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polydispersity index (PDI) of 1.64. Similarly, using ethane-1,2-
diol or but-2-yne-1,4-diol, polymers characterized by a M
of 790 g mol and a PDI of 1.5 or a M of 3200 g mol and
n
n
−
1
−1
a PDI of 1.9 were obtained. In the case of but-2-yne-1,4-diol, a
solution polymerisation reaction in THF/CH Cl = 1 : 1 ([DVS]
2
2
−
1
=
0.5 mol L ) was performed, yielding a polymer character-
ized by a Mn of 6400 g mol and a PDI of 1.7 in 70% yield
−
1
(
conversion was quantitative, cf. ESI†).
Multifunctional alcohols like propane-1,2,3-triol and
2-ethyl-2-(hydroxymethyl)propane-1,3-diol gave insoluble yet
cross-linked polymer networks. In these cases, solvent-free
conditions were applied resulting in fast and exothermic
reactions, thus mixing of the three components is hardly pos-
sible (also because of the poor solubility of PPh in the neat
3
Scheme 2 Mechanistic rationale.
alcohols – ideally, the nucleophile should be dissolved in the
1
5
alcohol and this solution should then be mixed with DVS ).
Therefore, these reactions are preferably mediated with
alcohol-soluble DMAP (0.05 eq.). Mixing of the DMAP/alcohol
solution with DVS led to a somewhat retarded polymerisation
reaction with a pot life of approx. 30 s. The formulation was
transferred into Teflon moulds (22 × 5 × 3 mm) and speci-
mens for dynamic mechanical analysis (DMA) were produced
by curing for 4 h at 80 °C. The use of propane-1,2,3-triol
resulted in stiff and brittle specimens which break upon
mounting into the sample holder of the DMA machine.
2-Ethyl-2-(hydroxymethyl)propane-1,3-diol-based polymers gave
specimens with a storage modulus of 3300 MPa at 10 °C and
a Tg of 28 °C (determined to be the maximum of the loss
modulus curve).
In conclusion, we demonstrated that the nucleophile-
mediated oxa-Michael reaction between alcohols and divinyl
sulfone is particularly fast and efficient under solvent-free
conditions, allowing for the preparation of polymers. The
reactivity of the alcohols decreases in the order primary >
secondary > phenol > tertiary alcohols, and allylic, benzylic
forming the corresponding phosphonium alkoxide B. The
conjugate addition of the generated alkoxide to DVS forms
ion pair C. Protonation of the carbanion by another alcohol
results in the formation of the β-alkoxy sulfone derivative
and phosphonium alkoxide B to complete the catalytic cycle.
The rate-determining step of the reaction is believed to be
the proton transfer from the alcohol to carbanion A (state-
ment based on mechanistic studies of related thiol-Michael
3
,12
reactions).
The values of entropy of activation (measured
in a related system) are very negative suggesting the necessity
of a precise arrangement of PPh , the electron-deficient olefin
3
1
3
and the proton donor for the reaction to occur. The follow-
ing results support the briefly sketched mechanistic picture.
Deuterium incorporation in α-position to the sulfone group
was found upon performing the reaction with MeOH-d
4
or in
CDCl as the solvent (cf. ESI†), suggesting that a strong base
3
31
1
is generated during the reaction. P-{ H}-NMR monitoring of
the reaction revealed that a phosphorus signal for zwitterion
A is not observable. Only upon addition of the alcohol signals
at 24.5 ppm and 24.4 ppm (relative to 85% H PO ) tentatively
3
4
assigned to the phosphonium-containing ion pairs B and C
1
4
formed. The reaction becomes faster by (a) using more
acidic alcohols forming alkoxides with sufficient nucleophi-
licity (cf. Table 2), (b) lowering the reaction temperature (opti-
mum about 10 °C, cf. ESI†) and (c) increasing the concentra-
tion (cf. ESI†). The latter finding implies to carry out the
reaction under solvent-free conditions. The reaction of DVS
and 2-propanol is a good showcase. Under the conditions, as
mentioned in Table 2 (entry 5), only 13% of the diadduct were
formed after 24 h. Optimized reaction conditions (using 26
3
eq. of 2-propanol and 10 mol% PPh at room temperature, cf.
ESI†) gave the diadduct in 75% isolated yield after column
chromatographic purification.
Switching to di- and trifunctional alcohols allowed for
the preparation of polymers (cf. Fig. 1). Reacting an equi-
molar formulation of 4-(2-hydroxyethyl)phenol and DVS in
water (5 eq.) upon adding 10 mol% PPh
3
(stock solution in
CH Cl ) at 25 °C gave a polymer characterized by a number
2
2
Fig. 1 Results from polyaddition reactions of di- and trifunctional
alcohols to DVS.
−
1
average molecular mass (M
n
)
of 780
g
mol
and
a
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