Poly polymer tacticity

A complete quantitative description of the thermodynamics of polymer-polymer solutions also might need to include the effects of polymer tacticity. As demonstrated recently by Schurer et al. (50), changing the stereo configuration of poly (methyl methacrylate) from isotactic to syn-diotactic causes it to become miscible with PVC. These results suggest the importance of the spatial articulation of interacting segments in the polymer. 

Fig. 14 3D- H, C, F correlation of poly(l-chloro-l-fluoroethylene) with slices at 5( F) = -98.2 (a), -99.4 (b), -100.7 (c) (d) schematic illustration of the 3D spectrum with relative positions of the shoes. Each shce in (a)-(c) displays two correlation signals representing the two CHj groups adjacent to a CF moiety. The polymer tacticity can be determined as follows the splitting of each signal reveals the stereochemistry of individual CFCI-CH2-CFCI diads, the two protons at a methylene carbon being chemically inequivalent in meso (m) and chemically equivalent in racemic (r) diads. Consequently, the fluorine atoms can be assigned to rr (a), rm (b) or mm triads (c). The possible tetrad structures can be determined by looking for identical C-H cross-peaks in different F slices e.g. the A cross-peaks in (a) do not occur in the other two sUces therefore, the methylene group A shows only correlations with F atoms in mm triads and must be centred in an mmm tetrad, etc. Reproduced, with permission, from ret 100. Copyright 1996 American Chemical Society.

Representative poly( 1-pentene) tacticity data is provided in Table 4.4 representative polypropylene tacticity data is provided in Table 4.2. Poly(pentene) was produced using neat 1-pentene poly(propylene) was produced using neat propylene or a 50/50 (v/v) mixture with toluene. These scandocenes and yttrocenes provide atactic polymers. The tacticity values range from [r] = 28.62% to [r] = 52.12% for poly( 1-pentene) samples, and from [r] = 48.77% to [r] = 61.13% for poly(propylene) samples. Within experimental error, polymerization temperature and monomer dilution appear to exert a minimal effect on polymer tacticity. 

Poly(7-MeNBD) prepared with 10b and 11a contains between 47% and 52% trans C=C units, which are atactic. It is reported that the cis sequences in these samples comprise more r than m dyads. A model has been proposed suggesting that the change in polymer tacticity from syndiotactic to atactic is due to switching from a mode of coordination of the previously formed C=C double bond (similar to structure E in Scheme 20.15) to a mode of noncoordination. 

The syndiotactic-rich polymer, poly[3-(2,2,2-triphenylacetylamino)prop-ionic acid] (syndfo-poly[( -Ala-OH)-MA]), and the isotactic-rich polymer, (/so-poly[(j0-Ala-OH)-MA]), were examined to investigate the effect of polymer tacticity involving the side chain of the ligand upon CaCOs crystallization (Fig. 20). The diad contents for sy/7dm-poly[(y0-Ala-OH)-MA and zso-poly[(j0-Ala-OH)-MA] were determined as 77% and 70%, respectively, determined by the NMR spectra in solution of the ester-formed polymers. An extremely broad amide NH H-NMR signal for deproto-... 

The photodegradation of poly(alkylacrylate)s and poly(methacrylate)s under UV irradiation (248 nm) in solution was studied for the first time by TR EPR by Harbron et Well-resolved spectra of oxo-acyl radicals from the ester side chain and of main-chain polymeric alkyl radicals were used to show the side-chain cleavage via the Norrish I process. The methacrylate spectra are strongly influenced by the stereoregularity of different polymer tacticity, the temperature and the solvent. The relations of these dependences on the conformational motion in the polymer chain are discussed. 

The better definition of spectra from crystalline polymers can give an indication of polymer tacticity since atactic polymers are generally non-crystalline and there are certain polymers such as poly(methyl methacrylate) and polypropylene in which specific absorption bands can be assigned to the presence of molecules with particular types of tacticity. Some idea of the tacticity for samples of these polymers can, therefore, be obtained from measurements of the strength of the absorption of the relevant bands, but in general this method is not as accurate as n.m.r. for tacticity determination. 

