Reaction of Na2O2NN' [H2O2NN' = (2-C5H4N)CH2N[2-HO-3,5-C6H2(t)Bu2]2] with M(BH4)3(THF)3 afforded the dimeric, rare-earth borohydride compounds [M(O2NN')(mu-BH4)(THF)n]2 [M = Y(III), n = 0.5 (1-Y); M = NdIII, n = 1 (1-Nd); M = SmIII, n = 0 (1-Sm)]. For comparison the chloride analogues [M(O2NN')(mu-Cl)(THF)n]2 (2-M; M = La(III) or Sm(III), n = 0; M = Nd(III), n = 1) and the corresponding pyridine adducts [M(O2NN')(mu-X)(py)]2 [X = BH4 (3-M) or Cl (4-M); M = La(III), Nd(III), or Sm(III)] were prepared and structurally characterized for 4-La. Compounds 1-M initiated the ring-opening polymerization of epsilon-caprolactone. The best molecular weight control (suppression of chain transfer) for all three monomers was found for the samarium system 1-Sm. The most effective heterotactic enrichment (Pr) in the polymerization of rac-lactide was found for 1-Y (P(r) = 87%). Compound 1-Nd catalyzed the block copolymerization of epsilon-caprolactone and L- and rac-lactide provided that epsilon-caprolactone was added first. Attempted block polymerization by the addition of L-lactide first, or random copolymerization of a ca. 1:1 mixture of epsilon-caprolactone and L-lactide, gave only a poly(L-lactide) homopolymer.

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Research toward bio-based and biodegradable polymers has received a lot of attention these last years, due to environmental concerns and the need to replace fossil resources. Starch is in this frame an interesting starting material due to its abundance and low cost. Native starch displays however, poor mechanical properties and is highly hydrophilic. New starch-based thermoplastic materials can be prepared to overcome these drawbacks by chemical modification, and notably graft copolymerization. A large range of polymers can be grafted on starch by ring-opening and radical polymerizations of various monomers in order to modulate the properties of the final product. Polymers can also be directly grafted onto starch using coupling strategies. This review presents the state of the art in the field.

The synthesis and ring-opening polymerization (ROP) capability of bis(phenolate)amine-supported samarium borohydride and amide complexes are reported, together with a DFT study. Reaction of Na 2 -O 2 N L (L=OMe, NMe 2 , py, or Pr) with Sm(BH 4 ) 3 (THF) 3 gave the borohydride complexes Sm(O 2 N L )-(BH 4 )(THF) (L = OMe (2), NMe 2 (3), or py (4)) or Sm(O 2 N Pr )(BH 4 )(THF) 2 (5). Compounds 4 and 5 lost THF in vacuo, forming phenolate O-bridged dimers 1 and 6, respectively. Reaction of H 2 O 2 N L with Sm{N(SiHMe 2 ) 2 } 3 (THF) 2 formed monomeric Sm(O 2 N L ){N(SiHMe 2 ) 2 }(THF) (L = OMe (7), NMe 2 (8), or py ( 9) 12) or py (13); n=0, L=NMe 2 ( 14)) were prepared by reaction of Sm(O 2 N L )(BH 4 )(THF) with KN(SiMe 3 ) 2 . The X-ray structures of 2, 5, 6, 7, 10, 13, and 14 were determined. The borohydrides 2-5 were very efficient initiators for the ROP of ε-CL, giving linear dihydroxytelechelic poly(ε-CL). Selected amide initiators were also assessed but gave poorer control, as judged by broad PDI (M w /M n ) values and significant amounts of cyclic poly(ε-CL)s. Of the borohydrides, only 2-4 were active for the ROP of rac-LA, and activity increased in the order O 2 N L = O 2 N OMe ≈ O 2 N py < O 2 N NMe2 . The latter ligand also gave the best control of the ROP, as judged by the PDIs and M n values. All gave heterotactically enriched poly(rac-LA) with P r values in the range 0.82-0.84. The ROP of rac-LA with the amides 7, 9, and 12 was faster but much less well controlled. Overall, the borohydride initiators were superior for the ROP of both ε-CL and rac-LA when compared to otherwise identical amide initiators. MALDI-ToF MS analysis of the poly(rac-LA) formed with 3 showed both -CH(Me)CHO and -CH(Me)CH 2 OH end groups originating from the insertion of the first LA monomer into the Sm-BH 4 moiety of 3. In contrast, 2 and 4 formed only R,ω-dihydroxy-terminated polyesters with -CH(Me)CH 2 OH and -CH(Me)OH end groups. DFT calculations on Eu(O 2 0 N NMe2 )(BH 4 ) found two mechanisms for the initial ring-opening step of LA by the borohydride group, giving pathways leading to either aldehyde-or alcohol-terminated poly(lactide)s. Of these two pathways, the one giving R,ω-dihydroxy-terminated polymers was the most favored, in agreement with experiment. (Ligand abbreviations: O 2 N L = RCH 2 N(CH 2 -2-O-3,5-C 6 H 2 t Bu 2 ) 2 where R = CH 2 OMe, CH 2 NMe 2 , py, or Et for L = OMe, NMe 2 , py, or Pr, respectively; O 2 0 N NMe2 = Me 2 -NCH 2 CH 2 N(CH 2 -2-O-C 6 H 4 ) 2 .

Abstract.Highly stereospecific polymerization of isoprene was achieved using borohydrido neodymium complexes. In combination with stoichiometric amounts of dialkylmagnesium, Nd(BH 4 ) 3 (THF) 3 (1) and Cp*'Nd(BH 4 ) 2 (THF) 2 (2) (Cp*' = C 5 Me 4 nPr) afford very efficient catalysts. Activity reaches 37300 g polyisoprene/mol Nd/h. Half-lanthanidocene 2 gives rise to polyisoprene 98.5 % trans-regular, the highest content yet described for a homogeneous organometallic catalyst. NMR experiments argue for the formation of bimetallic Nd(-BH 4 )Mg active species.

The reaction of Ln(BH(4))(3)(THF)(3) or LnCl(3)(THF)(3) with 1 equiv of KCp*' ligand (Cp' = C(5)Me(4)n-Pr) afforded the new monocyclopentadienyl complexes Cp*'LnX(2)(THF)(n) (X = BH(4), Ln = Sm, n = 1, 1a, Ln = Nd, n = 2, 1b; X = Cl, Ln = Sm, n = 1, 3a) and [Cp*'LnX(2)](n') (X = BH(4), n' = 6, Ln = Sm, 2a, Ln = Nd, 2b; X = Cl, Ln = Nd, 4b). All these compounds were characterized by elemental analysis and (1)H NMR. Crystals of mixed borohydrido/chloro-bridged [Cp*'(6)Ln(6)(BH(4))(12-x))Cl(x)(THF)(n')] (x = 10, n' = 4, Ln = Sm, 2a', Ln = Nd, 2b'; x = 5, n = 2, Ln = Sm, 2a' ') were also isolated. Compounds 2a, 2b, 2a', 2b', and 2a'' were structurally characterized; they all exhibit a hexameric structure in the solid state containing the [Cp*(3)Ln(3)X(5)(THF)] building block. The easy clustering of THF adducts first isolated is illustrative of the well-known bridging ability of the BH(4) group. Hexameric 2a was found to be unstable in the presence of THF vapors; this may be correlated to the opening of unsymmetrical borohydride bridges observed in the molecular structure.

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Highly cis-1,4 polymerisation of isoprene and syndiospecific polymerisation of styrene were conducted using the same borohydrido half-sandwich scandium pre-initiator.