Important note in this synthesis: It is very important that all glassware, tubing, etc., in this synthesis is kept as dry as possible. I usually dry all glassware in the 80C oven prior to the synthesis.
Another note: if PEG is to be methacrylated, use a molecular weight of 104.5 g/mol and density of 1.07 g/ml, as well as a different purity (depending upon company – usually written on the bottle) in the calculations in step 7 below.
This protocol is for the synthesis of 10 grams of diacrylated product.
1) Obtain a dry, 3-neck, round-bottom flask (250 ml).
2) Place 10 grams of poly(ethylene glycol) (PEG) into the flask. Various molecular weights are available, the most common being PEG3400 and PEG4600.
3) Add 30 ml methylene chloride to the flask, and set up the apparatus as depicted below:
4) Calculate the amount of triethylamine (TEA) required. Usually, 12 moles of TEA are added per mole of PEG. For a ten-gram synthesis of PEG3400DA, this is 4.92 ml. In general, to calculate amount of TEA required, use the following formula (note that different molar ratios of TEA to PEG can be used):
Amt. TEA = (10 g PEG)/(3400 g/mol PEG)*(12 mol TEA/mol PEG)*(101.2 g/mol TEA)/(0.726 g/ml)
5) Before adding the triethylamine, it is necessary to purge the flask to remove any water vapor that may be present. To do this, connect an argon purge needle to one of the necks of the flask using a rubber septum. The purge needle tip should be submerged in the solution. On the third neck of the flask, place a septum with a vent needle (see Figure above). Purge the flask for 5 minutes
6) Add the appropriate amount of TEA (4.92 ml for 10-g synthesis).
7) In the addition funnel, combine 15 ml of MeCl2 with acryloyl chloride (AC). Usually 10 moles of AC are added per mole of PEG. To calculate the amount of acryloyl chloride to add, use the following formula (note that different molar ratios of acryloyl chloride to PEG can be used):
Vol. AC = (10 g PEG)/(3400 g/mol)*(10 mol AC)/(mol PEG)*(90.5 g/mol AC)/(0.96 purity)/(1.114 g/ml)
For a 10-g synthesis, this amount is 2.50 ml.
8) Make sure the apparatus is setup as shown in the Figure above. It is very important to make sure the flask sits in an ice bath and make sure that the ice bath remains ice-cold during the duration of the following step.
9) Very SLOWLY, drip the acryloyl chloride solution into the PEG/MeCl2/TEA solution. When I say very slowly, this means that there should be between 5-15 seconds between each drop! The solution will turn yellow or orange if the solution is added too quickly (the solution will actually probably turn light yellow regardless).
10) Let the acryloyl chloride solution drip for 45-60 minutes, then cap the flask and place on a stir plate in the cold room (4C) and stir overnight.
Note: it is not uncommon for the solution to be colored, even fairly red colored. This is caused by the formation of small byproducts in the reaction mixture. Through purification procedures (like washing with water or dialysis), the color can be removed.
11) The following day, the product is precipitated in ethyl ether. Instead of precipitating the entire product (and wasting time and expensive ethyl ether), I usually will precipitate maybe a quarter of the product and verify substitution using NMR. To do this, I precipitate 10-15 ml into a beaker containing a stir bar and 200-300 ml ethyl ether (on ice). The precipitate can then be filtered using Whatman paper and a ceramic filter, and the product is placed in a watch glass or glass Petri dish for drying in the vacuum oven at 50C for several hours.
12) Once the product has been dried, it can be evaluated for ability to polymerize and can be characterized using NMR.
Note: if the substitution is low, the product can be re-acrylated or remethacrylated.
13) If substitution/ability to polymerize is satisfactory, the remainder of the product can be precipitated. Usually, a 10:1 ratio of ether:product is used for precipitation. Keep in mind that ethyl ether is expensive and also must be disposed of through EH&S, so try to minimize waste generated through judicious experimental design.
14) Clean up your mess.