Polymer tacticity is the relative arrangement of side-chains with respect to the macromolecular C—C backbone (Figure 1). Tacticity is an important aspect of microstructure that has a dramatic influence on many of the polymer s physical properties including melting point, mechanical strength, and crystallinity, as well as its ability to assemble into ordered nanostructures. For instance, the melting point of atactic poly(propylene) (<0 °C) is more than 130 °C lower than either the isotactic (165 °C) or syndiotactic form... 

Del Vdlano L, KeUand MA, Miyake GM, Chen EYX. Effect of polymer tacticity on the performance of poly(Af,Af-dialkylacrylamide)s as kinetic hydrate inhibitors. Energy Fuels. 2010 24 2554-2562. 

Figure 7.10 Nuclear magnetic resonance spectra of three poly(methyl methacrylate samples. Curves are labeled according to the preominant tacticity of samples. [From D. W. McCall and W. P. Slichter, in Newer Methods of Polymer Characterization, B. Ke (Ed.), Interscience, New York, 1964, used with permission.]...

In this stage of the investigation, poly(methyl methacrylates) (PMMAs) were selected as the polymeric probes of intermediate polarity. Polymers of medium broad molar mass distribution and of low tacticity (14) were a gift of Dr. W. Wunderlich of Rohm Co., Darmstadt, Germany. Their molar masses ranged from 1.6 X 10" to 6.13 X 10 g-mol. For some comparative tests, narrow polystyrene standards from Pressure Co. (Pittsburgh, PA) were used. 

Nuclear magnetic resonance has proved to be a valuable tool in determination of configurational sequences in poly(MMA) (14). In Figure 3 is shown the NMR of poly(MMA) synthesized with an anionic polymerization catalyst known to produce predominantly isotactic sequences. In these polymers, the NMR spectrum of the methylene units In the polymer backbone gives an unequivocal determination of tacticity. The methylene signal, occurring about 1.8... 

Iso tactic poly(methyl methacrylate) (it-PMMA) can form a stereocomplex with st-PMMA. Recent X-ray studies 179) of this material indicate that the two polymer chains probably interact to form a double helical structure. The it-PMMA chain forms the inner helix and is surrounded by the st-PMMA helical chain which winds around it. If subsequent work confirms this model, this material would constitute a most unusual inclusion compound involving only one monomeric substance. 

Polymers that incorporate steric centers into their backbones can display various types of tacticity. The three principal types of tacticity are isotactic, syndiotactic, and atactic, as illustrated in Fig. 1.8 for polypropylene. Other polymers that display tacticity include polystyrene and poly a-olefins,... 

The tacticity or distribution of asymmetric units in a polymer chain can be directly determined using NMR spectroscopy and infrared (IR) spectroscopy and has been studied for a variety of polymers. Figure 5(a) and 5(b) show the proton NMR spectra [26,27] and IR spectra [28,29], respectively, for the two stereoisomers of poly(methyl methacrylate) (PMMA), syndiotactic and isotactic PMMA. These two structures in a polymer like PMMA give rise to different signatures in both the techniques. In the case of the NMR spectra [26,27], the... 

To clarify the tacticity problem, trans-l,4-hexadiene and 5-methyl-l,4-hexadiene polymers were examined by X-ray diffraction. Fiber diagrams were obtained from samples stretched to four times their original lengths. Eight reflections from the poly(trans-1,4-hexadiene) fiber pattern may be interpreted on the0basis of a pseudo-orthorhombic unit cell with a = 20.81 + 0.05 A b =... 

