Irregular polymers stereochemistry

High-resolution and NMR, and recently, multidimensional methods have revealed the microstructures of complex polymers. In particular, multidimensional (2D- and 3D-) NMR have proven to be useful techniques to identify small amounts of irregular structures in synthetic polymers. In this entry, specific topics to be covered include the use of solution NMR methods to study polymer stereochemistry/ tacticity, monomer composition and sequence distribution, short-chain branches, and chain-end structure, as these parameters influence the material s mechanical, thermal, optical, and electrical properties. 

In studying polymer stereochemistry, the form of the NMR spectrum gives a rapid, unequivocal identification of the structure of stereoregular vinyl polymers. For stereo-irregular polymers, the resonance peaks are frequently split into several lines from sequences of a few monomer residues of different stereochemistry. The relative peak intensities give statistical information on... 

The chain irregularities in isotactic poly (methyl methacrylate) become apparent as the gain is increased in Figure 3.5. These results show that polymer stereochemistry can be characterised in detail, since many of the peaks can be assigned and the amount of each stereosequence can be determined from the peak intensity. Some of the peaks are assigned to stereosequences that have probabilities below the 1% level. 

Discuss the structure and stereochemistry of polymers in terms of both regular and irregular features. (Problems 24.24 and 24.35)... 

Stereochemistry of polymers plays an important role in determining their properties. Different isomers of a polymer can be obtained starting with the same monomer but having the monomer molecules connected in different ways. For example, a monosubstituted vinyl monomer, in theory, can polymerize head-to-tail (H-T), head-to-head (H-H), or irregularly, as shown below ... 

Although the same polymer may be synthesized by two or more different approaches, the product can be somehow different in a chain structure including a different molecular weight and distribution, a different stereochemistry and a different extent of irregularity along the chains. Special attention must be paid when polymer reactions are applied for the synthesis of polymers. Two factors worthy of consideration, the extent of reaction and formation of isomers, are demonstrated by the two polymer examples (3.39) and (3.40). 

The above-mentioned chemical catalytic routes lead to racemic AHA mixtures. For the direct use of LA (or its esters) as a solvent or platform molecule for achiral molecules like acrylic acid and pyruvic acid, stereochemistry does not matter. The properties of the polyester PLA, the major application of LA, however, suffer tremendously if d and l isomers are built in irregularly [28]. This is exemplified by atactic PLA, made from racemic LA, which is an amorphous polymer with low performance and limited application. However, when l- and D-lactic acid are processed separately into their respective isotactic L- and d-PLA, as discovered by Tsuji et al., a stereocomplex is formed upon blending these polymers. This polymer exhibits enhanced mechanical and thermal properties [28, 164]. A productive route to D-Iactic acid is, however, missing today. If the chemocatalytic routes to LA are to become viable, enantiomer resolution of the racemate needs to be performed. Given separation success, a cheap source of o-lactic acid will be unlocked immediately, providing an additional advantage over the fermentation route (cfr. Table 2). 

---