Stereoregularity

As already described, PLA can be manufactured to give a wide range of properties because of the chiral nature of lactide. The mechanical characteristics of PLA are known to depend on the choice and distribution of stereoisomers within the polymer chains. High-purity l- and D-lactide form stereoregular isotactic PLLA and PDLA, respectively, with equivalent physicochemical and mechanical properties. These are semicrystalline polymers with a high around 175-180°C and a Tg in the 60-70°C range. The racemic d,l-lactide and mc o-lactide, on the other hand, form atactic POLL A and mcio-poly(lactide), which are amorphous materials [30-32]. 

It has been observed that a 1 1 mixture of pure PLLA with pure PDLA yields a stereocomplex of the two polymers during crystallization or polymerization. The PLA stereocomplex consists of racemic crystalline strucmres in which l-PLA and d-PLA chains are packed side by side, with a 1 1 ratio of l d monomer units [4, 32, 33]. While the melting temperature of a- and P-crystalline forms of PLA falls in the range 170-180°C, the 7) of PLA stereocomplex is between 220 and 230°C [33]. The high of PLLA/PDLA stereocomplex makes it a difficult material for processing however, it is interesting to note that the comparison between PLLA/PDLA equimolar blends and the starting materials shows mechanical properties that are markedly improved. 

FIGURE 11.4 Stress-strain curves for blend film and nonblended PLLA film. The blend was a 1 1 mixture of PLLA with = 150,000g/mol and PDLA with = 150,000g/mol [34]. 

Sawai et al. studied the mechanical properties of a stereocomplex PLA film prepared by casting from a solution of an equimolar blend of PLLA and PDLA [35]. The film was uniaxially drawn by solid-state coextrusion and characterized by DMA. The optimum draw temperature resulting in the highest draw and mechanical properties was 200° C. The maximum achieved tensile modulus and strength, for the samples with an extrusion draw ratio of 16 and prepared by solid-state coextrusion of a highly crystalline stereocomplex film, were 9500 and 410 MPa, respectively. Furthermore, the PLA stereocomplex films with an extrusion draw ratio of 16 exhibited excellent thermomechanical stability as evaluated by the E measured as a function of temperature. The reported E values at room temperature, 100 and 200°C, were 9500, 7000, and 3000 MPa, respectively [35]. Equimolar amounts of PLLA and PDLA stereocomplex are therefore characterized, upon orientation, by the most relevant me- 

The introduction of plasticizers in the formulation of semicrystalline polymers such as PLA can in principle reduce not only the Tg of the amorphous phase but also the 

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Atactic polymer (Section 7 15) Polymer characterized by ran dom stereochemistry at its chirality centers An atactic polymer unlike an isotactic or a syndiotactic polymer is not a stereoregular polymer... 

Isopropyl group (Section 2 13) The group (CH3)2CH— Isotactic polymer (Section 7 15) A stereoregular polymer in which the substituent at each successive chirality center is on the same side of the zigzag carbon chain Isotopic cluster (Section 13 22) In mass spectrometry a group of peaks that differ in m/z because they incorporate differ ent isotopes of their component elements lUPAC nomenclature (Section 2 11) The most widely used method of naming organic compounds It uses a set of rules proposed and periodically revised by the International Union of Pure and Applied Chemistry... 

Syndiotactic polymer (Section 7 15) Stereoregular polymer in which the configuration of successive chirality centers alternates along the chain... 

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. ... 

In the last three chapters we have examined the mechanical properties of bulk polymers. Although the structure of individual molecules has not been our primary concern, we have sought to understand the influence of molecular properties on the mechanical behavior of polymeric materials. We have seen, for example, how the viscosity of a liquid polymer depends on the substituents along the chain backbone, how the elasticity depends on crosslinking, and how the crystallinity depends on the stereoregularity of the polymer. In the preceding chapters we took the existence of these polymers for granted and focused attention on their bulk behavior. In the next three chapters these priorities are reversed Our main concern is some of the reactions which produce polymers and the structures of the products formed. 

