“The Marshmallow Polymer”

Chen Group MeetingOctober 21, 2013

What is it?

• Xerogel = gel dried and shrinkage allowed to occur

• Organosilica• Separates oil from water by absorbing oil and

not water• Two step acid-base sol/gel reaction

N. Hüsing and U. Schubert, Angew. Chem., Int. Ed., 1998, 37, 23.

Materials

• Urea• Acetic acid• CTAC (n-hexadecyltrimethylammonium

chloride or cetyltrimethylammonium chloride)• Methyltrimethoxy silane (MTMS)• Dimethyldimethoxy silane (DMDMS)• Acid-Base two step sol/gel reaction

Materials

Si

CH3

H3CO OCH3

OCH3

Si

CH3

OCH3H3CO

CH3

O

NH2NH2

CH3

O

OH

CH3 N+CH3

CH3CH3 Cl

-

MTMS DMDMS

15 mL5 mM

5 g

0.8 g

2 mL : 3 mL

Acid hydrolysis

• Hydrolysis of alkoxysilane (water, acid catalyzed)

Si

CH3

H3CO OCH3

OCH3

Si

CH3

OH OH

OH

Si

CH3

OCH3H3CO

CH3

Si

CH3

OHOH

CH3

H+

H+

CH3 OH

CH3 OH

Base condensation

• pH has increased because of reduced [H+]• Urea acts as base (another H+ acceptor)

• CTAC prevents phase separation

Si

CH3

OH OH

OH

Si

CH3

OHOH

CH3

Si

CH3

OH

OHSi

CH3

OH

CH3

Si

CH3

OOH

Si

CH3

OO

CH3

CH3 OH H2O

Caveats

• Keep septum on• Reaction is beginning once you add the first

silane• Don’t wait too long to add the other silane• Stir very well (increasing viscosity = decreasing

diffusion rate = decreasing reaction rate)• Don’t let the reaction sit for too long (1h is

plenty)

Gelation at 80 degrees

• Oil bath• GC oven• Removing byproduct alcohols and some other

things by heating

Next steps

• Molar ratio of (MTMS+DMDMS) : water : acetic acid : urea : CTAC = 1.0 : 2.4 x 10 : 2.1 x 10-3 : 2.4 : 7.1 x 10-2

• MTMS:DMDMS = 5.9 : 4.1

• Ratios of MTMS and DMDMS changes marshmallow properties

Next steps

• Aged for 2 days in glass bottle in the base-catalyzed condition brought about by the hydrolysis of urea

• Wash in 1:1 water:2-propanol• Wash twice in 2-propanol at 8h intervals• Gel washed 3 times with n-hexane and dried

at RT is termed xerogel

Gen Hayase et al. J. Mater. Chem., 2011, 21, 17077.

Synthetic procedure p. 1

• Measure 0.8 g CTAC directly into round-bottom flask

• Add 5 g urea beads directly into flask• Add 5 mM CH3COOH(aq) to flask. Urea will

mostly dissolve, CTAC will mostly not dissolve• Plug flask with septum• Medium stir

Synthetic procedure p. 2

• Methyltrimethoxy silane (MTMS) and Dimethyldimethoxy silane (DMDMS) are volatile

• Use syringe needle under nitrogen to transfer silanes to reaction flask

• CTAC will dissolve, solution becomes colorless and clear

• Increase stirring speed as polymerization is beginning• Leave for 60 minutes (some say 30 minutes is good

enough)