Polymers stereoisomerism

Several other types of monomers are capable of yielding stereoisomeric polymer structures. Ordered structures are possible in the polymerization of carbonyl monomers (RCHO and RCOR ) and the ring-opening polymerizations of certain monomers. Thus, for example, the polymers from acetaldehyde and propylene oxide can have isotactic and syndiotactic structures as shown in Figs. 8-3 and 8-4. 

In connection with the aforementioned study on polymerization mechanism of MMA77,78), Miyamoto et al. developed a preparatory method of separating blends of isotactic and syndiotactic PMMA82 The principle was based on a competitive adsorption of these different stereoisomeric polymers from a nonpolar solution (chloroform) onto an adsorbent surface (silica gel). The procedure was quite simple, as described below A given polymer blend was dissolved in chloroform, in which no stereocomplex formation usually occurs, and silica gel was then dispersed in this solution for adsorptive equilibration with the polymer species. The isotactic species could be isolated as the adsorbed component. In practice, its purity was ca. 80—90%, which depended on the added amount of silica gel. By repeating the same procedure, the purity could be enhanced. 

Isobutylene does not give the kinds of stereoisomeric polymers (isotactic, etc.) that propylene does. Why not What can you say about 1-butene ... 

The polymers with trans-fused five-membered rings linked with a diisotactic head-to-tail sequence have chirality, although the polymers composed of the cis-fused ring are achiral. Scheme 10 summarizes the structures of the stereoisomeric polymers. The optically active zirconocene complex with a C2 symmetric structure catalyzes the enantioselective cyclopolymerization of 1,5-hexadiene (Eq. 20) [98, 99]. Although the polymer contains not only trans-fused ring but also cis-fused ring units (ca. 68 32), it shows optical rotation due to the main chain chirality. 

Fig. 2. Stereoisomerism in vinyl polymers (a) isotactic (b) syndiotactic and (c) atactic.

The thermal polymerization of S has a long history.310 The process was first reported in 1839, though the involvement of radicals was only proved in the 1930s. Carefully purified S undergoes spontaneous polymerization at a rate of ca 0.1% per hour at 60 C and 2% per hour at 100 °C. At 180 aC, 80% conversion of monomer to polymer occurs in approximately 40 minutes. Polymer production is accompanied by the formation of S dimers and trimers which comprise ca 2% by weight of total products. The dimer fraction consists largely of cis- and trans-1,2-diphenylcyclobutanes (90 and 91) while the stereoisomeric tetrahydronaphthalenes (92 a nd 93) are the main constituents of the trinier fraction.313... 

Detailed discussion of polymer tacticity can be found in texts by Randall,2 Bovey,1-3 Koenig,4-5 Tonelli6 and Hatada.7 In order to understand stereoisomerism in polymer chains formed from mono- or 1,1-disubstiluted monomers, consider four idealized chain structures ... 

Stereochemistry Coordination Polymerization. Stereoisomerism is possible in the polymerization of alkenes and 1,3-dienes. Polymerization of a monosubstituted ethylene, such as propylene, yields polymers in which every other carbon in the polymer chain is a chiral center. The substituent on each chiral center can have either of two configurations. Two ordered polymer structures are possible — isotactic (XII and syndiotactic (XIII) — where the substituent R groups on... 

Figure 1.1 shows the typical representation proposed to distinguish easily between the different types of stereoisomerism present in vinyl polymers. Viewing the main chains as lying down on a plane, there are (i) isotactic sequence of configurations, (ii) syndiotactic sequence of configurations, and (iii) no order present, atactic sequence of configurations. 

The conformation assumed by polymer chains in the crystalline state depends on the configuration of the stereoisomeric centers present along the chains and is defined by two basic principles1,2,14 16 ... 

