Stereochemistry of Polymers

Abstract

When a polymer has stereochemical isomerism within the chain, its properties often depend on the stereochemical structure. Thus the analysis of the stereochemistry of polymers is important and NMR spectroscopy has been the most valuable tool for this purpose. Definition of TacticityIn polymers of vinyl monomers CH 2 =CH-X or vinylidene monomers CH 2 =CXY, the main-chain carbons having substituent group(s) are termed "pseudo-asymmetric" since, if the chain ends are disregarded, such carbons do not have the four different substituents necessary to qualify for being truly asymmetric. Nevertheless, they have the possibility of relative handedness. The simplest regular arrangements along a chain are the isotactic structure, in which all the substituents are located on the same side of the zigzag plane representing the chain stretched out in an all-trans conformation. The structure is often represented by a "rotated Fischer projection" in which the main-chain skeleton is represented by a horizontal line (Eq. 3.1). RNK. Hatada et al., NMR Spectroscopy of Polymers © Springer-Verlag Berlin Heidelberg 2004 74 3 Stereochemistry of PolymersFor a vinyl polymer, two types of diads should be considered, which are designated as meso (m) and racemo (r). Using these notations, a sequence in an isotactic polymer can be represented as -mmmmmm-and that in a syndiotactic polymer as -rrrrrrrr-. In reality, however, purely isotactic or syndiotactic polymers are rarely obtainable but the extent of regularity is always the question to be analyzed. Tacticity [1] is the term used for defining such stereochemical features of polymers. 1

Synthesis of Fluorine-Containing Polymers by Functionalization of cis-1,4-Polyisoprene with Hypervalent Iodine Compounds

“…Several peaks were observed for the tertiary alkyl fluoride F atom (F-i) over a relatively broad spectral region (−134.0 to −146.0 ppm), which was plausible because of the formation of repeat units with different configurations at the newly formed asymmetric carbon atoms and the coexistence of several stereosequences. The complexity in the 19 F NMR spectra of the fluoropolymers with random stereochemistry is well-documented. − Based on integration of the signals in the NMR spectra, it was determined that the repeat units with the secondary alkyl fluoride groups were less than 5 mol % of the total.…”

Organic Lewis Pairs Based on Phosphine and Electrophilic Silane for the Direct and Controlled Polymerization of Methyl Methacrylate: Experimental and Theoretical Investigations

“…The tacticity of this TTMPP/Me 3 SiNTf 2 -derived PMMA was also evaluated based on the signals from CH 3 groups (Supporting Information, Figure S3) . The result was as follows: syndiotacticity (% of triad rr ) = 62%, heterotacticity (% of triad mr ) = 34%, and isotacticity (% of triad mm ) = 4%.…”

“…The tacticity of this TTMPP/Me 3 SiNTf 2 -derived PMMA was also evaluated based on the signals from CH 3 groups (Supporting Information, Figure S3). 77 The result was as follows: syndiotacticity (% of triad rr) = 62%, heterotacticity (% of triad mr) = 34%, and isotacticity (% of triad mm) = 4%. These values are somewhat comparable to those observed in the Me 3 SiNTf 2 -catalyzed GTP of MMA with a SKA as initiator (% rr/mr/mm =72/27/1 at 25 °C in CH 2 Cl 2 ).…”