Head to tail placement

Isomeric polymers can also be obtained from a single monomer if there is more than one polymerization route. The head-to-head placement that can occur in the polymerization of a vinyl monomer is isomeric with the normal head-to-tail placement (see structures III and IV in Sec. 3-2a). Isomerization during carbocation polymerization is another instance whereby isomeric structures can be formed (Sec. 5-2b). Monomers with two polymerizable groups can yield isomeric polymers if one or the other of the two alternate polymerization routes is favored. Examples of this type of isomerism are the 1,2- and 1,4-polymers from 1,3-dienes (Secs. 3-14f and 8-10), the separate polymerizations of the alkene and carbonyl double bonds in ketene and acrolein (Sec. 5-7a), and the synthesis of linear or cyclized polymers from non-conjugated dienes (Sec. 6-6b). The different examples of constitutional isomerism are important to note from the practical viewpoint, since the isomeric polymers usually differ considerably in their properties. 

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... 

Contrary to the above-shown four propagation modes, a head-to-tail placement strongly predominates. This is true of most free-radical vinyl polymerizations. Such placement is shown in reaction 1. It is consistent with the localized energy at the a-carbon of the monomer. Also, calculations of resonance stabilization tend to predict head-to-tail additions. ... 

Before proceeding with the development of theory and instruments, a simple but important example of chain configuration is given. This involves the difference between head-to-head and head-to-tail placement of the monomeric units, or mers, during polymerization. The head-to-tail structure of polystyrene may be written... 

Although similar studies have not been reported for polyepi-chlorohydrin, there is no reason to expect any differences in mechanism of polymerization for this epoxide. It can be assumed that polymerization leading to crystalline polymer occurs predominantly by inversion at the asymmetric carbon atom and results in head-to-tail placement of the monomer. Mainly isotactic polymer will be formed. Our studies and previous work have born this out (8, 9, 14, 15). 

The total difference in activation energies is such, that at temperatures normally encountered in polymerizations the propagation proceeds almost entirely by a head-to-tail placement. 

It was recognised that intramolecular excimer configurations might be expected to be attained as a result of interactions between adjacent chromophores upon the polymer backbone which, in head to tail placements, satisfy the n=3 rule" of Hirayama [3] (which states that excimer formation is expected between aromatic substituents attached to an alkane chain only when the chromophores are separated by three carbon atoms. The limitations of the rule are briefly noted in section 2). 

Different molecular microstructures arise from there being several possible modes of propagation. The possibility of head-to-tail and head-to-head placements of the repeat units has been encountered already, with the observation that for both steric and energetics reasons the placement is almost exclusively head-to-tail for most polymers. Therefore in the subsequent sections dealing with the stereochemistry of propagation only head-to-tail placements will be considered. 

For each mode there is the possibility of head-to-head or head-to-tail placement, and for 1,2- and 3,4-addition the additional complication of isotactic or syndiotactic placement. 

One simple but important configuration type is the head-to-head or head-to-tail placement of repeating units along the polymer chain. Figure 2.4 show the... 

The necessary condition for chainlike molecules to fit into a crystal lattice is that they demonstrate an exactly repeating molecular structure along the chain. For vinyl polymers, this prerequisite is met only if they have predominantly head-to-tail placement and are highly tactic. When these conditions are satisfied, polymers can, indeed, form highly crystalline domains in the solid state and in concentrated solution. There is even evidence of the formation of microcrystalline... 

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