STRUCTURE OF MACROMOLECULES

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1 STUTUE F MMLEULES Introduction Life is polymeric in its essence: the most important component of living cell (proteins, carbohydrates and nucleic acids) are all polymers. Nature uses polymers both for construction and as part of complicated cell machinery. 1 In definition, a polymer is a substance consisted of macromolecules. It is well known, that all secrets and mystery of life are hidden in an extremely complex structure of natural macromolecules DN and N ones. owever, besides those of natural origin, humans know nowadays synthetic macromolecules, which are produced in polymerization processes to form polymers and plastics, i.e. polymer-based artificial materials, usually destined to substitute natural ones. oth natural and synthetic polymers exhibit a strong structureproperties relationship and theretofore a knowledge of structure of macromolecules is of a great importance in modern polymer chemistry and technology. This chapter will focus on microstructure of a synthetic polymer and its determination by NM spectroscopy. Some general considerations In general, we can distinguish a molecular structure of polymers, i.e. structure of individual macromolecules (intermolecular structure, microstructure), and supermolecular structure of polymers, i.e. positioning, ordering and interactions of macromolecules in a polymeric material (intermolecular structure, macrostructure, morphology, texture, etc.). To describe fully molecular structure of a polymer, we have to consider: - molecular weight and molecular weight distribution; - chemical composition; - shape of macromolecules; - regioselectivity (enment, entailment, sequentional isomerism); - configuration of a polymer ; - conformation (microconformation in a short parts of a ; macroconformation of a whole macromolecule). Molecular weight and molecular weight distribution are obviously very important features of all polymeric materials. Usually, to estimate them, GP and MLDI-TF techniques are employed. Unless polycondensates are concerned, chemical composition of a polymer is identical with that of initial monomer. owever, composition of a copolymer must not necessarily be the same as that of a comonomers mixture. 1 NM spectroscopy and I spectrophotometry are most useful methods to determine above. The most important structural features of polymer molecules are given in subchapters below. Shape of macromolecules Polymers may consist of individual, separate macromolecules, or may be crosslinked, i.e. macromolecules are interconnected to form a network. single macromolecule is usually linear or branched, sometimes cyclic. Some special shapes of macromolecules are known as well: ladder, star, comb, brush and dendrimer ones. 1 Igor Y.Galaev, o Mattiasson, Tibtech ugust 1999, vol.17,

2 linear branched comb - like crosslinked cyclic ladder brush - like star dendrimer Figure 1. Typical shapes of macromolecules. egioselectivity egioselectivity (sequentional isomerism) is the directionality of addition along a polymer and relates to regular and irregular polymers. For instance, poly(vinyl chloride), (- 2 l-) n, may have regular, i.e. head to tail : l- 2 -l- 2 -l-... or irregular one, where additionally head to head and tail to tail linkages exist: l-l l-... In most systems the head to tail configuration is strongly preferred due to steric reasons. When head to head inversions appear, the structure of a macromolecule becomes quite complex since regioirregularity superimposes on tacticity. This happens most frequently for polymerisation of vinyl fluoride and vinylidene fluoride, sometimes in other vinyl polymers; usually in an extent of 1-2%. Microstructure of a synthetic polymer onfiguration of a polymer is considered when sites of stereoisomerizm are present and here the term microstructure is often used. Those are usually chiral (prochiral) carbon atoms or carbon-carbon double bonds. The latter case concerns polydienes, where cis-1,4, trans-1,4, 1,2 and sometimes 3,4 isomeric configurational units are formed, dependly on the catalyst used. The most rich variety of structural features of a polymer exist when chiral carbon atoms are present. Let us consider a polymer formed via polymerization of a monomer having a general structure 2 = 1 2 to yield a general structure of a ( ) n. 2 is often hydrogen and this corresponds to a wide group of important industrial polymers, as polypropylene, polystyrene, poly(meth)acrylates, vinyl polymers, etc.. The substituents may assume two different positions along the fully extended, as demonstrated in figure 2.

