms by which the branch form. It is also possible tocalculate the degree of branching by referencing theseabsorptions to those of nuclei in the repeat unit.

    NMR Spectroscopy:Copolymer Analysis13C NMRand 1H NMRare power tools to characterise the estructure and composition of copolymers as well.

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    Lec 28Molecular Weight and Branching

    Polymers contain mixtures of different molecular sizes. Oneway of obtaining the weight is to separate the polymer into ano. of fractions each of which has a narrow size distribution ofmolar mass. The weight and molar mass fraction of eachpolymer chain is then determined and molar mass distributionis displayed as a histogram.Size exclusion chromatography allows a rapid and powermethod to determine molar mass distributions. Gel PermeationChromatography (GPC) is a type of SEC that is used todetermine the polymer molar mass distribution in a solvent.

    Instrumentation of GPC

    A solvent or a mixture of solvents is pumped through thepacked columns. A polymer solution is injected at injection portat some point in the solvent stream. After passing through thecolumns, adetector measures the amount of polymer in what iscalled the eluant stream (by measuring the refractive index ofthe solution).

    Each column is packed with beads of a porous gel (e.g. Si, orzeolite). The pore size of gel may be 5-10 5 nm. The smallsolvent moleculepass both through and around the beads.The smallest polymer molecules are able to pass through mostof the pores in the beads and thus have a relatively long flowpath through the column before exit. Therefore small moleculesreach to the end of column in much longer time compared tolar er size molecules.

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    SEC

    Instrumentation of GPC

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    GPC Mechanism of Separation

    Polymer with a range of chain length , each one followdifferent path lengths through the column according to its sizeand are eluted at different times.

    Decreasing chain size

    GPC

    The concentration of polymer in the eluate is monitoredcontinuously and a chromatogram is obtained which is a plot ofconcentration against the elution volume or time, which providesa qualitative distribution of the molar mass in a give polymer.There may be just a small amount of

    very high MW material that comes off first,and small amounts of low MW materialthat come off last, and in-between a curvethat reflects the MW distribution of thesample.

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    Separation by Size Exclusion inGPC

    In order to obtain a MW distribution andto relate MW to the eluted volume,columns must becalibrated by usingfractions of known MW (standardmonodispersed PS samples).

    A calibration curve can then beconstructed by plotting the log of theMW against the elution volume for thesesamples. Once calibrated, the instrumentcan be used to determine the MWdistribution of an unknown sample.

    Separation by Size Exclusion inGPCThe volume of the solvent contained in a GPC system from point of injectionto the point of concentration detection can be considered as the sum of avoid volume Vo (volume of solvent outside the porous beads) and an internalvolume Vi (volume of solvent inside the porous beads).

    The volume of solvent required to elute a particular polymer species fromthe point of injection to the detector is known as its elution volume Ve asfollows,

    Where Kse is the fraction of the internal pore volume penetrated by certainpolymer molecules.

    For small polymer molecules that can penetrate all of the internal voulmeKse=1 and therefore

    For very large polymer molecules that are unable to penetrate the pores Kse=0, and therefore

    i seoe V K V V

    ioe V V V

    oe V V

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    Separation by Size Exclusion inGPC

    SEC data obtained for apolydisperse, linear, atacticpolystyrene sample appears as thedistribution shown in Fig.If we cut the curve into narrow slicesof equal increments of elution volume, V, the polymer eluting in each ofthese elements may be regarded asmonodisperse and the total area ofthe curve can be regarded as the sumof the heights of the individual slices.The weight fraction of any slice is thengiven as; i

    ii

    h

    hw

    Calculation of MW Distribution inGPCThe MW of the ith species is then obtained from the calibrationcurve at point Vi and the MW averages can be calculated as;

    Polymers of the same MW, butof a different architecture, will elutefrom SEC column at different times.

    For instance, linear PS will beeluted at a different time to astar-shaped PS .

    iiw

    i

    i

    n

    M w M

    M

    w M

    1

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    GPC:Hydrodynamic Volume

    Calculating the MW distribution and MWaverages is valid as long as monodisperse orvery narrow molecular weight standards ofthe same polymer are available.This is because the size of a molecule insolution, itshydrodynamic volume dependsupon the nature of the polymer/solvent pairand the temperature.A linear polystyrene with the same number ofsegments as a star-shaped sample occupies alarger hydrodynamic volume.

    GPC:Hydrodynamic VolumeFractionation of molecules in SEC is governed by hydrodynamicvolume rather than by molecular weight.The largest polymers in the solution cannot penetrate the poreswithin the cross-linked gel beads, and so they will elute first asthey are excluded and their retention volume is smaller.

    The smallest polymer molecules in the solution are retained inthe interstices (or the voids) within the beads, and so requiremore time to elute as their retention volume is larger.

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    Column Packing

    Different types

    Semi-rigid, cross-linked macromolecular polymersRigid, controlled-pore-size glasses or silica

    Semi-rigid polymers:-these materials swell slightlycare must be taken during uselimited to a maximum pressure of 300 psi due to bedcompressibility

    e.g. styrene divinylbenzene polymers (for compounds of MWrange of 100-500 million) & suspension polymerization of 2-hydroxyethyl methacrylate with ethylene dimethacrylate (canwithstand pressure upto 3000 psi)