Polymer structure tacticity

Abstract The use of methylaluminoxane (MAO) as cocatalyst for the polymerization of olefins and some other vinyl compounds has widely increased the possibilities for more precisely controlling the polymer composition, polymer structure, tacticity, and special properties. Highly active catalysts are obtained by different transition metal complexes such as metallocenes, half-sandwich complexes, and bisimino complexes combined with MAO. These catalysts allow the synthesis of polyolefins with different tacticities and stereoregularities, new cycloolefins and other copolymers, and polyolefin composite materials of a purity that cannot be obtained by Ziegler-Natta catalysts. The single-site character of metaUocene/MAO or other transition metal/ MAO catalysts leads to a better understanding of the mechanism of olefin polymerization. 

There is also some evidence that the ionic liquid medium affects polymer structure. Biedron and Kubisa150 reported that the tacticity of PMA prepared in the chiral ionic liquid 19 is different from that prepared in conventional solvent. It is also reported that reactivity ratios for MMA-S copolymcrization in the ionic liquid IS161 differ from those observed for bulk copolymerization. 

Experimental and theoretical results are presented for four nonlinear electrooptic and dielectric effects, as they pertain to flexible polymers. They are the Kerr effect, electric field induced light scattering, dielectric saturation and electric field induced second harmonic generation. We show the relationship between the dipole moment, polarizability, hyperpolarizability, the conformation of the polymer and these electrooptic and dielectric effects. We find that these effects are very sensitive to the details of polymer structure such as the rotational isomeric states, tacticity, and in the case of a copolymer, the comonomer composition. 

The first report of ROMP activity by a well-characterized Mo or W species was polymerization of norbornene initiated by W(CH-t-Bu)(NAr)(0-f-Bu)2 [122]. In the studies that followed, functionality tolerance, the synthesis of block copolymers, and ring-opening of other monomers were explored [30, 123]. Two important issues in ROMP concern the cis or trans nature of the double bond formed in the polymer and the polymer s tacticity. Tacticity is a consequence of the presence of two asymmetric carbons with opposite configuration in each monomer unit. The four ROMP polymers (using polynorbornene as an example) that have a regular structure are shown in Scheme 3. 

The variation in polymer structures is observed when the backone of the polymer molecule contains a carbon atom attached to two different side groups. Such polymers can have different configurational arrangements or tacticity. 

Magnetic resonance (ESR, NMR) Chemical structure, tacticity, conformation, polymer mobility (NMR) Radical, triplet state structure and behaviour (ESR)... 

There are, however, things about the polymer structure which are not known for certain. We assume that the reaction occurs in a random manner along the polymer backbone but there is little evidence at all concerning this problem and a detailed analysis must await future research. In addition, we know very little about the effects of polymer tacticity on the reaction shown in Equation 21. This also remains to be studied. On the other hand, we are confident that this reaction does not lead to a novel crosslinking reaction sequence since these polymers are soluble in a number of different solvents (Table II). 

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],... 

The polymerization of 1,2-disubstituted ethylenes, RCH=CHR, such as 2-pentene (R = — CH3, R = —C2H5), presents a different situation. Polymerization yields a polymer structure II in which there are two different stereocenters in each repeating unit. Several possibilities of ditacticity exist that involve different combinations of tacticity for the two stereocenters. Various stereoregular structures can be defined as shown in Fig. 8-2. Diisotactic structures occur when placement at each of the two stereocenters is isotactic. 

Propylene content in EPM rubber can be determined with the help of IR spectra. A propylene band near 1155 cm 1 has been widely used [79] for EPM analysis, frequently in combination with the polyethylene band at 721 cm"1. Tacticity is important in EPM rubber, and the bands at 1229 and 1252 cm"1 are characteristic of syndiotactic and isotactic structures, respectively, (both bands are present in atactic polypropylene as well). Polymer structure may vary in the relative tactic placement of adjacent head to tail propylene units and in the sequence distribution of base units along the chain. Some of them can be identified [80] by infrared spectra, such as isolated or head to tail propylene units ... 

Epoxide polymers exhibit stereoisomerism originating from the chirality of tertiary carbon atoms present in the polymer main chain. The stereoisomers of epoxide polymers are therefore tactic polymers their tacticity is connected with the structure of epoxide monomers undergoing polymerisation. Epoxide... 

