304
M. Hong
of the series (Zn-1: P m = 0.99, Zn-2–4: P m = 0.95–0.96, [L-manOCA] 0 :[Zn] 0 :[3F-Py] 0 :[L-ML] 0 = 50:1:1:1, TOF = 77–100 h
−1 ) [70]. However, when switching to
Zn-5 (R 1 = R 2 =
t Bu, R 3 = H), the ROP occurred accompanied by serious epimerization (P m = 0.54), presumably attributed to the more open environment of the zinc
center because of the small R 3 (hydrogen atom) that makes the inversion of the chiral
center of manOCA easier. In all cases, M n s (4.2–24.2 kg/mol) of the obtained polymers are close to the calculated values with low Ð values (1.08–1.10), indicative of
good controllability of these LPs over MW. In contrast, the polymerization by Zn-1 or
3-F-Py alone only led to negligible or sluggish activity. The most selective Zn-1/3-FPy LP was also exploited for stereoselective ROP of rac-manOCA (Scheme 8.14), but
it produced atactic poly(rac-manOCA) at RT (P m = 0.60). Remarkably, decreasing
the polymerization temperature from RT to –50 °C can significantly enhance the
isoselectivity, generating a highly isotactic poly(rac-manOCA) with P m up to 0.92
(M n = 5.3 kg/mol, Ð = 1.12) due to the chain-end control mechanism, which represents the first stereoselective ROP of rac-manOCA. A noticeable increase in the T g
value from 84 to 92 °C was observed when the isotacticity of poly(rac-manOCA)
increased from P m = 0.60 to 0.92. A T m peak at 116 °C appeared for isothermally
crystallized isotactic poly(rac-manOCA) (P m = 0.92) on account of stereocomplex
formation, in comparison to no obvious T m for atactic poly(rac-manOCA) (P m =
0.60). When enantiopure poly(L-manOCA) and poly(D-manOCA) were mixed at
1:1 ratio, T m value of the resulting stereocomplex material can reach 173 °C after
isothermal crystallization.
LP catalysts have also been applied for the preparation of high-MW
poly(propylene oxide) (PPO) via ROP of propylene oxide (PO) as reported by
Naumann et al. in 2019 [71]. Due to the existence of chain transfer to monomer side
reaction (Scheme 8.15, top), the synthesis of high-MW PPO via conventional anionic
polymerization still remains a challenge. It was found that a cooperative LP catalyst
based on NHO-9 LB and novel magnesium bis(hexamethyldisilazide) [Mg(HMDS) 2 ]
LA can address this challenge (Scheme 8.15, middle). A PPO with a M n of 520 kg/mol
and a relatively low Ð of 1.24 was produced within 81 h through performing the
polymerization in pentane at –36 °C with a [PO] 0 :[Mg(HMDS) 2 ] 0 :[NHO-9] 0 ratio
of 5000:10:1. A zwitterionic polymerization mechanism has been confirmed, where
the polymerization is initiated by the nucleophilic attack of NHO-9 to Mg(HMDS) 2 -
activited PO and propagated via a bimolecular, activated monomer mechanism
(Scheme 8.15, bottom). The role of Mg(HMDS) 2 , which is critical for obtaining
high-MW PPO, is trifold: (1) activating PO to render the formation of zwitterionic initiated species and the occurrence of propagation, since neither NHO-9 nor
Mg(HMDS) 2 alone was able to initiate polymerization; (2) forming stable CLA
with NHO to enable a high ratio of monomer to initiator (i.e., low I
* ) for high-MW
product; (3) stabilizing the oxyanion species to suppress transfer-to-monomer side
reaction for high-MW product. Compared with NHO-9, pairing Mg(HMDS) 2 with a
more basic NHO-1 (Scheme 8.2) led to a UHMW PPO with a M n up to 1400 kg/mol
and a Ð of 1.55 in 144 h ([PO] 0 :[Mg(HMDS) 2 ] 0 :[NHO-1] 0 = 5000:20:1), probably due to even lower I
* . Besides, the combination of Mg(HMDS) 2 with the other
NHOs [NHO-10 and NHO-11 (Scheme 8.15, middle)] can all promote a smooth
