degassed three times using a freeze–pump–thaw cycle and N2
was introduced into the reaction vessel. The reaction vessel was
immersed in an oil bath preheated to the desired temperature
whereby polymerization occurred. The occurrence of poly-
merization was indicated by a high increase in the viscosity of
the reaction medium. Upon cooling, linear polymers (PMMA)
were dissolved in a small amount of methylene chloride and
precipitated by adding the solution to a large amount of
methanol, filtered and dried under vacuum.
The
crosslinked
polymers,
poly(TEGDMA)
and
poly(EBPADMA), were obtained by the same polymerization
method as described. At the end of the reaction, and upon
cooling, because the crosslinked polymer was extremely hard
and would not dissolve in any solvent, the Schlenk tube was
broken to remove the polymer.
Fig. 5 The structures of the monomers.
software for data collection and analysis. The sample (7 mg)
and dried KBr (1mg) were added in a mortar and ground to a
smooth powder. The ground powder was loaded into a
metallic sample holder and pressed into a transparent wafer.
A background spectrum was recorded with an empty sample
holder inserted in the path of the FTIR beam. Thereafter, the
metal holder bearing the sample wafer was inserted into the
path of the FTIR beam to record the sample spectrum.
As a typical procedure: to a Schlenk tube were added a
magnetic stirrer, initiator adduct 1 (0.6 g, 1.143 mmol) and
MMA (11.4 g, 114.3 mmol). The mixture was cooled in liquid
nitrogen and sulfuric acid (5.22 mmol) was added to it. The
solution was degassed three times using a freeze–pump–thaw
cycle. The tube was then filled with nitrogen, capped and
immersed in an oil bath preheated to 60 uC with stirring. The
viscosity of the solution increased as the polymerization began
and the reaction was stopped when the viscosity increased to a
point whereby the stirring magnet would no longer turn. After
the reaction cooled to room temperature, methylene chloride
(60 mL) was added to dissolve the polymer. The polymer
solution was added to 500 mL of methanol to precipitate it.
The precipitate was filtered and dried under vacuum to give
General procedure for the preparation of
N-acyloxytriakylammonium salts
All reactants, solvents and a magnetic stirring bar were placed
in a one-necked 500 mL reaction flask. The mixture was
reacted at 25 uC for 30 h (method 1) or at 220 uC for 4 h
(method 2). The product (precipitate) was filtered out and
dried under vacuum at room temperature and the yield was
calculated. The dried product was recrystallized from an
acetone/water (1 : 1) mixture and characterized.
1
PMMA (5.13 g, 45%). H NMR (CDCl3, 300 MHz): d (ppm),
3.5(s, 3H), 2.1–1(br, 2H), 1–1.6(d, 3H). 13C NMR (CDCl3,
300 MHz): d (ppm), 180, 53, 45, 20, 16. GPC: Mw = 127 000,
Mn = 120 000, PDI = 1.06. The Mn obtained (120 000) is
higher than the Mn expected from theoretical predictions and
this may be due to that fact that the initiator efficiency may be
lower than 100%.
As
a typical example, N-benzoyloxytriethylammonium
tetraphenylborate 1, was prepared by method 1 as follows.
Triethylamine (0.886g, 8.76 mmol), benzoylperoxide (2.12g,
8.76 mmol), sodium tetraphenyl borate (3g, 8.76 mmol),
ethanol (50 mL) and toluene (50 mL) were added into a
500 mL one-necked round bottom flask. To the mixture was
added a magnetic stirrer. The flask was immersed in ice to
which had been added sodium chloride to bring the
temperature to 25 uC. The reaction mixture was reacted at
25 uC for 30 h. The white precipitate was filtered out and dried
under vacuum at room temperature to give 1.52 g of product
(33% yield). The product was recrystallized from acetone/water
(1 : 1) mixture. Melting point: 128 uC. IR (KBr wafer, cm21):
1770, 1050, 3100, 750. Anal. Calcd for C37H40BNO2: C, 82.1; H,
7.45; N, 2.6; B, 2.06. Found: C, 82.41; H, 7.65; N, 2.63; B, 2.06.
The details on the experimental procedure for the preparation of
the other salts are in the ESI{.
Attempted photochemical polymerization
A mixture of the initiator (1 mol%) and monomer were placed
on a watch glass and irradiated in the Dentsply 2000 Triad
Curing1 apparatus for ten minutes.
Results and discussion
In making new derivatives of N-acyloxytrialkylammonium
salts, all the modifications that we made in reaction 2 (route 2),
led to the intended products (compounds 3, 4, 5, 6, 8 and 9) in
very good yield (. 70%). However, when similar modifications
were made in reaction 1 (route 1), only one reaction (reaction
7), led to the intended product (compound 7), all other
reactions failed. The results suggest that route 2 is the more
effective method for preparing new derivatives of the salts. The
UV and IR traces of 1 and 2 are shown as representative
spectra in Fig. 6 and 7 respectively.
General procedure for the thermal polymerization reactions
All polymerization reactions were done under bulk conditions.
A 100 mL Schlenk tube with a Teflon valve was used in all the
reactions. The pressure inside the Schlenk tube was kept
slightly higher than atmospheric. The initiator, chosen from 1
through 9, was dissolved in the monomer. The reaction
mixture was cooled in liquid nitrogen. At this stage H2SO4 was
added to the reaction mixture, for those reactions that would
be promoted by the acid. The reaction mixture was then
All characterization parameters for the polymers initiated
under thermal conditions (60 uC) namely, Tg, Td, PDI and
MWD were all augmented under H2SO4 promotion. The
results are shown in Table 1 and 2. The initiators show
effectiveness in crosslinking of the dimethacrylates as
This journal is ß The Royal Society of Chemistry 2007
J. Mater. Chem., 2007, 17, 4499–4507 | 4503
Ansong, Omari E.