3 the m (meso) diad the r (racemo) diad Figure 2. The structure of two configurational diads in a polymer of ( ) n type. The carbon backbone is situated at the plane of the figure, whereas the substituents are either in front of (thickened lines) or behind (dotted lines) the plane. Thus, the segment may assume two possible configurations, denoted in polymer chemistry as meso (m) and racemo (r) diads, i.e. the simplest possible stereochemical (configurational) sequences. This m/r notation derives from the terms used originally in describing the chirality of molecules (it must be noticed that the 1 2 carbons become chiral, or rather pseudochiral when hybridization changes from sp 2 to sp 3 during conversion of monomeric molecules to a polymer ). In meso diad both chiral carbons have the same configuration (i.e. or SS) whereas in racemo diad configuration alters (S or S). If the consists exclusively of the carbons bearing the 1 and 2 substituents in the same configuration, i.e. all the diads are the m ones, the polymer is termed an isotactic one. n the other side, when the configuration alters regularly along the, i.e. all the diads are the r ones, a polymer is syndiotactic. In most cases, configuration changes more or less randomly in individual s and such a polymer is termed an atactic one. oth m and r diads are present along the in an amount depended on the polymerisation mechanism. ccording to the ernoulian statistics (totally random ), the fractions of both the diads are equal to 0.5. If we consider a set of three monomeric units in the same polymer, i.e. the triad (consisting of two diads), four possible configurational arrangements are allowed, as it is shown in a schematic form in figure 3. mm triad (isotactic) mr = rm triad (heterotactic) rr triad (syndiotactic) Figure 3. The structure of configurational arrangements of triads in a polymer of ( ) n type. It is evident that stereoregular isotactic polymers have mm (isotactic) triads only along the whereas syndiotactic ones have rr (syndiotactic) triads. In atactic polymers additional mr=rm (heterotactic) triads are allowed; all the possible triads are distributed more or less randomly along a. gain, for an ideally random probabilities are equal for each triad and amount to 0.25.

4 fragment of a consisting of three diads is a tetrad (mmm, mmr, mrr, rmr, etc.) whereas four diads form pentads (mmmm, mmrr, rmrm, rrrr, etc.), higher stereosequences are hexads, heptads, octads, etc.. It is well known in polymer chemistry that NM spectroscopy is an unique and powerful tool to investigate the structure of a polymer. oth in particular diads and triads, as well as in longer sequences, nuclei ( 13, 1 and others) experience different chemical shift in NM spectra due to different shielding/deshielding effect of neighbouring atoms and groups of atoms. Thus, usually we observe two separate signals of diads for nuclei of 2 group and three triads signals for those of 1 2, the latter is illustrated in figure 4. Due to equivalency of heterotactic triads (mr=rm), three triad signals are observed for the methyl group signal in poly(vinyl acetate): mm, mr+rm, rr. Since the in this polymer is not ideally random, the intensities deviate to some extent from 1:2:1 ratio Figure 4. fragment of 1 NM spectrum of poly(vinyl acetate) - three lines of a 1:2:1 intensity ratio of the signal of the methyl group correspond to the four possible configurational triads where the heterotactic triads are equivalent (mr=rm). high-field side triad signal is split slightly due to pentad sensitivity. owever, when a repeating unit as above is separated by ether or ester bond, etc., (polyepoxides, some polyesters, etc.), mr triad is not equal to rm one and four corresponding signals can be observed, as it is seen in figure 5 for the signal of the methine carbon in poly(propylene oxide), which is split into four equal peaks corresponding to four possible triads Figure 5. fragment of the proton-decoupled 13 NM spectrum of poly(propylene oxide) - four lines of equal intensity of the methine carbon correspond to the four possible configurational triads.

