Bis(b-diketiminate) rare-earth borohydride complexes were reported by Shen (Shen et al., 2012). Reaction of RCl3(Ln¼Y, Yb) with the sodium salts of the b-diketiminate afforded the bis(b-diketiminate) rare-earth chlorides.
According to the ligand system used, these chloride complexes were either isolated or prepared in situ. Reaction of these latter compounds with 1 equiv of NaBH4, afforded the bis(b-diketiminate) monoborohydride complexes [{(2,6-iPr2C6H3)NC(Me)CHC(Me)N(C6H5)}2RBH4] (R¼Y (36Y), Yb (36Yb)) and [({N(2-MeC6H4)C(Me)}2CH)2RBH4] (R¼Y (37Y), Yb (37Yb)) (Scheme 16).
Recently the lanthanide borohydride complexes [R(BH4)3(THF)3] (R¼La (1La), Nd (1Nd)) were grafted onto non-porous silica dehydroxylated at 700C, resulting in the bis(borohydride) surface species [(@SiO)R (BH4)2(THF)2.2] (R¼La (38La), Nd (38Nd)) (Ajellal et al., 2010b). The monoborohydride grafted on silica [(@SiO)La(BH4)] (39La) was also dis- cussed (Del Rosa et al., 2011a,b, 2012).
3. RARE-EARTH BOROHYDRIDE INITIATORS IN
[{(Z5:Z1-C5Me4SiMe2NCMe3)Y(THF)(m-H)}2] was demonstrated as being a single-site, single-component catalyst for the polymerization of ethylene, tert-butyl acrylate, and acrylonitrile (Arndt et al., 2002; Okuda et al., 2001), the only, to our knowledge, other rare-earth hydride complexes investigated in polymerization, namely the tetrasubstituted guanidinato yttrium and lute- tium complexes, [{[TMS2NC(NiPr)2]2R(m-H)}2] (R¼Y, Lu), evaluated in the ROP of CL, displayed some activity but no control (MnSEC79, 000gmol1, 1:8<Mw=Mn<3:5;Lyubov et al., 2008).
3.1. ROP of Lactones and Lactides, anda-Amino Acids N-Carboxyanhydrides
3.1.1. Trivalent Inorganic Rare-Earth Borohydride Initiators 3.1.1.1. e-Caprolactone
The homoleptic rare-earth trisborohydride complexes [R(BH4)3(THF)3] (R¼Y (1Y), La, (1La), Nd (1Nd), Sm (1Sm)) were the real first transition metal borohydride derivatives to be investigated in the ROP of a polar monomer, namely of CL, by Guillaume and coworkers in the early 2000s (Scheme 17;Guillaume et al., 2003; Palard et al., 2004, 2005, 2006) (Table 1).
ROP of CL proceeded rapidly in THF, DCM, or DCM/toluene at room tem- perature in a controlled process. As many as 377 CL units per metal-BH4 could be converted within 10 min using the samarium initiator1Smproviding PCL of molar mass up to MnSEC¼39, 600gmol1. The molar mass values MnSEC varied linearly with the feed ratio highlighting a “living” process.
However, at [CL]0/[BH4]0>150, assuming three growing polymer chains per metal, MnSEC deviated to lower values relative to Mntheo indicating the presence of some transfer reactions. The fairly narrow dispersity values (Mw=MnLa,Nd,Sm ca:1:3) similarly suggested the occurrence of side reac- tions, yet to some minor extent. In ROP of cyclic esters, such undesirable reactions involve the typical inter- or intra-molecular transesterifications, that is, reshuffling or backbiting processes respectively, or other transfer reactions (Albertsson and Varma, 2003). Intra-molecular transesterification reactions lead to the formation of cyclic polymers (alongside linear ones), ultimately reducing the molar mass of the polymer. Intermolecular transesterification reactions, or intermolecular chain transfer reactions, induce the broadening of the dispersity. The moderate dispersity can also be attributed to a rate of
n (1) [R(BH4)3(THF)3]
(2) H+
HO O
O OH
O O n + 2
HO–PCL–OH
5 5 5
O O
CL
SCHEME 17 Synthesis of HO–PCL–OH from the ROP of CL initiated by [R(BH4)3(THF)3] (1R).
