Tactic structures

The zinc complex of 1,1,1,5,5,5-hexafluoroacetylacetonate forms coordination polymers in reaction with either 2,5-bis(4-ethynylpyridyl)furan or l,2-bis(4-ethynylpyridyl)benzene. The X-ray crystal structures demonstrate an isotactic helical structure for the former and a syndio-tactic structure for the latter in the solid state. Low-temperature 1H and 19F NMR studies gave information on the solution structures of oligomers. Chiral polymers were prepared from L2Zn where L = 3-((trifluoromethyl)hydroxymethylene)-(+)-camphorate. Reaction with 2,5-bis(4-ethy-nylpyridyl)furan gave a linear zigzag structure and reaction with tris(4-pyridyl)methanol a homo-chiral helical polymer.479... 

Polymerizations that yield tactic structures (either isotactic or syndiotactic) are termed stereoselective polymerizations. The reader is cautioned that most of the literature uses the term stereospecific polymerization, not stereoselective polymerization. However, the correct term is stereoselective polymerization since a reaction is termed stereoselective if it results in the preferential formation of one stereoisomer over another [IUPAC, 1996]. This is what occurs in the polymerization. A reaction is stereospecific if starting materials differing only in their configuration are converted into stereoisomeric products. This is not what occurs in the polymerization since the starting material does not exist in different configurations. (A stereospecific process is necessarily stereoselective but not all stereoselective processes are stereospecific.)... 

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],... 

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). 

If the value of this probability parameter is a <= 1, an essentially iso tactic structure is obtained. If, on the other hand a 0, almost all the monomer units in the chain are in syndiotactic placements. If the polymer is capable of crystallization and the crystallization takes place under equilibrium conditions, then the limitation of this model is that a small melting point depression implies also a high degree of percent crystallinity. Although there are a number of systems, for example stereoregular methyl methacrylate (2, 8), in which this is true and this model is valid, this is not the case for polymers of propylene oxide from different catalysts that we discuss in another chapter (1). 

In the first two cases, fc-sequences are present in the NMR spectrum due to partial inversion of ini-tial dichloro-derivative. All polymers with different tacticity types displayed high yields, except for the polymer of cis-tactic structure, the yield of which is much lower due to intramolecular cyclizati-on reaction, which induces formation of tricyclosiloxane (in the case of methyl groups at silicon atoms). 

Propene and the higher 1-alkenes can be polymerized to chains with the required degree of tacticity from almost atactic up to very highly tactic structures. However, a syndiotactic polymer can only be obtained from propene, mostly on soluble catalysts. The main factors determining controlled tactic addition are complexation, cis or trans addition, and primary or secondary addition. Most authors agree on the point that the interaction of the alkene molecule with the transition metal atom of the active centre leads to complex formation immediately before monomer insertion into the metal—polymer bond. The assumed existence of the complex is based on indirect experimental evidence and on theoretical considerations. 

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 structures of stereoregular and tactic polymers described above are ideal. Real polymers are always irregular they do not have perfect steric or tactic structures. Thus, the mean sequence and composition of steric and configurational diads, triads, etc. must be given by suitable statistical parameters, in analogy to the case for constitutional diads, triads, etc. 

The initiator and monomer control not only the rates and degrees of addition and insertion polymerizations, but also the overall configuration of the macromolecule produced. Stereoregular polymers, that is, those with a regular tactic structure, can be produced only under quite specific conditions. Polymerizations that lead to stereoregular polymers are called stereospecific. 

Figure 1.17 The basic tactic structures of a vinyl polymer, (a) Isotactic (b) syndiotactic (c) atactic. Methylene protons are equivalent in syndiotactic dyads but not in isotactic dyads.

Tactic PS. Isotactic (iPS) and syndiotactic (sPS) PSs can be obtained by the polymerization of styrene with stereospecific catalysts of the Ziegler-Natta-type. Aluminum-activated TiCls yields iPS while soluble Ti complexes [eg, ( j -CsHslTiCls] in combination with a partially hydrolyzed alkylaluminum [eg, methylalumoxane] yield sPS. The discovery of the sPS catalyst system was first reported in 1986 (33). As a result of the regular tactic structure, both iPS (phenyl groups cis) and sPS (phenyl groups alternating trans) are highly crystalline. Samples of iPS quenched from the melt are amorphous, but become... 

PSSA may also be prepared by sulfonation of polyst5rrene or by hydrolysis of poly(re-propyl p-vinylbenzenesulfonate). The latter cases allow preparation of tactic structures. Copol5uners can also be prepared by free-radical copolymerization of appropriate monomers or post-reaction. Poljuners are purified by precipitation of aqueous solutions with methanol, alkaline methanol, or other alcohols. Controlled structure PSSA homopolymers and block copolymers may also be prepared directly in aqueous media via RAFT using 4-cyanopentanoic acid dithiobenzoate as the CTA and V-501 as the radical source (48). NMP has been successfully... 

To this date, poly((C02Me)2NBD) prepared with the catalyst system MoCl5/Me4Sn/dioxane stands out as a rare example of an isotactic, all-cis polymer obtained with a conventional transition metal halide catalyst. This microstructure contrasts with the atactic and moderately syndio-tactic structures of poly(NB) and poly(5,5-Me2NB), respectively, that are obtained with the MoCl5/Me4Sn/dioxane system. 

There is one important class of polymers that do exhibit strong optical activity, as opposed to the above tactic structures. These are the polymers in which the chiral center is surrounded by different atoms or groups, and a true local center of asymmetry exists. 

If the repeating unit of the chain contains two distinguishable diasteric atoms, five different tactic structures can be envisioned ... 

Quantitative determination aims at the establishment of the composition of both copolymers and blends, and also of the microstructure of macromolecules, branching, tacticity, structural defects, and so on. In this respect, the surface, height, and other values of the corresponding peak of a particular component (denoted by c in Table 7 and italics in Table 8) are determined. 

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