Vinyl polymers asymmetric atoms

Tertiary carbon atoms along the chain have been defined as asymmetric (22-25, 34-37), pseudoasymmetric (6, 10, 38-40), stereoisomeric centers (30, 31), and diasteric centers (41). The first two terms put the accent on chirality and are linked to the use of models of finite and infinite length, respectively the last two consider only phenomena of stereoisomerism. Note the relationship between these last definitions and Mislow s and Siegel s recent discussion (42), where the two concepts—stereoisomerism (or stereogenicity) and chirality—are clearly distinguished. The tertiary carbon atoms of vinyl polymers are always stereogenic whether they are chinotopic or achirotopic (42) depends on stmctural features and also on the type of model chosen (43). 

As we have previously observed, since tertiary carbon atoms of the main chain of vinyl polymers synthesized by the usual processes cannot give rise to appreciable optical activity because of intra- and inter-molecular compensations, polymers from racemic vinyl monomers result optically active only when containing in the side chains an excess of asymmetric carbon atoms having R or S absolute structure. 

The macromolecules of optically active linear head-to-tail vinyl polymers contain asymmetric carbon atoms both in the principal and in the lateral chains. However, as the asymmetric carbon atoms in the principal chain cannot give a high contribution to the rotatory power (7), the optical activity of the known vinyl polymer is mainly related to the presence of asymmetric carbon atoms in the lateral chains. 

Such a model is in agreement with all the experimental findings till now ascertained in the field of optically active vinyl-polymers in fact it explains, in the case of polymers having asymmetric carbon atoms in a or j position with respect to the principal chain, the relationships between absolute structure of monomers and sign of the rotatory power of polymers, and the high rotatory power observed in isotactic polymers. The rapid and reversible variation of the optical rotation with temperature (105) is probably connected with the existence of a conformational equilibrium that is rapidly attained at each temperature. 

The 2,2,4- (or 2,4,4)-trimethylhexamethylenediamine has a head and a tail. Formally it can be incorporated into the chain according to principles known from vinyl polymers—e.g., in a head-to-head arrangement to the dicarboxylic acid or head-to-tail arrangement. It is quite probable that our melt condensates have a statistical distribution of structure. The different reactivities of the two ends of the diamine may suggest that certain conditions could be visualized under which identical monomers can arrange to macromolecules of different structures. In addition to the modifications by the head-tail principle, the asymmetric carbon atom creates optical isomers, such as the l and the d form or a mixture of both. 

When the repeat units of a polymer chain are themselves asymmetric, on account of their containing a carbon atom with four different substituents, the large number of possible permutations of right-handed d) and left-handed (/) units represents a large number of. sfm c-isomers. This kind of isomerism in polymers is called tacticity. When the arrangement along the chain is completely ordered, the polymer is said to be stereoregular. In a substituted vinyl polymer like polypropylene, - CH2-CH(CH3))- , there are three main types of steric isomer ... 

Stereoisomerism occurs in vinyl polymers when one of the carbon atoms of the monomer double bond carries two different substituents. It is formally similar to the optical isomerism of organic chemistry in which the presence of an asymmetric carbon atom produces two isomers which are not superimposable. Thus glyceraldehyde exists as two stereoisomers with configurations shown in 4-13. (The dotted lines denote bonds below and the wedge signifies bonds above the plane of the page.) Similarly, polymerization of a monomer with structure... 

Vinyl polymers contain many pseudoasymmetric sites, and their properties are related to those of micromolecular compounds which contain more than one asymmetric carbon. Most polymers of this type are not optically active. The reason for this can be seen from structure 4-15. Any has four different substituents X, Y, and two sections of the main polymer chain that difl er in length. Optical activity is influenced, however, only by the first few atoms about such a center, and these will be identical regardless of the length of the whole polymer chain. This is why the carbons marked are not true asymmetric centers. Only those centers near the ends of macromolecules will be truly asymmetric, and there... 

If every tertiary carbon atom in the chain is asymmetric, one might expect the polymer to exhibit optical activity. Normally, homoatomic carbon chains show no optical activity because two long chains constitute part of the group variatious as these become longer (and more alike) in relation to the chiral center, the optical activity decreases to a vanishingly small value. Vinyl polymers derived from (CH2=C XY) monomers fall into this category as they are centrosymmetric relative to the main chain, and the tertiary carbons arc thrai only pseudo-asymmetric. 