TABLE 1 Rare-Earth Borohydride Complexes Used as Initiators in the (Co)Polymerization of Lactones and Experimental Data
Rare-earth borohydride complex
Lactone
([Lactone]0/[BH4]0
highest value)a
Solvent Temp. (C)
Reaction time Conv.b
Mnmaxc
(Mntheo)a,d
g mol1 ÐMe
Polymer Tacticity
(%) Copolymer
Divalent complex [Sm(BH4)2(THF)2] (2Sm,Scheme 2)
CL (232)
(Jaroschik et al., 2010)
THF 25C
5 min 100%
24,600 (13,250)
1.59
[{(Cp*)Sm(BH4)(THF)2}2] (4Sm,Scheme 5)
CL (234)
(Jaroschik et al., 2010)
THF 25C
3 min 84%
20,400 (22,400)
1.36
[Tm(BH4)2(DME)2] (2Tm0,Scheme 2)
CL (233)
(Momin et al., 2011)
THF 25C
19 min 74%
12,400 (19,600)
1.34
[(TptBu,Me)Tm(BH4)(THF)]f
(15Tm) CL (100)f
(Momin et al., 2011)
THF 25C
3 min 100%
5700f (11,400)
1.44f
[(Ap*)YbI(THF)2]2
(16Yb)/NaBH4g
CL (568)
(Guillaume et al., 2007) THF 23C
4 h 98%
43,600 (63,400)
1.56
Trivalent complex [Y(BH4)3(THF)2.5]f (1Y0,Scheme 1)
VL(200)f
(Nakayama et al., 2009) THF 60C
60 min 3.3%f
4300f (660)h
1.6f
[Y(BH4)3(THF)3]f (1Y,Scheme 1)
PDL (50)f
(Nakayama et al., 2011) THF 60C
1 min 66%f
11,000f (7920)h
1.9f
[R(BH4)2(THF)5][BPh4] R¼Y (3Y), La (3La), Nd (3Nd), Sm (3Sm) (Scheme 3)
CL (250)
(Robert et al., 2008)
THF 19C
30 s 100%
17,400 (28,500)
1.33
[(iPr(TMS)NC(NiPr)N(CH2)3NC (NiPr)N(TMS)iPr)R(BH4)(DME)]
R¼Y, Nd, Yb
CL (5000)
(Zhang et al., 2012)
Tol 20C
5 min 100%
22,800 (45,800)
1.45
[{(O2NNpy)Y(BH4)(THF)0.5}2]f (26Y,Scheme 13)
CL (290)
(Bonnet et al., 2005a)
Tol/THF 25C
2 min 84%f
24,700f (27,800)
1.74f
[{(DAB)Y(BH4)2}{Li(DME)3}]
(35Y,Scheme 25)
rac-BL(250)
(Mahrova et al., 2009)
THF, Tol 20C
48 h 14%
4600 (3530)
1.23 Atactic PHB [La(BH4)3(THF)3]f
(1La,Scheme 1)
rac-BL (33)
(Ajellal et al., 2010b)
Tol 20C
24 h 91%f
2700f (2610)h
1.75f Pm 50
[La(BH4)3(THF)3]f (1La,Scheme 1)
rac-BL (300)
(Guillaume et al., 2013) Tol 25C
92 h 31%
8500 (8000)
1.08 Pm 56
[La(BH4)3(THF)3] (1La,Scheme 1)
VL (300)
(Nakayama et al., 2009)
THF, Tol 60C
30 min 94%
15,000 (16,200)h
1.43 PLLA-co-PVL
PCL-co-PVL (Nakayama et al., 2009) [La(BH4)3(THF)3]
(1La,Scheme 1)
CL (251)
(Palard et al., 2005, 2006)
THF 21C
15 min 100%
24,450 (28,610)
1.16 PCL-co-PVL
(Nakayama et al., 2009) [La(BH4)3(THF)3]
(1La,Scheme 1)
PDL (50)
(Nakayama et al., 2011) THF 60C
1 min 83%
14,000 (2880)h
2.8
[(Ap*)LaBr2(THF)3]
(17La,Scheme 24)/NaBH4g CL (450)
(Guillaume et al., 2007) THF 23C
4 h 99%
46,300 (50,800)
1.45
[{CH(PPh2NTMS)2} La(BH4)2(THF)]