Double bonds present along a polymer chain are stereoisomeric centers, which may have a cis or trans configuration. Polymers of 1,3-dienes with 1,4 additions of the monomeric units contain double bonds along the chains and may contain up to two stereoisomeric tetrahedral centers. Stereoregular polymers can be cis or trans tactic, isotactic or syndiotactic, and diisotactic or disyndio-tactic if two stereoisomeric tetrahedral centers are present. In the latter case erythro and threo structures are defined depending on the relative configurations of two chiral carbon atoms.1... 

But even when the constitution of the polymer may be regarded as essentially homogeneous, considerations analogous to the previous ones may apply at the stereochemical level. For this reason the study of macromolecular stereoisomerism calls for a twofold approach. On the one hand, one must turn to ideal models that permit the identification of the type of structure existing in the polymer on the other, statistical criteria must be used to determine to what... 

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). 

A polymer is defined as isotactic when the bonds issuing from the successive stereoisomeric centers (conventionally indicated by a vertical line) always have the same disposition when observed in the same direction -F —F —f- —1- —h — or— -l—1+—13—13— + itis defined as syn-diotactic when the disposition of the signs is inverted at each stage -t- --I++I----- -t-+ —. An isotactic sequence is, therefore, represented by... 

The combination of cis-trans isomerism with iso-syndio and erythro-threo dispositions gives complex stractures as exemplified by the 1,4 polymers of 1-or 4-monosubstituted butadienes, such as 1,3-pentadiene (72, 73), and 2,4-pentadienoic acid (74, 75) and of 1,4-disubstituted butadienes, for example, sorbic acid (76). This last example is described in 32-35 (Scheme 6, rotated Fischer projection). Due to the presence of three elements of stereoisomerism for each monomer unit (two tertiary carbons and the double bond) these polymers have been classed as tritactic. Ignoring optical antipodes, eight stereoregular 1,4 structures are possible, four cis-tactic and four trans-tactic. In each series (cis, trans) we have two diisotactic and two disyndiotactic polymers characterized by the terms erythro and threo in accordance with the preceding explanation. It should be noted that here the erythro-threo relationship refers to adjacent substituents that belong to two successive monomer units. 

It has been known for some time that dimethylketene produces three different types of polymers with polyketonic (47, X = Y), polyacetalic, 48, and polyester, 49, structures (88-90) (Scheme 11). The use of nonsymmetric ketenes like methylisopropylketene should produce an iso-syndio stereoisomerism in the ketonic form (47, X Y, only one of the possible stereoisomers is shown) and an orientational isomerism in the polyacetalic form, 48 in the polyester form, 49, there would be a combination of the two possibilities, analogous to those described for polyallenes. 4-Methylpentamethyleneketene offers the reverse possibilities iso-syndio isomerism in the polyacetalic form, 50, orientational isomerism in the polyketonic form, 51, and the combination of the two in the polyester form, 52 (Scheme 12). The three polymeric forms (ester, acetal, ketone) of 2- and 3-methylpentamethyleneketene each show both of these types of isomerism, one along and the other perpendicular to the chain. 

A line of research that has aroused much interest in recent years is the study of head-to-head, tail-to-tail polymers (96-98). Their direct synthesis has little likelihood of being successffil as head-to-tail sequences usually predominate in vinyl polymerization. One possibility for their preparation is through the chemical modification of suitable preformed polymers. In the case of the head-to-head, tail-to-tail polypropylene, different stereoisomeric forms have been isolated, depending on the method of preparation. In the general scheme, the precursor is an unsaturated polymer obtained by polymerization of the disubsti-tuted butadiene (2,3-dimethylbutadiene or 2,4-hexadiene) then, by chemical or catalytic reduction, this polymer is converted into the desired polypropylene, whose stmcture can then be examined by NMR spectra. Head-to-head, tail-to-... 

This term describes the phenomena of stereoisomerism observable in short chain segments. Microtacticity considerations can be applied to stereoregular polymers, to sequences of partially regular polymers, and to highly disordered polymers, both from the qualitative and quantitative points of view. This approach is the most appropriate and complete for the structural study of real polymers. 

