Synthesis of Heterobifunctional Carboxylic Acid PEG Polymer Silane

Jonathan Duhon

Introduction:

Heterobifunctional molecules possess a structure that allows for binding at two different

sites. Applications for these compounds include the linkage of macromolecules to surfaces and

site-specific targeting of drugs and the functionalization of nanoparticles for drug delivery. These

compounds are used extensively due to their lack of toxicity. When bound to other molecules,

PEG increases their solubility in aqueous environments to improve circulation times in vivo.

Experimental:

Obtain a 250 mL round bottom flask and set it atop a hot plate at 90 °C, spinning at 360

rpm with a stir bar. Weigh out 3 grams of PEG and place inside the round bottom flask. Clamp

the round bottom flask with a vacuum tube on the top, and a stopper on its side. Turn vacuum on

and let the PEG dehydrate for 24 hours. After 24 hours have past, let oil bath cool to 55 °C. Turn

off vacuum and hook up a bottle of THF to the nitrogen tank to ensure that no water gets inside

the round bottom flask. Syringe out 100 mL of the THF into the round bottom flask. Next,

weight out 181.6 mg of NaH and place inside round bottom flask. Let mixture react for 30

minutes, then gather .55 mL of tert-butyl-bromoacetate to put inside mixture. Let mixture react

for 24 hours at room temperature, spinning constantly.

After mixture has reacted at room temperature for 24 hours, transfer contents to another

round bottom flask that is 250 mL. Obtain a trap flask and clamp it to the Rotoevaporator, with

the new round bottom flask attached at the bottom. Turn on Rotoevaportor, vacuum, and turn the

faucet on low. Put the RPM to max, and the temperature of the Rotoevapor on 50 °C. To speed

up process, turn the temperature up. Once precipitation is done, take off the round bottom flask

(not the trap flask) and fill it with ether until mixture is observed. Retrieve a large filter sheet and

fold in half twice, then shape filter sheet into a cone. Place cone filter that was previously made

into a funnel, and filter into a 600 mL beaker. Set aside this filtered cone with solid precipitate.

Once precipitate is set aside, obtain clean column, DCM and MeOH. Get a 1000 mL

graduated cylinder and pour 900 mL of the DCM and 100 mL of the MeOH into the graduated

cylinder. Obtain two beakers and weigh out 245 grams of silica. Place half of the silica in one

beaker, with the other half of the silica in the other beaker. Pour mixture of DCM and MeOH

into the two beakers with silica, then pour this mixture into the column. Keep pouring DCM and

MeOH mixture into column until there is a uniform band that forms in the column. Take

precipitated product in cone filter, and scape into a round bottom flask. Dissolve this precipitate

with the DCM and MeOH solution. Once precipitate is dissolved, obtain pipette and rubber

stopper to slowly drip mixture alongside of the column until a ring forms in column. Fill the rest

of the column up with the DCM and MeOH mixture. Make fractions of this solution in the

column into around 75-100 test tubes. Use TLC plates to locate the product in the reaction by

using four spots on each TLC plate. Use DCM and MeOH compound in TLC plates. Once

chromatography is done, place plates in (amber bottle) and use blowtorch.

From the use of TLC plates, the product in the fractions can easily be located. Place the

test tubes with the product inside of a 250 mL round bottom flask and place in Rotoevaporator

machine again at the same RPM and temperature. Once complete, take round bottom flask and

mix chloroform inside of it. Take mixture and place inside NMR tube to see if the correct

product was synthesized.

Observations:

250 mL round bottom flask atop hot plate with PEG, THF, NaH, and tertbutylbromoacetate

RotoEvaporator Machine

Column with DCM and MeOH

Fractions

Results:

PEG IR

Structure C-H C-O-C OH

Frequency 3000 Broad 1750 Broad 3300

PEG Carbon NMR

PPM 27-30 65 66 70

Structure Tert-butyl CH2 H2 CH2

PEG Proton NMR

PPM 1.4 4.0 3.6 3.7

Structure Tert-butyl

hydrogens

CH2 CH2 CH2

Discussion:

3 𝑔𝑟𝑎𝑚𝑠 𝑃𝐸𝐺 (1 𝑚𝑜𝑙 𝑃𝐸𝐺

2000 𝑔𝑟𝑎𝑚𝑠 𝑃𝐸𝐺) (

1 𝑚𝑜𝑙 𝑃𝑟𝑜𝑑𝑢𝑐𝑡

1 𝑚𝑜𝑙 𝑃𝐸𝐺) (

2113 𝑔𝑟𝑎𝑚𝑠 𝑃𝑟𝑜𝑑𝑢𝑐𝑡

1 𝑚𝑜𝑙 𝑃𝑟𝑜𝑑𝑢𝑐𝑡)

= 3.1695 𝑔𝑟𝑎𝑚𝑠 𝑃𝑟𝑜𝑑𝑢𝑐𝑡

. 629 𝑔𝑟𝑎𝑚𝑠

3.1695 𝑔𝑟𝑎𝑚𝑠∗ 100 = 19.85 %

. 539 𝑔𝑟𝑎𝑚𝑠

3.1695 𝑔𝑟𝑎𝑚𝑠∗ 100 = 17.01 %

. 492 𝑔𝑟𝑎𝑚𝑠

3.1695 𝑔𝑟𝑎𝑚𝑠∗ 100 = 15.52 %

• The calculation for the percent yield for this reaction is shown above. Dimensional

analysis was used to find the theoretical yield of the synthesis. Once that was calculated,

a ratio was used to find the actual percent by dividing the actual yield by the previously

calculated theoretical yield then multiplying the decimal by 100. The three best percent

yields are shown above. Problems with this synthesis include:

• Incorrect vacuum setting

• Incorrect cooling temperature

• Incorrect round bottom flask placement

• Incorrect product filtration solvent

• Incorrect uniform ring formation of product in column

• All of the above could have the potential in receiving a low percent yield for the synthesis

due to possible error

Infrared spectroscopy was used to determine the different functional groups inside the

molecule. An alcohol, ester, and carbon-hydrogen stretching are present due to the broad peak at

3000, broad peak at 1700, and small peaks at 3000 respectively. To retrieve more information on

the structure of the molecule synthesized, proton NMR was used to differentiate the hydrogens

that are inside the molecule. The proton NMR showed important peaks at 1.4, 3.6, and 4.0. The

peak at 1.4 was determined to be the tert-butyl group at the end of the molecule. The methine

peaks at 3.6 and 3.7 are the hydrogens next to the alcohol at the end molecule. The last hydrogen

at 4 is the methine group next to the carbonyl. Carbon NMR was used to differentiate the carbons

that are contained inside the molecule. The significant peaks on the carbon NMR appeared at 28,

65, 66, and 70. The peak at 28 was determined to be the carbons on the tert-butyl group on the

molecule. The peaks at 65 and 66 were the carbons apart of the ester inside the molecule. The

last carbon had a peak at 70 which is the carbon next to the alcohol group in the molecule.