(20La,Scheme 11)
CL (150)
(Jenter et al., 2010)
THF, Tol 20C
1 min 98%
13,400 (16,800)
1.34
Continued
TABLE 1 Rare-Earth Borohydride Complexes Used as Initiators in the (Co)Polymerization of Lactones and Experimental Data—Cont’d
Rare-earth borohydride complex
Lactone
([Lactone]0/[BH4]0
highest value)a
Solvent Temp. (C)
Reaction time Conv.b
Mnmaxc (Mntheo)a,d
g mol1 ÐMe
Polymer Tacticity
(%) Copolymer
[Pr(BH4)3(THF)2]f (1Pr0,Scheme 1)
VL (200)f
(Nakayama et al., 2009) THF 60C
60 min 55%f
8100f (11,000)h
1.4f
[Nd(BH4)3(THF)3]f (1Nd,Scheme 1)
rac-BL (33)
(Ajellal et al., 2010b)
Tol 20C
24 h 100%f
3100f (2900)h
1.83f Pm 50
[Nd(BH4)3(THF)3]f (1Nd,Scheme 1)
rac-BL (300)
(Guillaume et al., 2013) Tol 25C
92 h 31%
9700 (8000)
1.06 Pm 54
[Nd(BH4)3(THF)3] (1Nd,Scheme 1)
VL (200)f
(Nakayama et al., 2009) THF 60C
60 min 55%f
7300f (10,960)h
1.5f
[Nd(BH4)3(THF)3] (1Nd,Scheme 1)
CL (221)
(Guillaume et al., 2003;
Palard et al., 2005, 2006)
THF, DCM 21C
15 min 100%
17,000 (25,200)
1.29 PCL-b-PBLG
PBLG-b-PCL- b-PBLG (Schappacher et al., 2006) PCL-co-PLLA (Nakayama et al., 2007) [Nd(BH4)3(THF)3]
(1Nd,Scheme 1)/aldehyde, ketone
CL (157) (Wu et al., 2008)
THF/
acetone 60C
14 min 90%
21,100 (nd)h
1.48
[Nd(BH4)3(THF)3] (1Nd,Scheme 1)
PDL (200)
(Nakayama et al., 2011) THF 60C
8 h 87%
39,000 (41,760)h
1.53
(Bonnet et al., 2005a) [{(O2N2)1Nd(BH4)2Li(THF)}2]
(31Nd,Scheme 14)
rac-BL (50)
(Sinenkov et al., 2011)
THF Tol 20C
12 h 61%
5000 (2625)
1.09 Atactic PHB
[(O2N2)2Nd(BH4)2Li(THF)2] (32Nd,Scheme 14)
rac-BL (50)
(Sinenkov et al., 2011)
THF, Tol 20C
12 h 24%
2500 (1050)
1.11 Atactic PHB [Sm(BH4)3(THF)3]f
(1Sm,Scheme 1)
rac-BL (300)
(Guillaume et al., 2013) Tol 25C
72 h 22%
8300 (5700)
1.05 Pm 52
[Sm(BH4)3(THF)3] (1Sm,Scheme 1)
CL (377)
(Palard et al., 2004, 2005, 2006)
THF, DCM, DCM/Tol 21C
10 min 96%
39,600 (41,500)
1.24 PCL-b-PMMA
PMMA-b- PCL-b-PMMA (Schappacher et al., 2007) [Sm(BH4)3(THF)3]
(1Sm,Scheme 1)
VL (200)
(Nakayama et al., 2009) THF 60C
60 min 26%f
4800f (5140)h
1.5f
[(Cp*)2Sm(BH4)(THF)]
(5Sm)
CL (565)
(Palard et al., 2004, 2006)
THF 21C
30 min 99%
39,150 (63,850)
1.43
[{(N2NNTMS)Sm(BH4)}2]f (24Sm,Scheme 13)
CL (250)f
(Bonnet et al., 2005b)
Tol/THF 25C
1 min 96%f
24,000f (27,400)
1.17f
Continued
TABLE 1 Rare-Earth Borohydride Complexes Used as Initiators in the (Co)Polymerization of Lactones and Experimental Data—Cont’d
Rare-earth borohydride complex
