Homopolymer tacticity

These polymers have been widely studied by NMR. In general, there are several types of information available, depending on the polymer in question, stereochemistry, e.g., homopolymer tacticity regiochemistry, e.g., normal or inverted monomer addition comonomer sequence placement (in copolymers) conq)ositional and tacticity heterogeneity... 

Figure 1.2 Schematic illustration of different types of configuration of homopolymers tacticity...

The classical representation of a homopolymer chain, in which the end groups are disregarded and only one monomer residue is considered, allows no possibility for structural variation. However, possibilities for stercoscqucnce isomerism arise as soon as the monomer residue is considered in relation to its neighbors and the substituents X and Y are different. The chains have tacticity (Section 4,2.1). Experimental methods for tacticity determination are summarized in 4.2.2 and the tacticity of some common polymers is considered in 4.2.3. 

There is also clear evidence that penultimate group effects are important in determining the stereochemistry of addition in many homopolymerizations and copolymerizations. This is made evident from the fact that most homopolymers have tacticity (i.e. P(/ i)f0.5, Section 4.2). Indeed, for some homopolyinerizations there is evidence that the configuration of the pcnpcnultimatc unit may also influence the stereochemistry of addition/9 If penpen- and penultimate units... 

Homopolymers made with multi-site catalysts tend to have composite tacticities, with each site generating a distinctive set of intensities in accordance with certain reaction probabilities. Such polymers can be regarded as in-situ blends of several components, each one bearing a different tacticity. Fractionation would then separate the polymer into different fractions, and the tacticities of each fraction would reflect the different proportions of the various conqponents. The MMR data on the fractions potentially contain Information on the weight percent of each component as well as the reaction probabilities. 

As an example, polybutylene is a commercially inq ortant polymer made with Ziegler-Natta catalysts. Such catalysts frequently produce more than one catalytic site, and the resulting polymer is a blend of several homopolymers differing only in propagation statistics. Previously, the two-site E/B model has been used to analyze the tacticity of this polymer.(11,12) Reasonably good fits with experimental data were observed. 

A few reviews have dealt with the identification of synthetic polymers by Py-GC/MS [76]. In addition to compositional studies, applications of pyrolysis to synthetic polymers include sequence length characterization in copolymers [77] and tacticity measurements in stereoregular homopolymers [78]. 

In the case of homopolymers, the tacticity of the chains directly influences the material properties. Atactic PHB has an oily consistency and is of less use for plastic industry, whereas completely isotactic PHB shows comparably interesting... 

In summary, the rate and the final state of biodegradation are strongly dependent on the tacticity in the case of homopolymers, or of the copolymer composition. This implies that it is possible to adjust the material s lifetime by controlling (/ )-units in the polymer or by changing the ratio of 3-HB to co-monomer units. 

Another way of preparing stereoblock polymers is the use of binary catalyst systems, where the exchange of growing alkyl chains with different tacticity might be expected.The alkyl chains are presumably carried by alkylaluminums and transferred between zirconocenes, and the products are a mixture of homopolymers from each catalyst and a stereoblock polymer containing both segments. 

Natta carried out the anionic polymerization of methyl sorbate, a 1,3-diene, with an optically active initiator and obtained an optically active homopolymer with main-chain chirality. The high molecular weight crystalline polymer produced with (P)-2-methylbutyllithium had a tritactic (di-iso-rra/w-tactic) structure. This was probably the first metal-catalyzed asymmetric polymerization 134). Polymerization of other dienes was attempted by using various asymmetric methods 135). 

It is convenient to classify these polymers on the basis of the tacticity of the main chain only thus, polymers obtained from one antipode, in which all the asymmetric carbon atoms of the lateral chains have the same absolute configuration, will be considered as homopolymers. On the contrary, polymers made up by macromolecules in the side chains of which asymmetric carbon atoms having R or S absolute configuration alternate more or less regularly, can be considered as copolymers of the two antipodes of the monomer having respectively R and S absolute structure. 

In proton spectra of polymers knowledge of the exact values of the coupling constants and the chemical shifts makes it possible to simulate the experimental spectra by computer calculations. In this way it is possible to interpret quantitatively the complex spectra of many homopolymers and copolymers in terms of configuration (or "tacticity") and monomer sequences (copolymers). The obtained sequence lengths can be directly related to the polymerisation mechanism. 

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). 

Homopolymers are made from the same monomer, but may differ by their microstructure, degree of polymerization or architecture. Examples of different microstructure of homopolymers such as tacticity, structural or sequence isomerisms were described in Section 1.2. Throughout this book we demonstrate that the degree of polymerization N tor the number of backbone bonds n) of macromolecules is a major ... 

The synthesis of alternating copolymers from carbon monoxide (CO) and olefins using palladium catalysts is currently an area of intense research. In cases where a-olefins are used, the regiochemistry (head/tail orientations) and stereochemistry (tacticity) of olefin insertion have a strong influence on the physical and mechanical properties of the polymers. Unlike regioregular a-olefins homopolymers, these copolymers have a directionality along the polymer backbone due to the incorporation of CO. Therefore isotactic, regioregular CO/a-olefin polymers are chiral by virtue of their main-chain stereochemistry (Scheme 11). 

If there are more than two components in a mixture (as in a blend of a homopolymer with a copolymer), binary interaction parameters can be combined into a composite % parameter to describe the overall behavior of the system. For example, Choi and Jo [11] showed how the effects of copolymer sequence distribution in blends of polyethylene oxide) with poly(styrene-co-acrylic acid) can be described by an atomistic simulation approach to estimate the binary intermolecular interaction energies which are combined into a total interaction parameter for the blend. Their paper [11] also provides a list of the many preceding publications attempting to address the effects of copolymer composition, tacticity, and copolymer sequence distribution on polymer blend miscibility. In addition to the advances in computational hardware and software which have made atomistic simulations much faster and hence more accessible, work in recent years has significantly improved the accuracy of the force fields [12] used in such simulations. 

Finally, it is worth noting that Suzuki et al [47] have provided a theoretical analysis of the effects of tacticity variations on Tg, treating homopolymers containing triads of different tacticities as steric copolymers . 

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