Tacticity

The tacticity of polymers has important effects on their physical structure and properties. Atactic polymers usually are amorphous unless the side group is extremely polar and allows some crystalhnity. On the other hand, both isotactic and syndiotactic stractures can crystalize due to their regularity along the polymer 

For instance, the Tg of PP depends strongly on its tacticity. Atactic PP has a Tg—19 C and is used in bitumen and hot-melt adhesive formulations, while for isotactic PP, it is approximately -8°C. The latter is a tough, semicrystalline plastic used in packaging films. The Tg of syndiotactic form of PP can be as high as ISO C Sophisticated catalyst systems even aUow synthesis of polymers that have ordered blocks of atactic materials and isotactic materials on the polymer chains. 

FIGURE 3 Illustration of the stereochemistry in a vinyl polymer. Below each structural formula is an illustration of the stereochemistry with a ball and stick model for polypropylene. 

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. L8 for polypropylene. Other polymers that display tacticity include polystyrene and poly a-olefins. 

When adjacent monomers in a backbone share the same stereoconfiguration, the placement is kno vn as a meso diad. When adjacent monomers have opposing stereoconfigurations, the placement is known as a racemic diad. Thus, a purely isotactic polymer comprises all meso placements, and a syndiotactic polymer consists of all racemic placements. 

In practice, the monomers comprising a polymer are never arranged in a purely isotactic or syndiotactic fashion. All stereoregular polymers contain some misinsertions. The polymerization catalyst and polymerization conditions control the level of stereoregularity. 

In iso tactic polymers the configuration of the steric centers on the backbone is identical. The net result is that all side groups are positioned on the same side of the chain, as illustrated schematically in Fig. 1.8 a). 

Stereodefects reduce the overall regularity of an isotactic polymer chain and hinder its ability to crystallize. As the concentration of defects increases, the degree of crystallinity falls, resulting in reduced density, reduced melting temperatures, lower heat distortion temperatures, reduced modulus, and reduced yield stress. 

In a perfectly random, atactic polymer, the position of pendant groups varies randomly along the chain. Such a random distribution is said to be BemouUian or to be a zero-order Markov chain. There are also intermediate structures, such as hemiisotactic and isotactic stereoblock polymers, which have randomly occurring, short rims of isotactic or syndiotactic structures. A detailed explanation of tacticity is given by Koenig [34]. 

Isotactic and syndiotactic polymers can crystallize, while atactic polymers cannot. Polymers other than polypropylene that have tacticity include polystyrene, polyfvinyl chloride), and poly(methyl methacrylate). Thus there are crystalline and non-crystalline forms of these polymers. By use of a copolymer, the number of side groups, and thus the crystallinity can be precisely controlled. An important example is the copolymerization of ethylene and a higher alpha-olefin, such as butene or octane, to make linear low-density polyethylene (LLDPE), in which the crystallinity is governed by the fraction of comonomer incorporated into the chain. Tacticity can affect important physical properties such as the intrinsic viscosity and thus must be taken into accoimt in characterization methods such as gel permeation chromatography. 

Jones et al. [35] used small-angle neutron scattering to study the chain dimension of syndiotactic polypropylene and found that the s-PP chain is substantially larger than that of i-PP. This implies that the s-PP molecule is stiffer than that of i-PP, which results in significant differences in the rheological and thermodynamic behavior of the two forms [35, 36]. The effect of tacticity on rheological properties is discussed in Chapter 5. 

A parameter that describes the level of branching in a general way is the branching frequency A, which is the average number of branch points per 1000 backbone carbon atoms. This is 

However, for a complex branching structure, this parameter provides no information about the topology of the chains. The various distribution functions mentioned above are required for a hill description of such a structure. In various sections of this book, the effects of long-chain branching are discussed in several contexts. 

The Fischer projections show that isotactic placement corresponds to meso or m-place-ment for a pair of consecutive stereocenters. Syndiotactic placement corresponds to racemo (for racemic) or r-placement for a pair of consecutive stereocenters. The configurational 

Polymerizations that yield tactic structures (either isotactic or syndiotactic) are termed stereoselective polymerizations. The reader is cautioned that most of the literature uses the term stereospecific polymerization, not stereoselective polymerization. However, the correct term is stereoselective polymerization since a reaction is termed stereoselective if it results in the preferential formation of one stereoisomer over another [IUPAC, 1996]. This is what occurs in the polymerization. A reaction is stereospecific if starting materials differing only in their configuration are converted into stereoisomeric products. This is not what occurs in the polymerization since the starting material does not exist in different configurations. (A stereospecific process is necessarily stereoselective but not all stereoselective processes are stereospecific.) 

Stereoselective polymerizations yielding isotactic and syndiotactic polymers are termed isoselective and syndioselective polymerizations, respectively. The polymer structures are termed stereoregular polymers. The terms isotactic and syndiotactic are placed before the name of a polymer to indicate the respective tactic structures, such as isotactic polypro-pene and syndiotactic polypropene. The absence of these terms denotes the atactic structure polypropene means atactic polypropene. The prefixes it- and st- together with the formula of the polymer, have been suggested for the same purpose it-[CH2CH(CH3)] and st-[CH2 CH(CH3)] [IUPAC, 1966], 

Both isotactic and syndiotactic polypropenes are achiral as a result of a series of mirror planes (i.e., planes of symmetry) perpendicular to the polymer chain axis. Neither exhibits 

Polymerization of unsymmetric vinyl monomers generates asymmetric carbon centers within the polymer chain 

In order to describe the stereosequence of such chains, meso or m diads with identical and racemic or r diads with opposite configuration of succeeding asymmetric carbon atoms are defined  

Longer sequences are referred to as triads, tetrads, pentads, Taking the pentad as 

Seven other possible pentad sequences (mmmr, rrmr, mmrm, mmrr, mrmr, mrrm, and rmmr) are atactic. Residues R may be CH3 for polypropylene, C6H5 for polystyrene, Cl for polyvinyl chloride, CN for polyacrylonitrile, and C02CH3 for polyacrylic acid methyl ester. 