Lactone
([Lactone]0/[BH4]0
highest value)a
Solvent Temp. (C)
Reaction time Conv.b
Mnmaxc
(Mntheo)a,d
g mol1 ÐMe
Polymer Tacticity
(%) Copolymer
[{(O2NNpy)Sm(BH4)}2]f (26Sm,Scheme 13)
CL (275)
(Bonnet et al., 2005a)
Tol/THF 25C
0.5 min 90%f
26,920f (28,250)
1.76f
[(O2NL)Sm(BH4)(THF)]
L¼OMe (27Sm), NMe2
(28Sm), py (29Sm), (Scheme 13)
[(O2NnPr)Sm(BH4)(THF)2] (30Sm,Scheme 13)
CL (275)
(Dyer et al., 2010)
Tol/THF RT
2 min 100%
28,400 (31,381)
1.83
[(N2O2)Er(Cl)(DME)]/NaBH4g
(33Er,Scheme 14)
CL (1500) (Wu et al., 2009)
Tol/Hexane 65C
120 min 95%
33,300 (nd)h
1.43
[Yb(BH4)3(THF)3]f (1Yb,Scheme 1)
VL (200)f
(Nakayama et al., 2009) THF 60C
60 min 0%f
–f –f
[(N2O2)Yb(Cl)(DME)2]/NaBH4g
(33Yb,Scheme 14)
CL (1000) (Wu et al., 2009)
Tol/Hexane 65C
120 min 96%
36,400 (nd)h
1.33
[(Ap*)LuCl2(THF)2] (18Lu)/NaBH4g
CL (450)
(Guillaume et al., 2007) THF 23C
4 h 99%
45,600 (50,800)
1.53
[{(2,6-iPr2C6H3)NC(Me)CHC (Me)N(C6H5)}2RBH4] R¼Y (36Y), Yb (36Yb) [({N(2-MeC6H4)C (Me)}2CH)2RBH4] R¼Y (37Y), Yb (37Yb) (Scheme 16)
CL (1000) (Shen et al., 2012)
Tol 20C
3 min (98%)
9870 (11,170)
1.35
Supported complex [(@SiO)La(BH4)2(THF)2.2]f (38La)
rac-BL (50)
(Ajellal et al., 2010b)
Tol 20C
24 h (9%)f
600f (400)h
1.79f Pm 50f
[(@SiO)Nd(BH4)2(THF)2.2] (38Nd)
rac-BL (100) (Ajellal et al., 2010b)
Tol 20C
24 h (75%)
11,500 (6450)h
1.61 Pm 85
aNote that the [lactone]0/[BH4]0ratio is different from the [lactone]0/[initiator/catalyst]0value, the former being used in the calculation of the theoretical molar mass (Mntheo) assuming one growing chain per BH4group.
bMonomer conversion determined by1H NMR or by gravimetry and corresponding to the highest [lactone]0/[BH4]0value reported.
cNumber average molar mass (maximum value) determined by SEC–MALLS or by SEC versus polystyrene standards and corrected by a factor of 0.56 for CL (Save et al., 2002) and uncorrected for PHB.2
dTheoretical molar mass calculated from [lactone]0/[BH4]0monomer conversionMlactone, assuming one growing chain per BH4group, withMBL¼86 g mol1, MVL¼100 g mol1,MCL¼114 g mol1, andMPDL¼240 g mol1; note that a discrepancy between Mntheoand MnSECmay be indicative of a number of growing polymer chains different from the number of BH4groups in the initiating complex.
eCalculated average dispersity value as reported from SEC traces.
fOnly a single experiment is reported.
gPreparedin situ.
hValue not reported by the authors.