The case in which this site (e.g. a double bond or an asymmetric carbon atom) is repeated along the polymer main chain with the same configuration is much more frequent both in natural (natural rubber, natural peptides, poly-p-hydroxybutyrate) and synthetic polymers (isotactic vinyl polymers) than the case in which stereoregularity arises from ordered successions of different configurations. Indeed, up to now, only the ordered alternation of the two possible configurations of asymmetric carbon atoms (syndiotactic vinyl polymers and alternating copolymers of D- and L-amino acids) has been recognized. 

The possibility of tacticity is not, of course, restricted to vinyl polymers. Other types of polymer which contain asymmetric carbon atoms and which ha/e been obtained in tactic forms include poly(propylene oxide) and natural rubber hydrochloride ... 

Finally, it may be noted that some polymers have been obtained in which optical activity is ascribed mainly to conformational asymmetry. In these cases there is a predominance of either right-handed or left-handed enantiomorphs of helical polymer molecules, in contrast to the more usual situation wherein equal amounts of the two enantiomorphs are produced and there is no resultant optical activity. Optically active polymers of this type have been obtained from a-olefins possessing optically active side chains, e.g., 3-methylpent-l-ene, 4-methylhex-l-ene and 5-methylhept-l-ene. Isotactic polymers from these monomers have greatly enhanced optical activity compared to the monomer. Since these polymers are vinyl polymers this optical activity cannot be associated with the asymmetry of the carbon atom in the polymer backbone (for the reasons given above). Thus it is supposed that the presence of optically active side groups favours a particular screw sense of the helix so that the resultant polymer shows a large optical rotation. Optical activity of this type has not been observed when the side groups are not asymmetric. 

Fig. 1. Molar optical rotation ([0] vs polymerized monomer optical purity of isotactic vinyl polymers having the asymmetric carbon atom in the oc and 3 position to the main chain. -O poly-(5)-4-methyl-l-hexene - t- poly-(5)-3,7-dimethyl-l-octene poly- (5)-l-methylpropyl]-vinyl ether.

The simplest type of tacticity relates to vinyl polymers of structure [-CH2-CHR-] and [-CH2-CRR -] . In such polymers every substituted carbon atom is asymmetric, i.e. is attached to four different atoms or groups and may thus have two possible configurations. This situation arises because every carbon atom is joined to two residual portions of the polymer chain and these two residues will be different. If the two residues are denoted by A and B then the group -CHR- (and similarly -CRR -) may have either of the... 

Any of the four monomer residues can be arranged in a polymer chain in either head-to-head, head-to-tail, or tail-to-tail configurations. Each of the two head-to-tail vinyl forms can exist as syndiotactic or isotactic stmctures because of the presence of an asymmetric carbon atom (marked with an asterisk) in the monomer unit. Of course, the random mix of syndiotactic and isotactic, ie, atactic stmctures also exists. Of these possible stmctures, only... 

In an earlier discussion (254) polymers in which the chirality depends only on the presence of chiral side groups were said to be nonintrinsically chiral, in contrast with intrinsically chiral polymers where the chirality is independent of the internal structure of the substituent. Substituted carbon atoms in the polymers described in the next paragraphs are often indicated as true or classic asymmetric carbon atoms. In this way one can distinguish between carbon atoms whose four substituents are constitutionally different in the proximity of the atom under consideration, from the tertiary atoms of vinyl isotactic polymers. For these, only the different length of the two chain segments and/or the stmcture of tire end grmips make all the ligands different from each other. 

In the polymers of racemic vinyl monomers, each monomeric unit in general contains at least two asymmetric carbon atoms, one in the... 

As shown in Table 13, the sign of the optical activity of polymers of alkyl- or cycloalkyl-vinyl-ethers at 589 mp, in hydrocarbon solvent, can in general be foreseen on the basis of the absolute configuration of the asymmetric carbon atom that is the nearest to the ethereal group. A positive sign is observed when the asymmetric carbon atom of the monomer has absolute configuration (S), as in poly-a-olefins. 

Table 13. Relationship between the absolute configuration of the asymmetric carbon atom nearest to the vinyloxy group of some optically active vinyl-ethers and the sign of the optical activity of the corresponding polymers...

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