Further confirmation of the structure and tacticity of poly/5-methyl-l,4-hexadiene)was obtained from X-ray diffraction and u-NMR data of its hydrogenated polymer (Scheme 2). The hydrogenated polymer sample showed a highly crystalline pattern (Figure 7), with diffraction spots that were well defined. This pattern was identical to that of isotactic poly(5-methyl-l-hexene) as reported in the literature (26) (measured identity period, 6.2 A lit., 6.33 A). 

Stereoselective ROMP has been reported with monomer (213). Initiator (211) affords highly stereoregular polymer with >98% trans C=C bonds in the polymer backbone.534 However, when (210) is used, >98% cis-poly-(213) is obtained.535 A similar situation occurs for the diester monomer (214). Furthermore, a rapidly equilibrating mixture of (210) and (211) can be used to allow intermediate cis/trans contents to be manipulated by the stoichiometry of the initiator mixture. 13C NMR536 and dielectric analyses537 suggested that trans-poly-(213) is highly syndio-tactic (92% r dyad content). The ROMP of other fluorinated olefins has been recently reviewed.538... 

A third factor influencing the value of Tg is backbone symmetry, which affects the shape of the potential wells for bond rotations. This effect is illustrated by the pairs of polymers polypropylene (Tg=10 C) and polyisobutylene (Tg = -70 C), and poly(vinyi chloride) (Tg=87 C) and poly(vinylidene chloride) (Tg =- 19°C). The symmetrical polymers have lower glass transition temperatures than the unsymmetrical polymers despite the extra side group, although polystyrene (100 C) and poly(a-meth-ylstyrene) are illustrative exceptions. However, tacticity plays a very important role (54) in unsymmetrical polymers. Thus syndiotactic and isoitactic poly( methyl methacrylate) have Tg values of 115 and 45 C respectively. 

The configurational average in eqn (10) can also be performed (8) with eqn (1). In table III we present the results of our calculations (8) of eqn (10) for poly(p-nitrostyrene) versus the tacticity of the polymer. No experimental EFSHG results have been reported for... 

The measurement of polymer configuration was difficult and sometimes speculative until the early 1960 s when it was shown that proton NMR could be used, in several instances, to define clearly polymer stereochemical configuration. Bovey was able to identify the configurational structure of poly(methylmethacrylate) in terms of the configurational triads, mm, mr and rr, in a classic example (3). In the case of polypropylene, configurational information appeared available but was not unambiguously accessible because severe overlap complicated the identification of resonances from the mm, mr and rr triads (4). Several papers appeared on the subject of polypropylene tacticity but none totally resolved the problem (5). 

The poly(propylene oxide) obtained at low conversion has an enantiomeric purity of around 30%, is not homogeneous from the point of view of stereoregularity (or stereoselectivity), and can be separated into fractions of different tacticity and optical activity, the latter being greater for polymers of higher stereoregularity. 

The same type of addition—as shown by X-ray analysis—occurs in the cationic polymerization of alkenyl ethers R—CH=CH—OR and of 8-chlorovinyl ethers (395). However, NMR analysis showed the presence of some configurational disorder (396). The stereochemistry of acrylate polymerization, determined by the use of deuterated monomers, was found to be strongly dependent on the reaction environment and, in particular, on the solvation of the growing-chain-catalyst system at both the a and jS carbon atoms (390, 397-399). Non-solvated contact ion pairs such as those existing in the presence of lithium catalysts in toluene at low temperature, are responsible for the formation of threo isotactic sequences from cis monomers and, therefore, involve a trans addition in contrast, solvent separated ion pairs (fluorenyllithium in THF) give rise to a predominantly syndiotactic polymer. Finally, in mixed ether-hydrocarbon solvents where there are probably peripherally solvated ion pairs, a predominantly isotactic polymer with nonconstant stereochemistry in the jS position is obtained. It seems evident fiom this complexity of situations that the micro-tacticity of anionic poly(methyl methacrylate) cannot be interpreted by a simple Bernoulli distribution, as has already been discussed in Sect. III-A. 

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