In favorable cases therefore, H NMR spectroscopy constitutes an absolute method for the determination of the stereoisomeric structure of polymers. This affirmation holds true not ordy in the sense that the spectrum is in agreement with the proposed structure (isotactic for the first polymer, syndiotactic for the second) but also because a detailed examination of the whole spectrum permits the exclusion of other hypotheses that might reasonably be put forward. 

The problem is further complicated for vinyl polymers with their problems of stereoisomerism. The first descriptions of the conformational state of isotactic polypropylene in solution go back 25 years (178, 179, 192, 193). Corradini, Allegra, and Ganis proposed a model, still essentially valid today, according to which macromolecules possess a local helical structure analogous to that observed in the crystalline state. The helix segments are rather short, only a few monomer units, after which an inversion of the helix sense occurs, with simultaneous alteration of its direction (Figure 15). As a whole this disordered con-... 

The polymerization of enantiomerically pure monomers presents no relevant stereochemical problems when the asymmetric carbon atom is not involved in the reaction and no new centers of stereoisomerism are formed. This is the case, for example, in polycondensation of chiral diacids with diamines (274) and in ring-opening polymerization of substituted lactams (275) and A -carboxyanhy-drides of a-amino acids (276). Interest here lies mainly in the properties of the polymer. Accidental racemization may sometimes occur but is not necessarily related to the mechanism of polymerization. 

The absence from a formula of any one of the horizontal or vertical lines at a chiral or prochiral carbon atom (as in examples on pages 27 and 32), or of or Z designations at double bonds, indicates that the configuration of that stereoisomeric centre is not known. Also, as in our previous document [2], the convention of orienting polymer structures (and the corresponding constitutional and configurational units) from left to right is used. Thus, the two bracketted constitutional units in... 

As the definition of a stereoregular polymer (see Definitions 1.4 and 1.9) requires that the configuration be defined at all sites of stereoisomerism, structures (8) and (10) do not represent stereoregular polymers. The same is true of (11) and (12), which differ from (8) and (10) in that the sites of specified and unspecified configuration have been interchanged. 

Examples (4), (5), (6), (8), (10), (11) and (12) are tactic polymers. A stereoregular polymer is always a tactic polymer, but a tactic polymer is not always stereoregular because a tactic polymer need not have all sites of stereoisomerism defined. 

Note As the definition above indicates, a regular polymer, the configurational base units of which contain one site of stereoisomerism only, is atactic if it has equal numbers of the possible types of configurational base units arranged in a random distribution. If the constitutional repeating unit contains more than one site of stereoisomerism, the polymer may be atactic with respect to only one type of site if there are equal numbers of the possible configurations of that site arranged in a random distribution. 

A polymer such as -[-CH=CH-CH(CH3)-CH2-hr which has two main-chain sites of stereoisomerism, may be atactic with respect to the double bond only, with respect to the chiral atom only or with respect to both centres of stereoisomerism. If there is a random distribution of equal numbers of units in which the double bond is cis and trans, the polymer is atactic with respect to the double bond, and if there is a random distribution of equal numbers of units containing the chiral atom in the two possible configurations, the polymer is atactic with respect to the chiral atom. The polymer is completely atactic when it contains, in a random distribution, equal numbers of the four possible configurational base units which have defined stereochemistry at both sites of stereoisomerism. 

Polymerization in which a tactic polymer is formed. However, polymerization in which stereoisomerism present in the monomer is merely retained in the polymer is not to be regarded as stereospecific. For example, the polymerization of a chiral monomer, e.g., R)-propylene oxide ((i )-methyloxirane), with retention of configuration is not considered to be a stereospecific reaction however, selective polymerization, with retention, of one of the enantiomers present in a mixture of R)- and (S)-propylene oxide molecules is so classified. 

Tactic polymer that contains two sites of defined stereoisomerism in the main chain of the configurational base unit. 

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