Doug Schoon Has a good bit about this when he talks about inhibitors that are added to the monomer to prolong the shelf life and stop the liquid from gelling. PG137
Spot on research
Monomer does indeed contaminate and 'go off' easily
I've seen with my own eyes the result of a shipment left to sit on the dock in the sun... the entire batch had turned into a solid 'jelly' in the bottles...
Monomers are a very energetic molecule, they are so charged that they actually are constantly moving in the container and 'push away' from each other - the moment a reaction starts they grab ahold of each other one by one forming into millions of molecules in a long chain (solid)
In the container the inhibitors keep them from being able to start grabbing each other. These inhibitors only work for an average of 18 months (in a sealed container)
Opening the bottle and exposing oxygen will start to degrade them and shorten their protective nature.
Any storage container once opened will lead to the eventual breakdown and partial polymerisation and loss of energy from the molecule.
Never should old monomer be added to fresh monomer, it will already have an altered structure and will instantly weaken the fresh product... This is why Doug recommends only ever dispensing a small amount at a time from a small (eg 4 oz) container. With your bulk bottles only used to refill the small container when empty.
even the best pumps I've come across generally allow some form of forward and backward flow, which can cause contamination and breakdown of the monomer stored. If it works for you that's great, but dispensing to a small dappen dish ensures one less thing that 'could' cause you service breakdown.
Monomer evaporation with oxygen has a two fold problem -
one, the evaporation does indeed alter the formula, there is far more than just one type of monomer in the liquid, and different ingredients evaporate at different levels
two, oxygen and other contaminants degrade the formula by bonding themselves in and creating new molecules and damage to the original chemicals...
hope that all helps a little...