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The official solution is that A has branched / side chains, weak Van der Waals forces between the polymer chains, less compact packing than B, and large inter-chain distances. B has no branched chains, hydrogen bonds between the polymer chains, compact packing, and small inter-chain distances.

  1. What does a branched chain mean? How does this influence the density? Doesn't B also have side chains?

  2. What does 'compact packing' mean in this case? Does it mean that separate chains are packed closely together, or that in a single chain, the molecules are closer together?

Polymers

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First polymerisation proceeds through free radical mechanism, which may cause intermediate radicals to abstract H from other parts of growing chain, thus creating new branches in the process.
Refer Wikipedia for detailed mechanism.

On the other hand, second polymerisation proceeds through condensation reaction, where no such thing is possible (condensation can occur only at terminals).

Furthermore, the chains of B interact through H-bonding, which is much stronger than van der Waal interaction of polymer A. This causes chains to come closer and increases the overall density of polymer.

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  • $\begingroup$ So branching means that if I have 'branched' polyethene, I'll have a Hydrogen replaced by a chain? But wouldn't that make it denser than before? If it were unbranched, there'd be large distances between two chains because of the weak Van der Waals forces, but now, since the two chains are actually linked, they will be closer, and will thus take lesser space? $\endgroup$
    – John
    Apr 26, 2017 at 16:14
  • $\begingroup$ PP is very rarely made via FRP (free radical polymerisation), the result is a some gunk that looks rubbery, is actually liquid however. iPP via Z-N and derived catalysts can be made basically defect-free. It still doesn't crystallise as perfectly as nylon, because it needs to make a helical structure. $\endgroup$
    – Karl
    Oct 30, 2020 at 23:33
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“Branched chain” simply means that the main carbon backbone has other, smaller carbon backbones coming off of it. You’re correct, a hydrogen atom would have to be removed from a carbon atom in order to attach another polymer chain. Generally, branched chains decrease the density of a polymer because they fill space less efficiently than a perfectly straight chain. Imagine a pile of firewood - what would have more empty space? A pile of perfectly straight logs, or a pile of brush?

Polymer B does not have side chains. “Side chains” refers to units off the main chain with a carbon backbone, while the non-carbon atoms in polymer B are more accurately referred to as “functional groups”. Here, the N-C=O is known as a “ketone”.

Compact packing refers to the packing between chains, which is what determines the density of the polymer. Effects of interactions between chains are MUCH more significant than the small differences in interatomic bond length that can occur when you start adding functional groups.

As mentioned by jonsno, the hydrogen bonding of polymer B pulls the chains closer together than the VDW forces of polymer A.

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