Polarity, isotacticity

Tacticity of products. Most solid catalysts produce isotactic products. This is probably because of the highly orienting effect of the solid surface, as noted in item (1). The preferred isotactic configuration produced at these surfaces is largely governed by steric and electrostatic interactions between the monomer and the ligands of the transition metal. Syndiotacticity is mostly produced by soluble catalysts. Syndiotactic polymerizations are carried out at low temperatures, and even the catalyst must be prepared at low temperatures otherwise specificity is lost. With polar monomers syndiotacticity is also promoted by polar reaction media. Apparently the polar solvent molecules compete with monomer for coordination sites, and thus indicate more loosely coordinated reactive species. 

In order to generate stereoregular (usually isotactic) polymers, the polymerization is conducted at low temperatures ia nonpolar solvents. A variety of soluble initiators can produce isotactic polymers, but there are some initiators, eg, SnCl, that produce atactic polymers under isotactic conditions (26). The nature of the pendant group can influence tacticity for example, large, bulky groups are somewhat sensitive to solvent polarity and can promote more crystallinity (14,27). 

Substituents on the a-carbon atom restrict chain flexibility but, being relatively small, lead to a significantly higher Tg than with polyethylene. Differences in the Tg s of commercial polymers (approx. 104°C), syndiotactic polymers (approx. 115°C) and anionically prepared isotactic polymers (45°C) are generally ascribed to the differences in intermolecular dipole forces acting through the polar groups. 

Figure 8.1. (a) Spherulites growing in a thin film of isotactic polystyrene, seen by optical microscopy with crossed polars (from Bassett 1981, after Keith 196.3). (b) A common sequence of forms leading to sphertililic growth (after Bassett 1981). The fibres consist of zigzag polymer chains. 

Specialty waxes include polar waxes for more polar adhesive systems. Examples would be castor wax (triglyceride of 12-hydroxy stearic acid) or Paracin wax N- 2 hydroxy ethyl)-12-hydroxy stearamide) which are used in polyester, polyamide, or with high VA EVA copolymer-based systems. Other common polar waxes are maleated polyethylenes, which are used to improve the specific adhesion of polyethylene-based adhesives, and low molecular weight ethylene copolymers with vinyl acetate or acrylic acid, which are used to improve low temperature adhesion. High melting point isotactic polypropylene wax (7 155°C) and highly refined paraffin wax (7,n 83°C) are used where maximum heat resistance is critical. Needless to say, these specialty waxes also command a premium price, ranging from 2 to 5 times that of conventional paraffin wax. 

Problem 31.5 Polymers such as polypropylene contain a large number of chirality centers. Would you therefore expect samples of isotactic, syndiotactic, or atactic polypropylene to rotate plane-polarized light Explain. 

Figure 6 Spherulites of isotactic poly-l-butene (a, during growth) and of polyethylene (b, after completion) by optical microscopy (OM) under crossed polars. Reproduced from Ref. [3] with permission of John Wiley Sons, Inc.

Characteristic initiation behavior of rare earth metals was also found in the polymerization of polar and nonpolar monomers. In spite of the accelarated development of living isotactic [15] and syndiotactic [16] polymerizations of methyl methacrylate (MMA), the lowest polydispersity indices obtained remain in the region of Mw/Mn = 1.08 for an Mn of only 21 200. Thus, the synthesis of high molecular weight polymers (Mn > 100 x 103) with Mw/Mn < 1.05 is still an important target in both polar and nonpolar polymer chemistry. Undoubtedly, the availability of compositionally pure materials is a must for the accurate physical and chemical characterization of polymeric materials. 

A bulky methacrylate, triphenylmethyl methacrylate (TrMA), is a unique monomer which gives an almost 100% isotactic polymer in anionic polymerization with n-butyllithium both in nonpolar and polar solvents. Moreover, even free-radical polymerization affords a highly isotactic polymer from this monomer.23 The isotactic specificity of TrMA polymerization is ascribed to the helical formation of the main chain. When TrMA is polymerized in toluene at —78°C... 

This production of stereoregular structures has been known for sometime and is especially strong for vinyl ethers. Several general observations have been noted. First, the amount of stereoregularity is dependent on the nature of the initiator. Second, stereoregularity increases with a decrease in temperature. Third, the amount and type (isotactic or syndiotactic) are dependent on the polarity of the solvent. For instance, the isotactic form is preferred in nonpolar solvents, but the syndiotactic form is preferred in polar solvents. 