These centres are formed by the addition of monomer to a suitable anion. They are almost always simpler than their cationic reverse part. The counter ion is usually a metal cation able to interact with the electrons of the growing end of the macromolecule, and to bind in its ligand sphere monomer or solvent molecules or parts of the polymer chain. This changes the properties of the whole centre. Therefore, by selection of the metal, the stability of the centre, the tendency of the centres to aggregation, the position of the equilibrium between the contact and solvent-separated ion pairs and free ions, and the stereoselectivity of the centre [the ability to produce polymers with an ordered structure (tacticity, see Chap. 5, Sect. 4.1)] are predetermined. The chemical reactions of the metal cations are, however, very limited. Most solvents and potential impurities are of nucleophilic character. They readily solvate the cation, leaving the anion relatively free. The determination... 

This chapter discusses the dynamic mechanical properties of polystyrene, styrene copolymers, rubber-modified polystyrene and rubber-modified styrene copolymers. In polystyrene, the experimental relaxation spectrum and its probable molecular origins are reviewed further the effects on the relaxations caused by polymer structure (e.g. tacticity, molecular weight, substituents and crosslinking) and additives (e.g. plasticizers, antioxidants, UV stabilizers, flame retardants and colorants) are assessed. The main relaxation behaviour of styrene copolymers is presented and some of the effects of random copolymerization on secondary mechanical relaxation processes are illustrated on styrene-co-acrylonitrile and styrene-co-methacrylic acid. Finally, in rubber-modified polystyrene and styrene copolymers, it is shown how dynamic mechanical spectroscopy can help in the characterization of rubber phase morphology through the analysis of its main relaxation loss peak. 

The application of this technique to the determination of the polymer structure and tacticity is now almost universally practiced and needs no further discussion. 

With the development of polymer structural characterizations using spectroscopy, there has been a considerable effort directed to measurements of tacticity, sequence distributions and number average sequence lengths (59 65). Two methods have been traditionally used for microstructure analysis from polymer solutions. Vibrational spectroscopy (infrared) and Nuclear Magnetic Resonance (NMR). Neither of these techniques is absolute. The assignment of absorption bands requires the use of model compounds or standards of known structure. 

Nuclear Magnetic Resonance. The successful study of polymers in solution by high resolution NMR spectroscopy started with the pioneering work on the sequence structure of poly methyl methacrylate in 1960. Since then, an ever-increasing number of investigations have been carried out ranging from the elucidation of the statistics of homopolymer and copolymer structure to the study of conformation, relaxation and adsorption properties of polymers. The aspects of sequence length determination and tacticity have received considerable attention (Klesper 84, for example, reports more than 500 entries). Therefore, a detailed review will not be attempted. (For a detailed description of the NMR Theory and statistics of polymer structure, see Bovey 59, Randall 23, and Klesper 84). 

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. 

The cationic polymerization of several para-substi-tuted a-methylstyrenes initiated by various Friedel-Crafts catalyst-cocatalyst combinations has been studied for the effects of catalyst type, monomer substituent and reaction solvent polarity on polymer structure and properties. By using solvent mixtures, the tacticity of the resulting polymers could be varied over a wide range, the syndiotactic form being favored in the more polar mixtures. 

Quantitative data on polymer structure, such as tacticity, cis—trans isomerism and monomer sequences, can be obtained from relative intensities of responsible NMR signals for these structures. Since these quantitative data are not collected by other analytical means, the NMR measurements for the analyses should be performed with much higher accuracy and precision, compared with those for low molecular weight compounds, in which only approximate intensity ratios, such as CH3 CH2 = 3 2, are required. 

The molecular weight dependence of polymer structures such as tacticity or copolymer composition can also be easily evaluated by on-line GPC/NMR. Data on the molecular weight dependence of polymer structures are very useful for understanding the mechanism of polymerization or the properties of polymers, and are usually collected by the fractionation of the polymer and the subsequent structural analysis of each fraction. However, this method requires a lot of time and rather a large polymer sample. The on-line GPC/NMR method is very useful for this purpose it needs only a few hours and a very small sample (0.5 1.0mg). 

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