Dimethyl amine for methyl methacrylate polymerization
Answers
HEY MATE!
First of all lets break down PMMA,
PMMA = Poly(methylmethacrylate) or essentially a polymer (repeating chain of monomeric units) of methylmethacrylate.
Acrylic monomers are typically polymerized by generation of a radical and these can either be controlled or uncontrolled, but the key here is that in order to generate a radical something called an initiator needs to be used.
Initiation:
Industrially, something like AIBN is often used and when heated above a certain temperature it will generate a radical. This radical is so high in energy that it will attack the double bond of the methylmethacrylate monomer and generate a new radical.
Controlling how many radicals are generated is called controlled polymerization.
Propagation:
Now that our radicals are on the start of our poly(methylmethyacrylate) chains the polymer chains can grow and grow. This is dependent on there being sufficient monomer to consume and the viscosity of the polymerization stays low enough that a radical can find a new double bond. Typically mechanical stirring and solvents are used to ensure that high conversion of monomer can occur. When viscosity is too high to proceed we can think of this as vitrification.
Termination:
Eventually as all of the monomers are consumed the viscosity will become so high that a radical cannot find another monomer to react with or that radical will find another radical and link two polymer chains together. High molecular weight will give higher viscosity. Chain terminators can also be added to control the molecular weight of the polymer chain (how long it is).
Crystalline vs Amorphous:
PMMA is amorphous (clear) and something like high density polypropylene is semi-crystalline (opaque). The crystals essentially refract and scatter light because they are so large and they are also key in many polymer's strength.
Photopolymerization follows the same ideals of initiation. The photocatalyst generates a free radical with exposure to UV light and that radical starts the polymerization. Photopolymerization can encompass many other types of polymerization besides free radicals, but often there is no stirring and minimal solvent being used so because of this molecular weight growth is minimal and oftentimes crosslinking agents can be added to ensure better physical properties.
Photopolymerization is used widely in the electronics industry for epoxy resins and 3D lithography.
Photopolymerization is a useful technique for polymer coatings and can be applied to 3D printing.
Saeed also mentioned prepolymers which are also a route to achieve higher molecular weight growth, but can increase the viscosity of the polymer system dramatically so any photopolymerization that does occur is more to link the prepolymers together as opposed to actually growing polymer chains.
Hope that helps answer your question,