propagation which is faster than the rate of initiation (Penczek and Moad, 2008). The best control in terms of experimental and calculated molar mass agreement, and narrow dispersity was achieved with the largest metal, La>Sm>Nd. The quasi “living” character of the ROP process was later fur- ther demonstrated with the successful synthesis of block copolymers prepared by sequential copolymerization, poly(e-caprolactone)-b-poly(trimethylene carbonate), PCL-b-PTMC (Section 3.5.1;Table 1). The studies on the influ- ence of solvent revealed the detrimental effect on the polymerization rate of a coordinative polar solvent such as THF, in comparison to non-coordinative DCM or apolar toluene. Indeed, it is assumed that THF, because of its high affinity toward oxophilic rare-earth metals, competes with the polar CL monomer for coordination onto the metal center. This trend was later often observed in the ROP of cyclic esters promoted by rare-earth borohydride complexes, yet with some exception, as for instance with the higher CL polymerization rate reported in THF than in toluene using diaminobis (phenoxide)monoborohydride complexes (Bonnet et al., 2005a). The polyester recovered were established, based on extensive investigations by
1H and13C NMR, FTIR, Matrix-Assisted Laser Desorption/Ionization-Time- of-Flight Mass Spectrometry (MALDI-ToF MS) and elemental analyses, as a,o-dihydroxytelechelic PCL (Scheme 17). Note that dihydroxytelechelic polymers herein referred to as HO-polymer-OH are formally H-polymer-OH (Guillaume, 2013). Formation of such HO–PCL–OH, initially not foreseen, resulted from the reduction of the carbonyl group of the first inserted CL molecule (see below, Scheme 26,Section 3.1.4). This behavior is certainly distinct from that observed with the more common rare-earth alkoxide initiators which provide a,o-hydroxyalkoxytelechelic (see below, Scheme 27,Section 3.1.4; Palard et al., 2006). Detailed FTIR investigations revealed that the terminal -OBH2 group of the growing polymer was involved in van der Waals interactions leading to the cohesion of the experimentally observed “physical” gel. This route from rare-earth borohy- dride initiators represents a valuable strategy for the synthesis of highly desirable dihydroxytelechelic macroinitiators (vide infra; Section 3.1.5;
Guillaume, 2013). Interestingly, the residual amount of metal present in the recovered PCL samples was measured around 0.26–0.63% of the initial loading in metal (Palard et al., 2005). This issue is relevant considering the potential application of PCL in the biomedical field. Subsequent investiga- tions on thein vitrocytotoxicity of PCL samples prepared from various metal derivatives, including the rare-earth trisborohydride complexes, were carried out by Guillaume and coworkers (Schappacher et al., 2010). The results revealed the absence of toxic influence of the polyesters on human osteo- progenitor cells, making these polymers valuable candidates as biomaterials to be in contact with living human cells. Besides, the absence of toxicity of the residual rare-earth metal remaining in the isolated PCL was also demonstrated.
Handbook on the Physics and Chemistry of Rare Earths 24
Following their interest in cationic rare-earth organometallic species, Okuda and coworkers investigated the first ROP of a cyclic ester using a cat- ionic borohydride rare-earth complex, [R(BH4)2(THF)5][BPh4] (R¼Y (3Y);
La (3La), Nd (3Nd), Sm (3Sm)) (Robert et al., 2008) (Table 1). All initiators featured a higher activity toward the ROP of CL (Mn<17, 400gmol1 with 1:25<Mw=Mn<1:38) than the parent neutral complexes [R(BH4)3(THF)3] (R¼Y (1Y), La (1La), Nd (1Nd), Sm (1Sm); 100% conversion within 15 min; Palard et al., 2005), with a complete CL conversion reached in 30s whichever the metal ([CL]0/[BH4]0ca. 250). However, the number of growing chains per metal center always exceeded two (as anticipated from such bisborohydride initiators), presumably due to rapid gelation of the reac- tion mixture involving chain transfer reactions. These preliminary results next prompted theoretical investigations by Okuda, Maron, and coworkers on the ROP of CL initiated by [MeY(BH4)(THF)5][BPh4] (14Y; Susperregui et al., 2011; see belowSection 4 andTable 6).
As an extension of these studies on homoleptic rare-earth borohydride initia- tors, Sun and coworkers developed an elegant approach toward the synthesis of related end-capped PCLs (Wu et al., 2008) (Table 1). The so-called one-pot reduction/initiation strategy was successfully extended to the reduction (up to 90% within 14 min at 60C), by the rare-earth borohydride complexes [R(BH4)3(THF)3] (R¼Y (1Y), Nd (1Nd)), of the carbonyl group of an exogenous aldehyde or ketone RC(O)R0 (R¼alkyl, aryl, R0¼H, alkyl; see below, Scheme 28, Section 3.1.4). The resulting rare-earth alkoxide [R(OCHRR0)3] then triggered the ROP of CL with high efficiency, ultimately affording HO–PCL–OCHRR0 as evidenced by 1H NMR and FTIR analyses (2, 650<Mn<21, 100gmol1 with 1:25<Mw=Mn<1:76). Double or multi carbonyl-containing reagents similarly gavea,o-hydroxyalkoxytelechelic PCLs.
