# Polymerisation of a UV-curing resin

I have a photosensitive resin MSDS. It lists a mixture of 1,6-Hexanediol diacrylate ($$\ce{C_{12}H_{18}O_4}$$) with its proper CAS number and Polyurethane acrylate together with the CAS number of "Butyl acrylate/methyl methacrylate/methacrylic acid copolymer" with the formula (or rather, formulas) of $$\ce{(C7H_{12}O2.C5H8O2.C4H6O2)_x}$$. There's also the Photoinitiator 1-Hydroxycyclohexyl phenyl ketone listed. The additives of color and reaction inhibitor are most likely irrelevant for the actual question, so I leave them out.

My organic chemistry is somewhat rusty, but the first part of the compound I can identify as having the setup $$\ce{H2C=CH-CO-O-C6H12-O-CO-CH=CH2}$$. This appears to be a quite nice molecule that reacts evenly on both sides under polymerization. This material is also listed as photosensitive, which I interpret that it sheds something from both ends to activate the ends.

The other, of course is a mixture of three monomers: Butyl acrylate ($$\ce{H2C=CH-CO-O-C4H9}$$), methyl methacrylate ($$\ce{H2C=C(CH3)-CO-O-CH3}$$) and methacrylic acid ($$\ce{H2C=C(CH3)-COOH}$$). The first two have ester groups in them, the last has its precursor, the $$\ce{-COOH}$$ group (carboxylic acid). All these only seem to have one active side to me.

Assuming the four main chemicals are mixed somewhat evenly and in even numbers and what might be an ample supply of the starter Hydroxycyclohexyl-phenyl-ketone, they surely will react and polymerize under UV light of about 400 nm (+- 5), as this is how the mixture behaves in its native application, an SLA printer.

However I have a problem with these parts:

• What kind of reaction does happen between the compounds? I remember vaguely that some polymerizations create waste in the shape of water (was that condensation?), but I don't really see that happening here for most of the combinations and, as a result, I fail to see what the parts of the polymer might look like in their bonded state.
• What type of bonds does the resulting polymer have? This is pretty much the direct result of failing to see the reaction between the compounds. I thought I had seen a possibility to form an ester ($$\ce{R_1-CO-O-R_2}$$) between the methacrylic acid and the hexanediol diacrylate, but then realized the later misses an alcohol group ($$\ce{R-OH}$$) to attack and create water from to form the bond. So I am pretty sure that these don't bond as esters to one another, but what is the correct name for these bonds that elude me?!
• What would this polymer be classed as? I remember that some polymers are classed based on their monomers, manufacturing process, the chain length, bond type, and the macro-structure, putting each into several groups. These would allowing to estimate some vague properties about them. It's most certainly not a Homogenous one like pure PolyLacticAcid, as it is basically made up of 4 different monomers. It also should not form an elastic bond due to what I know from the MSDS and that the processed resin is a somewhat brittle product. But what groups it can be put into from the polymerization process?
• You need a photoinitiator to do this. Related article: Songklanakarin J. Sci. Technol. 2011, 33(2), 201-207 (thaiscience.info/Journals/Article/SONG/10761807.pdf). – Mathew Mahindaratne Aug 15 '20 at 19:15
• that would be, according to the MSDS, CAS 947-19-3, so... 1-Hydroxycyclohexyl phenyl ketone – Trish Aug 15 '20 at 19:20
• That'd work. Check with photo-curing coating. Similar reactanets and polymerization. – Mathew Mahindaratne Aug 15 '20 at 19:29
• in this case, it's an SLA resin, I think the chemistry might be quite similar. – Trish Aug 15 '20 at 19:30
• I don't get it. It's just an acrylic resin and you get polyacrylate! – Mithoron Aug 15 '20 at 23:14

## 1 Answer

Polymerizations expelling water, equally known as polycondensation (PET is a typical product), are a subset of polymerizations. The radical polymerization here is more likely to proceed differently with three reaction types running concurrently: initiation, propagation, and termination.

• During the initiation, light splits the phenylketone into two fragments with one unpaired electron each. This is symbolized by the dot next to the oxygen atoms, denoting that these chemicals are radicals:

• These radicals may react with one electron of the double bond of a neutral acrylate in the propagation reaction. Symbolized by blue is the newly formed single bond between the two reagents (first line). Simultaneously, the other electron of the former double bond represents a new site where again a neutral acrylate may react (second line). This growth of the polymer chain may continue again and again, or propagate (third line).

• As soon as there are radicals formed, there may be a number of reactions where these carbon-centered radicals are consumed. This cancels the further growth of the polymer chains. Among these termination reactions, hydrogen-transfer / redox reactions as well as dimerizations are the most prominent ones. Some which seem likely to occur are depicted below:

# Structure

Your starting material consists of a starter, and a blend of bifunctional acrylates. Thus the curation will yield polymer chains that are cross-linked with each other. The illustration below (for a different material) symbolized this by the blue zig-zag lines:

(Konuray et al., Polymers 2018, 10, 178 doi 10.3390/polym10020178, open access)

Because you started with multiple different monomers, it would be an example of branched copolymers.