Introduction

Reactions in that bonds are broken and made at a solo asymmetric atom (generally but not necessarily carbon), and which lead mainly to a single stereoisomer, are said to be stereo exact. If the configuration is changed in the procedure, the reaction is said to engage inversion of configuration. If the configuration continues the similar, the transformation happens through retention of configuration. In each polymer molecule, the atoms are bound mutually via covalent bonds.

Though, the divide molecules, or segments of the similar molecule, are magnetized to each other via weak 'intermolecular forces', as well termed 'secondary' or "Vander Waals" forces. In common, covalent bonds govern the thermal and chemical stability of polymers. On the other hand, secondary forces find out most of the physical properties we connect through exact compounds.

Polymer Stereochemistry 

The kind of monomer(s) constituting a polymer chain and the way they are bonded, via and huge, find out the properties of the polymer. Therefore there might be more than one kind of monomer and, or the monomers might be arranged to provide a linear, branched, cross-linked or network structure.

Instances -

• Linear Polymers: Polyethylene, poly (vinyl chloride) (PVC), polystyrene, polymethyl methacrylate (plexiglass), nylon, fluorocarbons (teflon).
• Branched Polymers:
• Many elastomers or rubbers.
• Cross-bonded Polymers: Thermosetting polymers, many elastomers or rubbers are as well cross-linked (vulcanized).
• Network Polymers: Epoxies, phenol-formaldehydes.

Such effect in differences in the polymer microstructure giving increase to isomerism in polymers. There are 4 significant kinds of polymer isomerism, namely; Structural (Architectural) isomerism, Configurational isomerism, Orientational isomerism and Geometrical isomerism.

Structural Isomerism

This happens from the fact that atoms or group of atoms are bonded differently in the formation of branches, side chains or network. Polymers derived from isomeric monomers, for example, poly (vinyl alcohol), poly (ethylene oxide) and poly acetaldehyde contain the similar chemical composition C₂H₄O but dissimilar atomic arrangements through side chains in 2 of them.

 Fig: Structural Isomerism

As we know the physical property of the polymers, for example, the glass transition temperature; T_g for poly (vinyl alcohol) is 350K, T_g for poly acetaldehyde is 243K and T_g of poly (ethylene oxide) is 206K.               

Orientational isomerism

This consequence from the different orientations of monomer addition at the active center during vinyl polymerization. Consider a vinyl monomer CH₂=CHX where X is any substituent. One carbon atom of the double bond might be arbitrarily labeled the head and the other the tail of the monomer, that is 

                            (tail) CH₂=CH (head)  

                                              X    

The active center can be shaped via 2 possible reactions-

 Fig: monomers

The monomers then add to the active centers leading to the formation of polymer through the subsequent chain structures:   

If route (1) is favored, the arrangement is recognized as head to tail configuration.    

                ~CH₂-CH-CH₂-CH-CH₂-CH-CH₂-CH~        (3)

                           X            X           X          X    

                                     Head to tail     

If route (2) is followed, that is chain enclosing a portion of head to head, tail to tail arrangement or a random structure enclosing both is attained.

                ~CH₂-CH-CH₂-CH-CH-CH₂-CH₂-CH-CH-CH₂-CH₂-CH~      (4)

                          X           X     X                  X    X                  X    

                                      head to head                             tail to tail

The real mode of addition depends on 2 factors; the stability of the product and the possible steric hindrance to the approach of R? caused by a large group X in the molecule. The reaction in route (1) is extremely favored, since:  

There is a greater possibility of resonance stabilization of this structure due to interaction between group X and the unpaired electron on the adjacent α- carbon atom.

The direction of radical attack is least impeded via substituent X. Thus the favored structure is the head to tail orientation (3) while the alternative structure (4) might occur occasionally in the chain, chiefly when termination via amalgamation predominates; the existence of a completely head to head orientation is unlikely unless synthesized via special route. It is experimentally proven that structure (3) (head to tail) is predominant in majority of polymers. 

