Stereoregular polymer synthesis

Topochemical [24-2] photoreactions of diolehn crystals has been reviewed. The reactions clearly depart from typical solution chemistry crystal-lattice control offers a unique synthetic route into photodegradable polymers, highly strained [24-2] paracyclophanes, stereoregular polymers, and absolute asymmetric synthesis. However, achieving the desired type of crystal... 

Several asymmetric polymerization of small molecules have been reported applying the asymmetric catalysis methodology to polymer synthesis. Unique physical properties due to the mainchain stereoregularity are envisioned. 

Mechanisms have been proposed to explain stereoselectivity in 1,3-diene polymerizations, but these processes are less understood than the polymerization of ethylene and 1-alkenes [Peluso et al., 1997]. The ability to obtain cis 1,4-, trans 1,4-, and st 1,2-polymers from 1,3-butadiene, each in very high stereoregularity, hy using different initiators has great practical utility for polymer synthesis even if it is not well understood why a particular initiator gives a particular stereoregular polymer. 

III. Contribution given by the research on optically active addition polymers to the knowledge of the synthesis and properties of stereoregular polymers... 

Butenyllithium and butenylmagnesium chloride were used as "dynamic allylic compounds. The former was selected because of the ability of lithium catalysts to provide high rates of diene polymerization and to give stereoregular polymers the latter was selected for its availability and simplicity of synthesis. 

The most self-evident method for formation of a stereoregular polymer is the polymerization of enantiomerically pure monomers with structural properties supporting the assembly of a confor-mationally uniform and stable secondary structure. Alternatives such as asymmetric polymerization generating stereogenic centers ( asymmetric synthesis polymerization ) or P- or M-he-... 

Staudinger, Hermann (1881-1965). Fundamental research on high-polymer structure, catalytic synthesis, polymerization mechanisms, resulting eventually in development of stereospecific catalysts by Ziegler and Natta (stereoregular polymers). Nobel Prize 1963. 

The primary structure of macromolecules is defined as the sequential order of monomers connected via covalent chemical bonds. This structural level includes features such as chain length, order of monomer attachment in homopolymers (head-to-head, head-to-tail placement), order of monomer attachment in various copolymers (block copolymers, statistical and graft copolymers, chemical composition of co-monomers), stereoregularity, isomers, and molecular topology in different branched macromolecules and molecular networks. Structure at this primary level can be manipulated by polymer synthesis [4]. With AFM it is possible to visualize, under certain conditions, single macromolecules (Fig. 3.2) and it is even possible to manipulate these (i.e. push with AFM tips). Characteristics of chain-internal... 

According to an early hypothesis21, stereoregular isotactic polymerization requires the presence of chirality within the catalyst. Thus, with achiral metallocenes mostly atactic polymers are obtained. Chiral ansa-metallocenes with substituted cyclopentadienyl ligands, or especially with indenyl and tetrahydroindenyl ligands, are effective in stereoselective polymer synthesis. 

The principal problem of polymer science is the mastering of methods for obtaining materials with predetermined properties. Consequently, the main task of polymer chemistry is the development of the methods of synthesis of polymers which exhibit, in principle, any theoretically possible primary structure of macromolecules. At present, many empirical data have been accumulated in the field of experimental chemistry, which in many cases, make it possible to synthesize polymers with a predetermined primary microstructure. In particular, the most interesting in this sense is the available experience of the synthesis of stereoregular polymers. On the whole, the successful fulfilment of this program requires a reasonable and harmonious combination of empirical and theoretical approaches. 

Since the initial advancements in the synthesis of stereoregular polymers by Natta et al.1J in the late fifties, attempts have been made to obtain stereoregular forms of many polymers. With the synthesis of these new polymers came a large number of reports on their properties, both in the bulk state and in solution. Among the properties studies have been crystallinity, and the effect of stereoregularity on the unperturbed dimensions. In this review we are concerned exclusively with the latter. 

---