While the reduction reactivity of aliphatic aldehydes is prior to that of ketones, the polymerization process from aliphatic aldehydes was less efficient than others.
3.1.1.2. d-Valerolactone
The ROP of the one-carbon smaller lactone,d-valerolactone (VL) was investi- gated by Nakayama, Shiono, and coworkers at 60C in THF or toluene from [R(BH4)3(THF)x] (R¼Y, x¼2.5 (1Y0); La,x¼3 (1La); Pr, x¼2 (1Pr0); Nd, x¼3 (1Nd); Sm, x¼3 (1Sm); Yb, x¼3 (1Yb)) with various metal centers (Nakayama et al., 2009) (Table 1). The catalytic activities observed were in the same order as the ionic radii of the ions LaPrNd>Sm>Y>Yb (Shannon, 1976), and better in toluene as opposed to THF. The linear variation of Mn values with the PVL yields along with Mw=Mnca:1:43 indicated a rather controlled polymerization in the case of1La. In comparison to CL, the ROP of VL in THF was considerably slower (30 min for 81% conversion at [VL]0/[BH4]0¼200) as expected for a less strained VL ring (Saiyasombat et al., 1998). 1H NMR and MALDI-ToF MS analyses supported the formation of a,o-dihydroxytelechelic
PVLs (Mn up to 15,000 g mol1with 1:3<Mw=Mn<1:7) following the same mechanism as that established for CL (Scheme 18).
3.1.1.3. o-Pentadecalactone
In the [R(BH4)3(THF)3] (R¼Y (1Y), La (1La), Nd (1Nd)) series, the neo- dymium complex was the most efficient (83% in 1 min at [PDL]0/ [BH4]0¼50) toward the ROP ofo-pentadecalactone (PDL) at 60C, as estab- lished byNakayama et al. (2011)(Table 1). The molar mass of the polymer synthesized from1Ndincreased with the yield (Mn up to 39,000 g mol1with Mw=Mnca:1:4) featuring a quite good control. The resulting polyesters were shown by 1H NMR and MALDI-ToF MS analyses as being a,o- dihydroxytelechelic PPDL, HO–PPDL–OH, thereby assuming a mechanism similar to that of CL (Scheme 19).
3.1.1.4. Lactide
The series of complexes [R(BH4)3(THF)x] (R¼Y, x¼2.5 (1Y0); La, x¼3 (1La); Pr,x¼2 (1Pr0); Nd,x¼3 (1Nd); Sm, x¼3 (1Sm); Yb,x¼3 (1Yb)) were also shown by Nakayama, Shiono, and coworkers to be active in the ROP of racemic-lactide (rac-LA,Nakayama et al., 2009) (Table 2). The same trend in catalytic activity related to the metal center (LaPrNd>Sm>Y>Yb) or solvent polarity (ROP proceeded slower in THF as compared to in toluene) was observed, as for VL (vide supra).
No stereoselectivity data have been reported for PLA (3, 800<Mn<30, 100gmol1 with 1:2<Mw=Mn<1:7). The ROP of (S,S)- lactide (L-lactide, LLA), initiated by 1Nd at [LLA]0/[BH4]0¼50, reached 76% in 30 min at 60C, affording highly crystalline PLLA (Tm¼162C, Mn16, 500gmol1) with 1:2<Mw=Mn<1:6 suggesting the absence of
n (1) [R(BH4)3(THF)3]
(2) H+ HO O
O OH
O O n + 2
HO–PVL–OH O
O
VL
4 4 4
SCHEME 18 Synthesis of HO–PVL–OH from the ROP of VL initiated by [R(BH4)3(THF)3] (1R).
n (1) [R(BH4)3(THF)3]
(2) H+
HO O
O OH
O O n + 2
HO–PPDL–OH O
O
PDL
14 14 14
SCHEME 19 Synthesis of HO–PPDL–OH from the ROP of PDL initiated by [R(BH4)3(THF)3] (1R).
Handbook on the Physics and Chemistry of Rare Earths 26
Rare-earth borohydride complex
Lactide
([Lactide]0/[BH4]0
highest value)a
Solvent Temp.