In the next group of chapters we shall discuss condensation or step-growth polymers and polymerizations in Chap. 5, addition or chain-growth polymers and polymerizations in Chap. 6, and copolymers and stereoregular polymers in Chap. 7. It should not be inferred from this that these are the only classes of polymers and polymerization reactions. Topics such as ring-opening polymeri-... 

Specifically chemical considerations are especially evident in Chap. 7, where copolymers and stereoregular polymers are discussed. Since two monomers are required for the formation of a copolymers, the differences in their reactivity affects both the composition of the product and the distribution of components in it. Likewise, the catalysts that produce stereoregularity are highly specific, highly reactive, and poorly understood chemical reagents. 

The bifunctionality of the bis-diene and bis-dienophile monomers is apparent from the condensation product, structure [XXI], which still contains a diene and a dienophile in the same molecule. This polymer is crystalline, indicating a high degree of stereoregularity in the condensed rings. It decomposes to a graphitic material before melting. 

Stereoregular polymerization. This is also taken up in Chap. 7. 

Stereoregular polymerizations strongly resemble anionic polymerizations. We discuss these in greater detail in Chap. 7 because of their microstructure rather than the ionic intermediates involved in their formation. 

In the discussion of these combined topics, we use statistics extensively because the description of microstructure requires this kind of approach. This is the basis for merging a discussion of copolymers and stereoregular polymers into a single chapter. In other respects these two classes of materials and the processes which produce them are very different and their description leads us into some rather diverse areas. 

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. 

Nuclear magnetic resonance (NMR) spectroscopy is another physical technique which is especially useful for microstructure studies. Because of the sensitivity of this technique to an atom s environment in a molecule, NMR is useful for a variety of microstructural investigations We shall consider the application to copolymers now and to questions of stereoregularity in Sec. 7.11... 

The use of NMR spectroscopy to characterize copolymer microstructure takes advantage of this last ability to discern environmental effects which extend over the length of several repeat units. This capability is extremely valuable in analyzing the stereoregularity of a polymer, and we shall have more to say about it in that context in Sec. 7.11. 

Before examining NMR applications to problems of stereoregularity, let us conclude the discussion of copolymers by considering some copolymer applications. 

Instead of devoting more space to copolymers, we turn next to stereoregular polymers, in which many of the descriptions of microstructure developed in Sec. 7.6 can also find application. 

Our discussion of stereoregularity in this chapter is primarily concerned with polymers of monosubstituted ethylene repeat units. We shall represent these by X... 

There are several topics pertaining to stereoregularity which we shall not cover to simplify the presentation ... 

Stereoregular copolymers. We shall restrict our discussion to stereoregular homopolymers. 

The statistical nature of polymers and polymerization reactions has been illustrated at many points throughout this volume. It continues to be important in the discussion of stereoregularity. Thus it is generally more accurate to describe a polymer as, say, predominately isotactic rather than perfectly isotactic. More quantitatively, we need to be able to describe a polymer in terms of the percentages of isotactic, syndiotactic, and atactic sequences. 

These protons show a single chemical shift in the NMR spectrum. This is called a racemic (subscript r) structure, since it contains equal amounts of D and L character. In the next section we shall discuss the NMR spectra of stereoregular polymers in more detail. 

It is not the purpose of this book to discuss in detail the contributions of NMR spectroscopy to the determination of molecular structure. This is a specialized field in itself and a great deal has been written on the subject. In this section we shall consider only the application of NMR to the elucidation of stereoregularity in polymers. Numerous other applications of this powerful technique have also been made in polymer chemistry, including the study of positional and geometrical isomerism (Sec. 1.6), copolymers (Sec. 7.7), and helix-coil transitions (Sec. 1.11). We shall also make no attempt to compare the NMR spectra of various different polymers instead, we shall examine only the NMR spectra of different poly (methyl methacrylate) preparations to illustrate the capabilities of the method, using the first system that was investigated by this technique as the example. 

The peaks centered at 5 = 1.84 ppm-a singlet in the syndiotactic and a quartet in the isotactic polymers-are thus identified with these protons. This provides an unambiguous identification of the predominant stereoregularity of these samples. 

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