1 mmmmm 2 mmmmmr 3 rmmmmr 4 nmunmrr 5 mnmmvrm 6 rmmmrr  

Syndiotactic polymers are also sufficiently regular to crystallize, not necessarily as a helix but rath as glide planes. 

Branching in the side group tends to stiffen the chain and raise as shown in the series poly(but-l-ene), = 399 K poly(3-methyl but-l-ene), = 418 K and poly(33 -dimethyl but-l-ene), 593 K. If the side group is flexible and nonpolar, is lowered. 

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Unfortunately this route gives only a 40% yield IJ. Amer. Cham. Soc 1951, 73, 3237) in the Grignard reaction, largely because benzyl Grignard reagents easily give radicals which polymerise. In any case, it s poor tactics to chop off carbon atoms one at a time, and a better disconnection would be ... 

Analysis The functional group is an acetal - once we ve removed this, we can follow straightforward tactics. 

Thus mixed aldol additions can be achieved by the tactic of quantitative enolate for matron using LDA followed by addition of a different aldehyde or ketone... 

Generally polymers involve bonding of the most substituted carbon of one monomeric unit to the least substituted carbon atom of the adjacent unit in a head-to-tail arrangement. Substituents appear on alternate carbon atoms. Tacticity refers to the configuration of substituents relative to the backbone axis. In an isotactic arrangement, substituents are on the same plane of the backbone axis that is, the configuration at each chiral center is identical. 

Figure 1.2 shows sections of polymer chains of these three types the substituent R equals phenyl for polystyrene and methyl for polypropylene. The general term for this stereoregularity is tacticity, a term derived from the Greek word meaning to put in order. ... 

Polymers of different tacticity have quite different properties, especially in the solid state. One of the requirements for polymer crystallinity is a high degree of microstructural regularity to enable the chains to pack in an orderly manner. Thus atactic polypropylene is a soft, tacky substance, whereas both isotactic and syndiotactic polypropylenes are highly crystalline. 

Figure 1.2 Sections of polymer chains of differing tacticity (a) isotactic (b) syndiotactic (c) atactic.

These differences do not arise from 1,2- or 3,4-polymerization of butadiene. Structures [XIII] and [XIV] can each exhibit the three different types of tacticity, so a total of six structures can result from this monomer when only one of the olefin groups is involved in the backbone formation. 

All polymer molecules have unique features of one sort or another at the level of individual repeat units. Occasional head-to-head or tail-to-tail orientations, random branching, and the distinctiveness of chain ends are all examples of such details. In this chapter we shall focus attention on two other situations which introduce variation in structure into polymers at the level of the repeat unit the presence of two different monomers or the regulation of configuration of successive repeat units. In the former case copolymers are produced, and in the latter polymers with differences in tacticity. Although the products are quite different materials, their microstructure can be discussed in very similar terms. Hence it is convenient to discuss the two topics in the same chapter. 

For both copolymers and stereoregular polymers, experimental methods for characterizing the products often involve spectroscopy. We shall see that nuclear magnetic resonance (NMR) spectra are particularly well suited for the study of tacticity. This method is also used for the analysis of copolymers. 

Use zero-order Markov statistics to evaluate the probability of isotactic, syndio-tactic, and heterotactic triads for the series of p values spaced at intervals of... 

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.]...

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. 

From appropriate ratios of these sequence lengths, what conclusions can be drawn concerning terminal versus penultimate control of addition The following are experimental tacticity fractions of polymers prepared from different monomers and with various catalysts. On the basis of Fig. 7.9, decide whether these preparations are adequately described (remember to make some allowance for experimental error) by a single parameter p or whether some other type of statistical description is required ... 

B. D. Nahloosky and G. A. Zimmerman, Thermoplastic Elastomers for S olid Propellant Binders, AERPL-TR-86-069, Aerojet Tactical Systems Co., Sacramento, Calif., Dec. 1986. 

J. E. Tormey, "Processing and Manufactuie of Composite Propellants," Proceedings of the AGARD Colloquim, Advances in Tactical Rocket Propulsion, CIRA Pub., Pelham, New York, 1968. 

National Academy of Sciences, Pesticide Resistance Strategies and Tactics for Management, Washiagton, D.C., 1986. 

R. Goldscheider, "The Art of Licensiag Out", Technology Management Eaw, Tactics, Forms, Clark Boardman Callaghan, New York, 1988, Chapt. 6. 

The nature of the alkyl group from the esterifying alcohol, the molecular weight, and the tacticity determine the physical and chemical properties of methacrylate ester polymers. 

Transformations in the Solid State. From a practical standpoint, the most important soHd-state transformation of PB involves the irreversible conversion of its metastable form II developed during melt crystallization into the stable form I. This transformation is affected by the polymer molecular weight and tacticity as well as by temperature, pressure, mechanical stress, and the presence of impurities and additives (38,39). At room temperature, half-times of the transformation range between 4 and 45 h with an average half-time of 22—25 h (39). The process can be significantly accelerated by annealing articles made of PB at temperatures below 90°C, by ultrasonic or y-ray irradiation, and by utilizing various additives. Conversion of... 

Specialty Polystyrenes. These include ionomers and PS of specified tacticity, as well as stabilized PS. 

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