(C)
Reaction time Conv.b
Mnmaxc
(Mntheo)a,d g mol1 ÐMe
Polymer Tacticity
(%) Copolymer
Trivalent complex [Y(BH4)3(THF)2.5]f (1Y0,Scheme 1)
rac-LA(133)f
(Nakayama et al., 2009) THF 60C
60 min 24%f
3800f (4550)g
1.7f ndg
[{(O2NNpy)Y(BH4)(THF)0.5}2] (26Y,Scheme 13)
rac-LA (200) (Bonnet et al., 2005a)
THF, 25C Tol, 70C
60 min 72%
10,600 (18,300)
1.52 Pr 87
[{(O2NNpy)Y(BH4)(THF)0.5}2] (26Y,Scheme 13)
LLA (200)f
(Bonnet et al., 2005a)
THF 25C
180 min 28%f
3850 (7780)
1.37f Isotactic PLLA [{(DAB)Y(BH4)2}{Li(DME)3}]
(35Y,Scheme 25)
rac-LA (75)
(Mahrova et al., 2009)
THF, Tol 20C
4 h 92%
13,400 (9950)
1.21 Atactic PLA [La(BH4)3(THF)3]
(1La,Scheme 1)
rac-LA (200)
(Nakayama et al., 2009) THF 60C
60 min 90%
30,100 (17,625)g
1.33 ndg PLLA-co-PVL
(Nakayama et al., 2009) [Pr(BH4)3(THF)2]f
(1Pr0,Scheme 1)
rac-LA (133)f
(Nakayama et al., 2009) THF 60C
60 min 65%f
12,700f (12,500)g
1.3f ndg
[Nd(BH4)3(THF)3] (1Nd,Scheme 1)
rac-LA (133)f
(Nakayama et al., 2009) THF 60C
60 min 64%f
12,200f (12,300)g
1.4f ndg
[Nd(BH4)3(THF)3] (1Nd,Scheme 1)
LLA (200)
(Nakayama et al., 2007) THF 60C
3 h 76%
16,500 (22,000)
1.2 Isotactic PLLA
PCL-co-PLLA (Nakayama et al., 2007)
Continued
TABLE 2 Rare-Earth Borohydride Complexes Used as Initiators in the (Co)Polymerization of Lactides and Experimental Data—Cont’d
Rare-earth borohydride complex
Lactide
([Lactide]0/[BH4]0
highest value)a
Solvent Temp.
(C)
Reaction time Conv.b
Mnmaxc (Mntheo)a,d g mol1 ÐMe
Polymer Tacticity
(%) Copolymer
[{(TMS)2NC
(NiPr)2}2Nd(BH4)2Li(THF)2]f (22Nd,Scheme 12)
rac-LA (200)
(Skvortsov et al., 2007a)
THF, Tol 20C
60 min
>98% 52,000
(28,220)
1.68 Atactic PLA
[{(TMS)2NC
(NiPr)2}2Nd(BH4)2Li(THF)2]f (22Nd,Scheme 12)
LLA (100)
(Skvortsov et al., 2007a) THF 20C
60 min 98%
17,300 (14,100)
1.54 Isotactic PLA
[1,8-C10H6{NC(tBu)N-2,6-Me2- C6H3}2]R(BH4)2Li(THF)2] R¼Nd, Sm
rac-LA (250)
(Yakovenko et al., 2012)
THF, Tol 20C
18 h 97%
18,700 (36,000)
1.57 Pr 62
[(iPr(TMS)NC(NiPr)N(CH2)3
NC(NiPr)N(TMS)iPr)R(BH4) (DME)]
R¼Y, Nd, Yb
rac-LA (1000) LLA (3000) (Zhang et al., 2012)
THF, Tol 20C
3–15 h 78%
30,500 (33,700)
1.55 Pr 72
[{(O2NNpy)Nd(BH4)(THF)}2] (26Nd,Scheme 13)
rac-LA (200) (Bonnet et al., 2005a)
THF, 25C Tol, 70C
120 min 33%
4800 (9500)
1.57 Pr 64 PCL-b-PLAf (Bonnet et al., 2005a) [{(O2NNpy)Nd(BH4)(THF)}2]
(26Nd,Scheme 13)
LLA (200)f
(Bonnet et al., 2005a)
THF 25C
180 min 34%f
6330 (9790)
1.31f Isotactic PLLA
PCL-b-PLLAf (Bonnet et al., 2005a)
[(O2N2)2Nd(BH4)(BH4)Li(THF)2] (32Nd,Scheme 14)
rac-LA (250)
(Sinenkov et al., 2011)
THF, Tol 20C
12 h 99%
15,000 (29,900)
1.60 Atactic PLA [Sm(BH4)3(THF)3]f
(1Sm,Scheme 1)
