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Sigma-Aldrich and used as-received. ACS Reagent-grade hex-
ane, tetrahydrofuran (THF), and methanol were purchased
from BDH and used as-received. ACS Reagent-grade methy-
lene dichloride (DCM) was purchased from BDH and dried
over molecular sieves. Butyraldehyde (BA) was purchased
from Alfa Aesar and purified by mixing with CaH2 over night
and distilled over molecular sieves.
For the in situ NMR of poly(phthalaldehyde), a solution of
0.75 M purified PHA in DCM-d2 was prepared with 0.05 mL
BF3-OEt2 in dry nitrogen glovebox. For the polymer yield
experiments, 2 g (15 mmol) of PHA was dissolved in 20 mL
DCM in a 100 mL round bottom flask in a nitrogen purged
glovebox. To the monomer solution, 0.03 mmol of BF3-OEt2
was added and the flask was sealed with a septum. The reac-
tion was cooled to the target temperature and allowed to
react for 30 min. The reaction was quenched with 0.12 mL
(1.5 mmol) pyridine to remove the BF3-OEt2. The solid was
precipitated by addition of methanol and filtered. The precip-
itate was redissolved in THF, and 0.05 mL (0.36 mmol) of
triethylamine was added per gram of precipitate to help
remove residual catalyst. The polymer was precipitated into
hexanes, filtered, and allowed to dry until constant weight.
Characterization Equipment
A Bruker DMX 400 MHz tool was used for 1H NMR experi-
ments. DCM-d2 was used as the reaction solvent with the
residual solvent peak at 5.32 ppm as the internal reference.
The temperature of the Bruker NMR cavity was calibrated
from the chemical shift differences of a pure solution of
methanol.20 In this calibration, the difference between the
CH3 and OH hydrogen peaks is measured in hertz. As tem-
perature cools, the hydrogens in the CH3 and OH moieties
move further apart. Each reaction was allowed to proceed at
temperature until no change was observed in the integrated
peaks.
A solution of 0.93 M purified PHA (0.56 mmol) and BA (0.21
mmol) in DCM-d2 was made for the in situ NMR of the BA–
PHA copolymer. The solution was added to an NMR tube
with 0.6 mL BF3-OEt2 (0.005 mmol) in a dry nitrogen glove-
box. Similarly, a solution of only distilled BA (0.56 mmol) in
DCM-d2 was prepared and added to an NMR tube with
0.05 mL BF3-OEt2 for the BA only polymerization.
Methods
Determination of the thermodynamic properties of the poly-
merization reaction requires that the polymerization reaction
be in thermodynamic equilibrium. The equilibrium polymeri-
zation reaction is shown in eq 1.
RESULTS AND DISCUSSION
The poly(phthalaldehyde) data was previously reported by
Schwartz et al.22 NMR of PHA monomer has peaks at 10.45
(a, 2H, aldehyde, singlet), 7.98 (b, 2H, phenyl, quadruplet),
and 7.79 (c, 2H, phenyl, quadruplet). BF3-OEt2 alone has
peaks at 4.21 (4H, ACH2A, quadruplet) and 1.40 (6H, ACH3,
triplet). The integration of the 1H peaks was normalized to
the residual methylene dichloride solvent peak at 5.32 ppm,
which remains constant within each experiment. Integrations
of the PHA aldehyde peak provided the equilibrium concen-
tration of monomer at each temperature. Equilibrium
between the PHA monomers and BF3-OEt2 catalyst was
reached within 15 min at each temperature. The tempera-
tures were chosen based on previous reports of the ceiling
temperature for poly(phthalaldehyde) (PPHA).12 Figure 1
shows the 1H NMR spectra for PHA in the PHA polymeriza-
tion between 225 8C and 235 8C. The peaks at 7.34 and
7.42 ppm are polymerized phthalaldehyde phenyl protons.
The peak at 6.45 ppm is the proton on the carbon in the
PPHA polymer backbone next to the ether linkage which is
complexed with BF3.
K
M1Mn $ Mn11
(1)
In eq 1, M is monomer, Mn is a propagating polymer chain of
length n units, K is the equilibrium constant for the reaction,
and Mn11 is a propagating chain of length of n 1 1 units. The
reaction can be described thermodynamically by assuming
equilibrium conditions for the propagating chain and assum-
ing n is large so that the length of Mn is approximately equal
to the length of Mn11. The resulting equation, eq 2, can be
used to determine the change in enthalpy and entropy of
polymerization.8
DHꢀ DSꢀ
2
5ln ð½MꢁÞ
R
(2)
RT
In eq 2, DHo is the change in enthalpy of polymerization in
J/mol, R is the ideal gas constant, T is the absolute tempera-
ture, DSo is the change in entropy of polymerization in J/
mol-K, and [M] is the monomer concentration. Extrapolation
of ln([M]) versus 1/T to the initial monomer concentration
(i.e., concentration of Mn being zero), allows calculation of Tc,
as shown in eq 3.19
The increase in the two doublet peaks at 7.34 and 7.42 ppm
(f and e) shows the formation of PHA polymer. At tempera-
tures slightly above the ceiling temperature, Figure 1, some
polymer yield is observed. This small polymer yield asymp-
totically approaches zero, as observed when the ceiling tem-
perature phenomenon was first reported.10 In this case, the
asymptotic approach could be due to the catalyst, which low-
ers the activation energy for both the polymerization and
the depolymerization reactions. At temperatures between
225 8C and 235 8C, the depolymerization reaction dominates
the polymerization reaction and only a small polymer yield is
observed in the peaks at 7.34 and 7.42 ppm (f and e).
DHꢀ
À
Á
Tc5
(3)
DSꢀ1R ꢂ ln ½Mꢁo
In eq 3, [M]o is the initial concentration of monomer and Tc
is the ceiling temperature. For addition polymerizations, DH
is exothermic on the order of tens to hundreds of kJ/mol,
and DS is exo-entropic on the order of tens to hundreds of
J/mol-K.12,21 Equation 3 is used for reaction conditions
where the initial monomer concentration is not 1 M.
222
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2018, 56, 221–228
Schwartz, Jared M.