5 number of stereosequences in a polymer increase infinitely as length of a sequence is extended. limitation lays just in capability of instrumentation provided nowadays to detect the signals. Modern NM spectrometers work usually at the magnetic field of up to 21.1 T, routine machines at 7.0 to 11.7 T. The latter enables observation of signals of sequences consisting of up to 8 9 repeating units, i.e. octads to nonads, in some polymers. Usually, carbon spectra enable more deep view at the polymer microstructure due to a broad chemical shift scale in respect to proton ones. Figure 6 presents an example of some more sophisticated microstructural analysis 2. The signal of carbonyl carbon in poly(methyl methacrylate) exhibits sensivity to odd sequences being split into several sublines which were assigned to respective pentads and partially to heptads. mmrrrr rmrrrr rmrrrm mmrrrm mrrrrr rrrrrr mmrm + rmrm rmrrmr mmrrmr mmrrmm mmrr + rmrr mmmm rmmm rmmr ppm Figure 6. The carbonyl region of the proton-decoupled 13 NM spectrum of poly(methyl methacrylate) with assignments of configurational sequences at the pentad (right part) and heptad (left part) level. number of possible lines in the respective spectra depends directly on the sequence length and on the structural features of a. For common polymers having the structure as in figure 2, a number of observable sequences consisting of n diads, N obs (n), is: N obs (n) = 2 n m - 1 (1) where: m = n/2 when n is even, m = (n + 1)/2 when n is odd When n = 2, for instance, we have 3 possible triads, 6 tetrads for n = 3, and so on. In hypothetic, relatively small, oligomeric molecule of PMM having MW = 1000, i.e. the containing 10 repeating units, 528 sequences (decads) are possible. (Usually, this polymer has MW in order of hundreds of thousands to milions). In polymers having the structure of repeating unit as 2 1 2, or 2 1 2, amount of possible sequences is higher due to spectroscopic nonequivalence of some of them, as mr rm, mmr rmm, etc.. n the other hand, microstructure of a polymeric and a number of sequences allowed may be governed by the structure of a monomer itself, e.g. as in the case of polylactides. 2 The data obtained by courtesy of prof. Marek Matlengiewicz from the Polish cademy of Science.

6 Monomeric lactide has two assymetric centers and if both they are the same (e.g. L-lactide both chiral carbons have S configuration), the polymer retains the same configuration along whole the macromolecule. The formed is a fully isotactic one and single signals are observed in the NM spectra as it is shown in figure 6a. owever, when racemic lactide, i.e. 1:1 mixture of L- and D-enantiomers was polymerized (S,S and, respectively), formation of the is governed by so-called pair-addition ernoulian statistics, where always double S and double must repeat along a (e.g. SSSSSSSS) and some sequences are forbidden, as S, SSSS, etc.. number of hexads allowed, for instance, is limited to 11, as it is shown in figure 7 (maximal number for this type of polymer is 32, as specified in table 1). b) a) Figure 7. arbonyl region of the proton-decoupled 13 NM spectra of polylactides: (a) poly (L-lactide) - pure isotactic polymer, one line is observed; (b) atactic polymer of racemic lactide - 11 lines of 11 possible hexads are visible. Notice that only one hexad experiences the same chemical shift as isotactic polymer and this is mmmmm hexad (denoted often as iiiii hexad in that type of polymers) drastic increase in an amount of possible sequences occurs in copolymers, say composed of and monomeric units. The sequences are termed copolymeric or compositional ones (diads:,, ; triads:,, etc.) and are distinguishable in NM spectra for the same reasons as in homopolymers exhibiting configurational isomerism. In simplest case, if both and are of the type and 1 = 2, there is no chiral centers in the backbone and thus no configurational sequences. number of copolymeric sequences composed of n monomeric units is given by: N obs (n) = 2 n k - 1 (2) where: n = 2k (even); n = 2k - 1 (odd) When both the comonomers form chiral centers in the, i.e. 1 differs from 2 in both and, N obs (n) is given by: N obs (n) = 2 n + 2 2n-2-2 n-1 (3) Each compositional diad may be meso or racemo and thus six of these simplest sequences are possible. Figure 8 presents a half of 20 allowed arrangements at the triads level.