rac-LA (133)f
(Nakayama et al., 2009) THF 60C
60 min 59%f
12,100f (11,400)g
1.3f ndg
[{(TMS)2NC
(NCy)2}2Sm(BH4)2Li(THF)2] (22Sm,Scheme 12)
rac-LA (200)
Skvortsov et al., 2007a)
THF, Tol 20C
16.5 h 95%
23,500 (13,820)
2.10 Atactic PLA
[{(TMS)2NC
(NCy)2}2Sm(BH4)2Li(THF)2] (22Sm,Scheme 12)
LLA (100)f
(Skvortsov et al., 2007a) THF 20C
60 min 12%
2200 (1700)
1.56 Isotactic PLAf
[{(O2NNpy)Sm(BH4)}2] (26Sm,Scheme 13)
rac-LA (200) (Bonnet et al., 2005a)
THF 25C Tol 70C
60 min 66%
15,700 (18,000)
1.53 Pr 71.5
[{(O2NNpy)Sm(BH4)}2] (26Sm,Scheme 13)
LLA (200)f
(Bonnet et al., 2005a)
THF 25C
45 min 74%f
15200f (14,400)
1.63f Isotactic PLLA [(O2NL)Sm(BH4)(THF)]
L¼OMe (27Sm), NMe2(28Sm), py (29Sm) (Scheme 13) [(O2NnPr)Sm(BH4)(THF)2] (30Sm,Scheme 13)
rac-LA (200) (Dyer et al., 2010)
THF RT
30 min 92%
21,700 (26,520)
1.58 Pr 84
[Yb(BH4)3(THF)3]f (1Yb,Scheme 1)
rac-LA (133)f
(Nakayama et al., 2009) THF 60C
60 min 0%f
–f –f ndg
Continued
TABLE 2 Rare-Earth Borohydride Complexes Used as Initiators in the (Co)Polymerization of Lactides and Experimental Data—Cont’d
Rare-earth borohydride complex
Lactide
([Lactide]0/[BH4]0
highest value)a
Solvent Temp.
(C)
Reaction time Conv.b
Mnmaxc (Mntheo)a,d g mol1 ÐMe
Polymer Tacticity
(%) Copolymer
[{(TMS)2NC
(NiPr)2}2Yb(BH4)2Li(THF)2] (22Yb,Scheme 12)
rac-LA, LLA (200) (Skvortsov et al., 2007a)
Tol 20C
16.5 h 94%
15,500 (13,700)
1.72 Atactic PLA PLLA
[{(2,6-iPr2C6H3)NC(Me)CHC (Me)N(C6H5)}2RBH4] R¼Y (36Y), Yb (36Yb) (Scheme 16)[({N(2-MeC6H4)C (Me)}2CH)2RBH4]R¼Y (37Y), Yb (37Yb) (Scheme 16)
LLA (1000) (Shen et al., 2012)
Tol 20C
4 min (96)
12,660 (13,760)
1.36 PLLA
aNote that the [lactide]0/[BH4]0ratio is different from the [lactide]0/[initiator/catalyst]0value, the former being used in the calculation of the theoretical molar mass (Mntheo) assuming one growing chain per BH4group.
bMonomer conversion determined by1H NMR or by gravimetry and corresponding to the highest [lactide]0/[BH4]0value reported.
cNumber average molar mass (maximum value) determined by SEC–MALLS or by SEC versus polystyrene standards and corrected by a factor of 0.58 for PLA (Save et al., 2002).
dTheoretical molar mass calculated from [lactide]0/[BH4]0monomer conversionMlactide, assuming one growing chain per BH4group, withMLA¼144 g mol1; note that a discrepancy between Mntheoand MnSECmay be indicative of a number of growing polymer chains different from the number of BH4groups in the initiating complex.
eCalculated average dispersity value as reported from SEC traces.
fOnly a single experiment is reported.
gValue not reported by the authors.