Notes and references
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Fig. 3 Copolymerization of rac-LA and rac-BBL initiated by
{Hf(2)OiPr}2.
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Due to their comparable rates it was hoped that a random
or alternating copolymer could be achieved under similar
conditions to the homopolymerizations via {Hf(2)OiPr}2 and
{Hf(4)OiPr}2 at 150 : 150 : 1 [LA] : [BBL] : [initiator], in one pot
with the BBL and LA added simultaneously. The copolymers
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suggest that a copolymer was produced (Mn = 31 350 g molÀ1
,
1
PDI = 1.61). From H NMR studies, Fig. 3, it is surmised
that the rate of insertion of BBL into a M-LA linkage is
significantly slower than into the M-BL linkage. This leads to
the conclusion that with these initiators and a 1 : 1 ratio of
BBL : LA a block copolymer results, albeit with a broad
molecular weight distribution. In line with previous reports
we have not observed melting points in the DSC for either the
homo or block polymers.12 We also observed no melting
temperature if an L-PLA-b-PHB copolymer is prepared in
the same manner (Mn = 64 000 g molÀ1, PDI = 1.59). The
copolymer was also prepared by sequential addition of the
monomers, 150 equivalents of one monomer was initially
homopolymerized (24 h, 80 1C) in toluene followed by addition
of 150 equivalents of the second monomer and left for a
further 24 h at 80 1C. If rac-LA is polymerized initially
followed by rac-BBL then Mn = 38 900 g molÀ1 and PDI
1.33 and a conversion of 99% of LA and 84% BBL is
observed. If the copolymer is prepared initially with rac-BBL
followed by rac-LA, Mn = 41 700 g molÀ1 and PDI 1.09 and a
conversion of 89% of LA and 99% BBL observed. The
sequential addition method yielded copolymers with signifi-
cantly lower PDIs.
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In conclusion, a series of Group 4 complexes of four related
amine tris(phenolate) ligands has been investigated for the
activity of ROP of both rac-LA and rac-BBL. Interestingly,
previously reported C3-symmetric amine tris(phenolate)
complexes that are highly selective for the ROP of LA showed
poor control over the ROP of rac-BBL. However with a slight
ligand modification, the Group 4 initiators reported here, that
are non-selective in the production of PLA, produce syndio-
tactic PHB. The complexes also demonstrate the ability to
form copolymers of LA and BBL.
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We thank EPSRC for funding a DTC studentship to BJJ
and Johnson Matthey for funding.
c
12330 Chem. Commun., 2011, 47, 12328–12330
This journal is The Royal Society of Chemistry 2011