As seen, the anionic and cationic polymerizations are analogous differing mainly on the nature of the active species. The stereochemistry associated with anionic polymerization is also similar to that observed with cationic polymerization. For soluble anionic initiators at low temperatures, syndiotactic formation is favored in polar solvents, whereas isotactic formation is favored in nonpolar solvents. Thus, the stereochemistry of anionic polymerizations appears to be largely dependent on the amount of association the growing chain has with the counterion, analogous with the cationic polymerizations. 

The effect of heterogeneous nucleation on the crystallization of isotactic polypropylene from the melt can be easily established as follows. A small amount of powdered polypropylene is well mixed with about 0.1 wt% of sodium benzoate in a mortar or by means of an analytical mill. Some of the mixture is transferred with a spatula to a microscope slide and melted at about 250 °C on a hot block. A cover slip is pressed on to the melt with a cork to obtain as thin a film as possible.The sample is held at 200-250 °C for some minutes and then allowed to crystallize at about 130 °C on the hot stage of the microscope an unadulterated polypropylene sample is crystallized in the same way. Both samples are observed under a polarizing microscope during crystallization,the difference in spherulite size between nucleated and untreated polypropylene can be seen very clearly. An ordinary microscope can also be used by placing polarizers on the condenser and eyepiece, and adjusting these to give maximum darkness. 

For example, it is possible to synthesize isotactic as well as syndiotactic polypropylene in high configurational purity and high yields. The same holds for syndiotactic polystyrene. Furthermore, metallocene catalysts open the possibility to absolutely new homopolymers and copolymers like, e.g., cycloolefin copolymers (COG) and even (co)polymers of polar monomers.The simplest metallocene catalyst consists of two components. The first one is a n-complex (the actual metallocene) that can be bridged via a group X and therefore can become chiral ... 

Many radical polymerization systems were examined hy Challa at a/. A number of papers were devoted to polymerization of methyl methacrylate, MM, in the presence of poly(methyl methacrylate). The role of tacticity of template was stressed. Isotactic PMM forms stereocomplex with syndiotactic PMM. Polymerizations of MM in the presence of isotactic PMM were carried out and the product (template + daughter polymer) was analyzed by NMR. When MM is polymerized in the polar solvent in the presence... 

The analysis shown in Figure 1 is however incomplete. The medium is a three-component system consisting of two polar components (monomer and THF) and one non-polar (toluene). Correlations in terms of two components only lead to incomplete characterization and, if extrapolated, invalid conclusions. In Figure 3 the meso (isotactic) dyad frequencies of polymers produced by t-butylmagnesium bromide at 225 are shown as a function of the mol fractions of the three components of the medium. It is apparent that monomer itself acts as a polar solvent component decreasing the isotactic content in comparison to toluene. It should be noted that the polymerizations carried out at =... 

Clustering (association or aggregation) as a result of dissimilar parts within one chain molecule is well-known in the field of biopolymers a recent example in the field of synthetic polymers is provided by poly (p-nitrophenylmethacrylate) which in dimethylformamide (105) forms threefold clusters, presumably because of the difference in polarity of backbone and ester groups. Liquori (118) has presented proof for a double-stranded helix of isotactic and syndiotactic polymethylmethacrylate. 

In the least polar solvents, when presumably largely tight ion pairs are present, Aso found for both a-methylstyrene (29) and isobutyl vinyl ether (28), that the degree of isotacticity was a maximum, with monomer attack being via the back side. As the solvent polarity increased and presumably the ion pair tightness decreased, possibly with some dissociation to free ions, back side attack becomes less favourable and isotactidty falls. 

Some information is available on other acrylates. N,N-disubstituted acrylamides form isotactic polymers with lithium alkyls in hydrocarbons (12). t-Butylacrylate forms crystallizable polymers with lithium-based catalysts in non-polar solvents (65) whereas the methyl, n-butyl, sec-butyl and isobutyl esters do not. Isopropylacrylate also gives isotactic polymer with lithium compounds in non-polar solvents (34). The inability of n-alkylacrylates to form crystallizable polymers may result from a requirement for a branched alkyl group for stereospecific polymerization. On the other hand lack of crystallizability cannot be taken as definite evidence of a lack of stereoregulating influence, as sometimes quite highly regular polymer fails to crystallize. The butyllithium-initiated polymers of methylmethacrylate for instance cannot be crystallized. The presence of a small amount of more random structure appears to inhibit the crystallization process1. 

Braun, Herner, Johnsen and Kern (19) have also studied the polymerization of methylmethacrylate by alkylmetals. The amount of isotactic structure decreased in the series alkyllithium > alkyl-sodium > alkylpotassium. The use of polar solvents also decreased the amount of isotactic structure (see Table 3). 

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