M. Hong
of the series (Zn-1: P m = 0.99, Zn-2–4: P m = 0.95–0.96, [L-manOCA] 0 :[Zn] 0 :[3F-Py] 0 :[L-ML] 0 = 50:1:1:1, TOF = 77–100 h
−1 ) [70]. However, when switching to
Zn-5 (R 1 = R 2 =
t Bu, R 3 = H), the ROP occurred accompanied by serious epimerization (P m = 0.54), presumably attributed to the more open environment of the zinc
center because of the small R 3 (hydrogen atom) that makes the inversion of the chiral
center of manOCA easier. In all cases, M n s (4.2–24.2 kg/mol) of the obtained polymers are close to the calculated values with low Ð values (1.08–1.10), indicative of
good controllability of these LPs over MW. In contrast, the polymerization by Zn-1 or
3-F-Py alone only led to negligible or sluggish activity. The most selective Zn-1/3-FPy LP was also exploited for stereoselective ROP of rac-manOCA (Scheme 8.14), but
it produced atactic poly(rac-manOCA) at RT (P m = 0.60). Remarkably, decreasing
the polymerization temperature from RT to –50 °C can significantly enhance the
isoselectivity, generating a highly isotactic poly(rac-manOCA) with P m up to 0.92
(M n = 5.3 kg/mol, Ð = 1.12) due to the chain-end control mechanism, which represents the first stereoselective ROP of rac-manOCA. A noticeable increase in the T g
value from 84 to 92 °C was observed when the isotacticity of poly(rac-manOCA)
increased from P m = 0.60 to 0.92. A T m peak at 116 °C appeared for isothermally
crystallized isotactic poly(rac-manOCA) (P m = 0.92) on account of stereocomplex
formation, in comparison to no obvious T m for atactic poly(rac-manOCA) (P m =
0.60). When enantiopure poly(L-manOCA) and poly(D-manOCA) were mixed at
1:1 ratio, T m value of the resulting stereocomplex material can reach 173 °C after
isothermal crystallization.
LP catalysts have also been applied for the preparation of high-MW
poly(propylene oxide) (PPO) via ROP of propylene oxide (PO) as reported by
Naumann et al. in 2019 [71]. Due to the existence of chain transfer to monomer side
reaction (Scheme 8.15, top), the synthesis of high-MW PPO via conventional anionic
polymerization still remains a challenge. It was found that a cooperative LP catalyst
based on NHO-9 LB and novel magnesium bis(hexamethyldisilazide) [Mg(HMDS) 2 ]
LA can address this challenge (Scheme 8.15, middle). A PPO with a M n of 520 kg/mol
and a relatively low Ð of 1.24 was produced within 81 h through performing the
polymerization in pentane at –36 °C with a [PO] 0 :[Mg(HMDS) 2 ] 0 :[NHO-9] 0 ratio
of 5000:10:1. A zwitterionic polymerization mechanism has been confirmed, where
the polymerization is initiated by the nucleophilic attack of NHO-9 to Mg(HMDS) 2 -
activited PO and propagated via a bimolecular, activated monomer mechanism
(Scheme 8.15, bottom). The role of Mg(HMDS) 2 , which is critical for obtaining
high-MW PPO, is trifold: (1) activating PO to render the formation of zwitterionic initiated species and the occurrence of propagation, since neither NHO-9 nor
Mg(HMDS) 2 alone was able to initiate polymerization; (2) forming stable CLA
with NHO to enable a high ratio of monomer to initiator (i.e., low I
* ) for high-MW
product; (3) stabilizing the oxyanion species to suppress transfer-to-monomer side
reaction for high-MW product. Compared with NHO-9, pairing Mg(HMDS) 2 with a
more basic NHO-1 (Scheme 8.2) led to a UHMW PPO with a M n up to 1400 kg/mol
and a Ð of 1.55 in 144 h ([PO] 0 :[Mg(HMDS) 2 ] 0 :[NHO-1] 0 = 5000:20:1), probably due to even lower I
* . Besides, the combination of Mg(HMDS) 2 with the other
NHOs [NHO-10 and NHO-11 (Scheme 8.15, middle)] can all promote a smooth