7 ompositional triads onfigurational triads mm mr rr Figure 8. Ten possible compositional/configurational triads. If one alters with, total number of sequences will be 20. Table 1 specifies numbers of sequences up to hexads to be observed in NM spectra for particular types of polymers discussed above. The highest variety of possible sequences is offered by the equation (3). The latter implies that in an oligomeric composed of 10 repeating units - 5 of type and 5 of type, both having chiral centre, and assuming ideally random distribution, amount of possible sequences (decades) is What will be in the case of a terpolymer composed of, and units? Table1. Numbers of configurational/copolymeric sequences in homopolymers and copolymers. The type of Number of sequences a polymer diads triads tetrads pentads hexads homopolymer ( ) n homopolymer ( ) n or( ) n poly(d,l-lactide) copolymer of, ; both of ( ) n type = 2 copolymer as above, onformation of macromolecules The most privileged conformation of two subsequent atoms in a of a polymer having general structure (- 2 -) n is the trans one (T). It is characteristic for polyethylene in crystalline state. owever, substituents and pendant groups (), especially bulky ones, force some distorsion to minimize interactions and in consequence gauche conformations (G + and G - ) exist as well.

8 2 2 2 trans (T) gauche + (G + ) gauche - (G - ) Figure 9. Newman projection of basic conformations of a set of neighbouring atoms in a polymer. If we consider three neighbouring atoms, e.g. forming configurational diads, a number of possible conformational arrangements is possible. Some of them are favorized due to minimal interactions of substituents and fragments whereas some are unprivileged due to steric reasons. meso diad conformation racemo diad T T T G + T G - G + G - G + G + Figure 10. Possible conformers of m and r diads. In the case of stereoregular polymers, repeating G conformation results in formation of a helical macroconformation of a macromolecule, especially in a solid state. In crystalline polymers a folded macroconformation is also frequent whereas rod-like one occurs in liquidcrystalline systems. In solution, when a good solvent is employed, macromolecules exist in unperturbed conformation of a whole. ddition of a non-solvent ( poor solvent) causes them to assume a globular form and, finally, to precipitate into a solid.

9 unperturbed rod like folded globular helical Figure 11. asic macroconformations of linear macromolecules. Usually, regioselectivity, stereoregularity and tacticity favour formation of crystalline phase in polymers. In atactic polymers and statistic copolymers, sequence distribution along a have considerable effect on properties as well. Theretofore, some basic knowledge on microstructure of a polymer is sometimes useful when working on use properties of polymeric materials. eferences eatley, F., 1993, Introduction to NM and its use in the study of polymer stereochemistry. In. N. Ibbett (ed.) NM Spectroscopy of Polymers (Glasgow: lackie cademic & Proffesional; n Imprint of hapman & all), pp Mark,. F., Gaylord, N. G. (eds), 1968, Encyclopaedia of Polymer Science and Technology. Vol. 8, (New York: Interscience Publishers / John Wiley & Sons), p.760. Klepser, E., Sielaff, G., 1974, igh esolution Nuclear Magnetic esonance Spectroscopy. In: D.. ummel (ed.) Polymer Spectroscopy (Weinheim: Verlag hemie), pp ero, M., Kasperczyk, J., Jedliński, Z. J., 1990, oordination polymerization of lactides, 1 Structure determination of obtained polymers. Makromolekulare hemie, 191, randolini,. J., ills, D. D., 2000, NM Spectra of Polymers and Polymer dditives. (Marcel Dekker, Inc.)

PHYSICS OF SOLID POLYMERS

PHYSICS OF SOLID POLYMERS PYSIS OF SOLID POLYMERS Professor Goran Ungar WU E, Department of hemical and Biological Engineering Recommended texts: G. Strobl, The Physics of Polymers, Springer 996 (emphasis on physics) U. Gedde,