Addition Polymers Branching

Study Notes

The unpaired electron is at the end of the last unit added to the chain, known as the propagating site.
The propagating site can react with multiple monomers, repeating the propagation steps hundreds or thousands of times.
The propagation steps are terminated in a termination step, which can occur through one of three pathways.

Chain combination: two chains combine at their propagating sites.
Disproportionation: one chain is oxidized to an alkene, and the other is reduced to an alkane.
Chain transfer: the growing chain reacts with a molecule XY, terminating the chain and initiating a new one.

The propagating site can remove a hydrogen atom from a chain, leading to branch growth.
Branching can occur through the removal of a hydrogen atom from a different polymer chain or from the same polymer chain.
Short branches are more likely to be formed than long ones, depending on the location of the removed hydrogen atom.
Branching affects the physical properties of the polymer, with unbranched chains packing together more closely than branched chains.

Linear polyethylene (high-density polyethylene) is a hard plastic, used for producing artificial hip joints.
Branched polyethylene (low-density polyethylene) is a more flexible polymer, used for trash bags and drycleaning bags.

The initiator is an electrophile that adds to the monomer, causing it to become a carbocation.
The initiator cannot be an acid such as HCl.
Elastomers, such as rubber and spandex, are formed through cationic polymerization.

The monomers commonly used are ethylene (ethene) and substituted ethylenes (CH2=CHR).
Polymers formed from ethylene or substituted ethylenes are called vinyl polymers.
Chain-growth polymerization proceeds by one of three mechanisms: radical, cationic, and anionic polymerization.
Each mechanism has three distinct steps: initiation, propagation, and termination.