Steric Arrangement in Cis and Trans Configuration

Polymers with double bonds in the repeat unit give rise to different geometric isomers. For example, 1,3-butadiene can be polymerized to give poly(1,2-butadiene) or either of two geometric isomers of poly(1,4-butadiene).(2) These two isomers are called cis and trans poly(1,4-butadiene). In the case of cis-polybutadiene, the first and the forth carbon lie on the same side of the central double bond, and in the case of trans, on opposite sides of the central double bond.

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Double bonds in the cis form reduce the energy barrier for the rotation of adjacent C-C bonds compared to those in trans from, which, in turn, reduces the glass transition temperature (Tg). The affect of cis-trans isomerie on the Tg is usually not very strong, that is, the Tg shift is only about 20 K for polybutadiene. If other substituents (-Cl, -CH3 etc.) are present, the energy barrier for the rotation of adjacent bonds will increase, and hence the Tg will increase as well. Bulky substituents will also reduce the energy barrier for rotation of the trans form relative to the cis form. For example, the observed Tgs of cis and trans polyisoprene are nearly the same, whereas the Tg of the cis isomer of poly(chloro-butadiene) is about 20 K larger than the Tg of the trans isomer (see table below).

 

Glass Transition Temperature of Polybutadienes

Polymer Tg,cis (K) Tg,trans (K)
Polybutadiene 170 (165) 187 (190)
Polyisoprene 204 207
Poly(chlorobutadiene) 252 234
The glass transition temperatures have been calculated with the software 3Ps-Tg from Triton Road and the values in parenthesis have been taken from reference (1).

 

Double bonds in the polymer backbone have a singnificant impact on the polymer properties. The double bonds in the polymer backbone prevent rotation that occurs in carbon-carbon single bonds. It is believed that the inability to rotate freely hinders the allignment of the polymer chains. Thus, unsaturation in the polymer chain reduces the crystallinity and increases the elasticity and flexibility of polymers.

Literature
  1. Robert O. Ebewele, Polymer Science and Technology, 1st Edition, 2000
  2. 1,2-Polybutadiene, which has a pending vinyl group, has very different properties. For example, its Tg is 260 K and more than 70 K higher than those of cis and trans 1,4-polybutadiene.
  • Summary

    PolyDiene Isomers

    Polymers with double bonds in the backbone exist in two different isomeric forms:

    CIS ISOMERS: First and forth carbon lie on the same side.

    TRANS ISOMERS: First and forth carbon lie on opposite sides.

  • Arrangements fixed by chemical bonds are known as configurations. The configuration of a diene cannot be altered unless bonds are broken and reformed.

  • Cis and trans polydienes have similar properties. Often, the trans form has a slightly higher glass transition temperature.

  • Polydienes, also known as rubbers, are often slightly cross-linked (vulcanized) to improve the mechanical properties.

  • Cis-trans isomerization

    During vulcanization, part of the cis 1,4 double bonds are converted to trans 1,4 double bonds.

  • Unsaturation or double bonds in the polymer backbone reduce crystallinity and increase elasticity.

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