Modes of Addition for Conjugated Monomers

Modes of Addition for Conjugated Monomers jls164

Thus far, we have only considered chain polymerization of monomers that have isolated reactive groups. But we also recall from general chemistry and organic chemistry that double bonds are conjugated and can participate in resonance (depending on the molecular structure). So what happens if we have a conjugated monomer? For example, 1,3-butadiene and isoprene are common monomers (shown below) which have conjugated double bonds. How do these monomers react?

Molecular diagrams of 1,3-butadiene and isoprene
Figure 7.9: 1,3-isobutadiene and isoprene
Source: Dr. Lauren Zarzar based on figures from Young, Robert J., and Peter A. Lovell.
Introduction to Polymers, Third Edition, CRC Press, 2011.

There are going to be multiple ways in which these unsaturated bonds can react. To keep track of which carbon is where, we need to number our carbons. Following tradition from the naming of organic molecules, we number our carbons along the carbon chain with the diene and start counting with the carbon nearest the substituent. Take this generic diene as an example, where "R" represents some substituent. We number the carbons as follows:

molecular diagram of a generic diene with numbered carbons
Figure 7.10: Generic diene with numbered carbons
Source: Dr. Lauren Zarzar based on figures from Young, Robert J., and Peter A. Lovell.
Introduction to Polymers, Third Edition, CRC Press, 2011.

It is very important that you number your carbons correctly, or you will end up drawing the wrong polymerization products. Please note that you do not necessarily number from left to right, it just depends on where that substituent "R" is located. We see we have two unsaturated bonds that we could polymerize: the 1,2 and 3,4 bonds. Figure 7.11 shows the products we get if we polymerize through those bonds:

Molecular diagrams showing 1,2 addition and 3,4 addition
Figure 7.11: Products of polymerizing through the 1,2 and 3,4 bonds
Source: Dr. Lauren Zarzar based on figures from Young, Robert J., and Peter A. Lovell.
Introduction to Polymers, Third Edition, CRC Press, 2011.

We see we get two very different polymer depending on whether we react the 1,2 or 3,4 bond. (Also, don't forget that each of these polymers can have tacticity!) But we aren't quite done, because these dienes have conjugated double bonds, which we know can have resonance. This means, if we generate an active center, that it can move through the molecule via resonance and this will also affect our products. To make things even more complicated, single bonds can rotate, and the bond rotation in the diene will also affect our polymer! As shown in Figure 7.12, because of these resonances and bond rotation we can also get cis 1,4 addition and trans 1,4 addition. (Remember the "cis" and "trans" geometric isomers from general chemistry and organic chemistry?).

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Figure 7.12: Polymerization of dienes showing the formation of the cis and trans 1,4 addition products. The * signifies the active center.
Source: Dr. Lauren Zarzar based on figures from Young, Robert J., and Peter A. Lovell.
Introduction to Polymers, Third Edition, CRC Press, 2011.

Just as tacticity was important to the polymer structure and properties, so is the mode of addition for dienes. Depending on whether you have primarily cis or trans addition, you can have polymers with very different mechanical properties.

PROBLEM

Shown below are (A) trans, (B) cis, and (C) vinyl polybutadiene (from left to right). Which one is most crystalline?

molecular diagrams of (A) trans, (B) cis, and (C) vinyl polybutadiene
  1. Trans polybutadiene
  2. Cis polybutadiene
  3. Vinyl polybutadiene

ANSWER

A. Trans polybutadiene

The regular zig zag structure of the backbone will allow the polymer to pack better giving it more crystallinity.