The anionic polymerization of (R)-β-butyrolactone
initiated with either 18-crown-6 complexes
of a potassium alkoxide or a simple carboxylate (reference initiator)
proceeded with inversion of
configuration. As a result (R)-β-butyrolactone formed
an isotactic poly((S)-β-hydroxybutyrate) with
these
initiators at room temperature. Polymerization of
(R,S)-β-butyrolactone under the same conditions
gave
an atactic polymer, but at lower temperatures the predominantly
syndiotactic form of
poly((R,S)-β-hydroxybutyrate) was produced from the racemic monomer.
Novel model block copolymers of (R,S)-β-butyrolactone
with pivalolactone (PVL) are prepared
in order to define the effect of crystalline domains provided by
poly(pivalolactone) on the biodegradability
of atactic poly(β-butyrolactone), a-PHB. The “living”
a-PHB is synthesized from racemic β-butyrolactone,
in the presence of potassium alkoxide/18-crown-6 complex, and such a
living polymer is applied for
polymerization of PVL, yielding block copolymers,
a-PHB-b-PPVL, of tailored molecular weight and
composition. The copolymers contain an amorphous phase with
T
g = 5 °C, associated with the
a-PHB
block, and a high melting crystalline phase, whose amount increases
with PPVL content. Films of
copolymers containing 9 (PVL9), 17 (PVL17), and 23 mol % of PPVL
(PVL23) are exposed to
PHB-depolymerase A from Pseudomonas
lemoignei (37
°C, Tris−HCl buffer pH = 8). While plain a-PHB
does not biodegrade, the biodegradation rate of a-PHB-b-PPVL
copolymers increases (PVL9 ≪ PVL17 <
PVL23) along with the increase of crystalline PPVL domains. The
biodegradation rate of PVL23 is similar
to that of natural (crystalline) PHB. On the basis of a comparison
of a-PHB-b-PPVL composition changes
(by 1H NMR) with weight loss during biodegradation
experiments, it is concluded that in the copolymers
studied only the a-PHB block is attacked by the enzyme and that the
crystalline block of nonbiodegradable
PPVL efficiently promotes enzymatic attack to a-PHB, by providing a
binding support to the enzyme.
The anionic polymerization of 2-oxetanone and 4-methyl-2-oxetanone initiated with the potassium hydride/l8-crown-6 complex was investigated. The a-proton abstraction of the monomer was found to proceed at the initiation step of this polymerization. The salt of the unsaturated carboxylic acid formed initiates further propagation, leading to functionalized polyesters with unsaturated, dead end-groups.
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