The most notable exceptions are poly (vinylidene fluoride) through 4 to 6% and poly (vinyl fluoride) by 25 to 35% head to head bonds noticed via N.M.R studies. The presence of head to tail structures can be revealed in different ways for instance, when a dilute solution of poly (vinyl chloride) in dioxane is heated through zinc dust, chlorine is abolished. This can proceed via 2 mechanisms: 

CH₂ 

(a)    ~CH₂-CH-CH₂-CH-CH₂~ + Zn          ~CH₂-CH-CH-CH₂~  +  ZnCl₂ 

                  Cl            Cl

      Head to Tail polymer 

(b)   ~CH2-CH-CH-CH2~ + Zn          ~CH2-CH=CH-CH2~  +  ZnCl2 

                 Cl   Cl

     Head to Head polymer 

Analysis of chlorine loss via route (a) shows that only 86.4% of the chlorine will react because elimination is a random procedure, the other 13.6% become isolated during the reaction and will remain in the chain. Elimination via mechanism (b) effects in total removal of chlorine. Poly (vinyl chloride) after treating through zinc dust indicated 84 - 86% chlorine elimination. This suggested that the polymer is approximately the head to tail orientation. 

Configurational Isomerism

If a tertiary carbon atom in a polymer chain has 2 dissimilar side groups connected to it, the spatial arrangement of bonds in a molecule will be different. The presence of the asymmetric carbon atom (or chiral center) provides increase to configurational isomerism. Therefore a group -CHX- can only have 2 possible configurations since carbon atom is tetrahedral.

                         X                                                        H

                  H---C              where R1≠ R2             X---C

                    R₁-----R₂                                            R₁-----R₂  

These configurations can't be interconverted merely via rotation about single bonds; bonds have to be broken and reformed. To differentiate them, they are arbitrarily allocated (d-) or (l-) configurations using the terminology of stereochemistry. As we know [The term merely terms to whether X is below or above the chain in a planar projection]. It is apparent that the distribution of (d-) and (l-) configurations find out the extent of regularity or order in the polymer. Using the nomenclature of Natta, polymers through extremely regular distribution of (d-) or (l-) are recognized as Tactic (or stereoregular) Polymers and those through random distribution as Atactic Polymers. Two kinds of tactic structures can take place: Isotactic and Syndiotactic.

In isotactic polymers, the asymmetric carbon atom in each repeat unit in the chain has the same configuration. That is, all the X substituents are located on one side, either all above or below the cabon- carbon polymer chain. Polymers in that all the successive asymmetric carbon atoms in the chain have opposite configuration are said syndiotactic polymers. 

A random arrangement of substituent groups in the chain effects in atactic polymers. The X substituents are placed alternating on opposite side of the plane of the chain. Such structures are symbolized below through Fischer projections: 

                                     X  X  X  X  X  X 

                                   ~C-C-C-C-C-C~           Isotactic polymer

                                     H  H  H  H  H  H 

                                     H  X  H  X  H  X 

                                   ~C-C-C-C-C-C~           Syndiotactic polymer

                                     X  H  X  H  X  H 

                                     H  X  H  X  H  H 

                                   ~C-C-C-C-C-C~            Atactic polymer

                                     H  H  H  H  X  X                                                                                                        

Fisher Projection of Isotactic, Syndiotactic and Atactic Polymers with repeating unit -CHX-

Geometrical isomerism 

When diene monomers enclosing conjugated carbon-carbon double bonds are polymerized, geometrical isomerism arises as a consequence of dissimilar configuration of substituents on the double bonds. The diene might be symbolized via the common formular - 

                                       X       

                              CH₂=C-CH=CH₂            1, 4-butadiene           

                                1     2    3     4

 1, 4 polymerisation, leads to a polymer with double in the chain and so can have either cis- or trans- configuration. In the cis- isomers, the chain parts are on the similar side of the double bond while on the trans- isomers, they are on opposite sides.                         

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