High cis-1,4-selectivity and activity coordination (co) polymerization of (polar) dienes by PNP-ligated rare-earth metal bis (aminobenzyl) complexes

The research on the synthesis of cis-1,4-selective poly (conjugated diene)s catalyzed by the transition metal complexes has been actively pursued in both industrial and academic laboratories until today. However, there has not been any reported rare-earth catalytic system that can achieve the cis-1,4-selective polymerization of multiple conjugated dienes. Polar and non-polar conjugated dienes, as well as copolymerization of different kinds of conjugated dienes affording novel copoly (diene)s, are included. Herein, the highly cis-1,4-selective homopolymerization of a series of (polar)dienes (such as IP, MY, OC, 2-MOPB, 2-FPB) was achieved by using PNP ligated rare-earth metal bis (aminobenzyl) complexes PNP-Ln (Ln = Ln = Sc, Lu, Y, Gd, Sm, 1-5)/[Ph3C][B(C6F5)4]/Al i Bu3 ternary catalyst systems. Moreover, the highly cis-1,4-selective copolymerization of (polar)dienes (such as 2-MOPB and IP, 2-FPB and IP, MEPB and IP, OC and IP as well as 2-FPB and MY) with a board range of comonomers contenting and random or block sequence distribution was also achieved.


Introduction
One of the essential rubbers is poly (conjugated dienes (CDs))s, which are involved in a wide range of applications in industry, medicine, agriculture, and so on.Due to the unique conjugated double bond structure, there are two addition modes (1,4-addition and 3,4 (or 1,2)-addition) to form cis-1,4-, trans-1,4-or 3,4 (or 1,2)-selective microstructures.Among them, cis-1,4-selective poly(CDs)s is important to provide products in the chemical industry [1] .Generally, a slight increase in cis-1,4-selectivity of poly(CDs)s usually leads to a great enhancement in the properties of elastic [2] .Furthermore, the incorporation of functional groups to poly(dienes) endows synthetic rubber with versatile properties such as hydrophilicity, printability, insulativity, and enhanced compatibility with inorganic fillers [3] .Rare-earth metal cationic alkyl active species with various ligands, including metallocene and nonmetallocene ligands, were reported to achieve high activity and cis-1,4-selectivity in isoprene polymerization [4] .Cis-1,4-selective polymyrcenes were obtained by using NdV 3 /Al i Bu 3 /AlEt 2 Cl ternary catalytic system [5] , cyclopentadienyl scandium complexes [6] , Flu-based CGC allyl-type monoalkyl complexes [7] , anilido-oxazoline non-metallocene complexes [8] , etc. Li first reported the ocimene cis-1,4-selective polymerization catalyzed by cyclopentadienyl ligand rare-earth metal complexes [9] .Moreover, Cui reported highly cis-1,4-selective polymerization of polar diene containing oxygen and fluorine groups by using β-diketiminato rare-earth dialkyl complexes [10] .Unfortunately, until now, there has not been any reported rare-earth catalytic system that can achieve the cis-1,4-selective polymerization of multiple conjugated dienes, including polar and non-polar conjugated dienes.Besides, the copolymerization of different kinds of conjugated dienes affording novel copoly(diene)s with different microstructures (random, alternating, and block) that possess unique physical and mechanical properties still remains a challenge.
Herein, we report the high cis-1,4-selectivity and active homopolymerization of a series of (polar)dienes (such as isoprene (IP), myrcene (MY), ocimene (OC), 2-(4-methoxyphenyl)1,3-butadiene In some cases, the living nature polymerization of IP or 2-FPB was achieved by using PNP-Lu complex 2 and PNP-Sm complex 5, respectively.Moreover, the highly cis-1,4-selective copolymerization of (polar)dienes (such as 2-MOPB and IP, 2-FPB and IP, MEPB and IP, OC and IP as well as 2-FPB and MY) with a board range of comonomers contenting were achieved by using PNP-Gd complex 4 and PNP-Y complex 3. Furthermore, the block copolymer P(IP-b-2-FPB) was realized based on the living nature of the PNP-Lu complex 2 ternary catalyst system in IP homopolymerization.

2.
Methods A specific (co)polymerization process was described as follows.In the glovebox and 25℃, the complexes 1-5 (10 μmol, 2 mL toluene) and 1 equiv. of [Ph 3 C][B(C 6 F 5 ) 4 ] (10 μmol, 9.2 mg) were added into a 25 mL flask.Then 10 equiv. of Al i Bu 3 (100 μmol, 100 μL, 1M in toluene) was added under stirring to give a yellow solution after several minutes, upon the addition of CDs monomers or the mixture of CDs comonomers.After the reaction for a period of time, the reaction mixture was poured into a large quantity of methanol (containing 5% BHT) and then dried under vacuum at 40°C to a constant weight.To evaluate the effect of tapes of the central metals and tapes of CDs on the regioselectivity and activity of CDs polymerization, a series of rare-earth metal PNP-ligated complexes (PNP-Ln 1-5, Ln = Sc, Lu, Y, Gd, Sm) were used to catalyze the homopolymerization of a serious of (polar)CDs.Some represent results are listed in Table 1.The polymerization of IP was carried out in toluene at 25°C using the 1-5 /[Ph 3 C][B(C 6 F 5 ) 4 ] /Al i Bu 3 ternary catalyst systems.The PIPs obtained by 1-5 had high cis-1,4selectivities (95% -99%), high molecular weights (M n : 5.2 -13.2 10 4 g•mol -1 ), narrow molecular weight distribution (M w /M n : 1.10 -1.74), and high activities (act.: 1.7 -3.5 10 5 g•mol cat.

Results and discussion
-1 •h -1 ) (Table 1 entries  1-5).The results demonstrated that PNP-ligated complexes 1-5 possess a wide coordination environment for cis-1,4-coordination and insertion mode of IP monomer.Moreover, the narrow molecular weight distribution of PIPs obtained by PNP-Sc and PNP-Lu complexes 1-2 suggested that a smaller ionic radius means larger steric hindrance.It limits the chain termination and chain transfer to achieve living fashion (Figure 1).Surprisingly, the polymerization of MY showed extremely high activities (act.: 3.2 -64.0 10 5 g• mol cat. -1 • h -1 ) and high cis-1,4-selectivity (96% -99%) by 1-5 /[Ph 3 C][B(C 6 F 5 ) 4 ]/Al i Bu 3 ternary catalyst systems (Table 1, entries 6-10).These results indicated that the catalyst active species have enough space for cis-1,4-polymerization of MY.The longer alkyl-vinyl group in MY may have a stronger electron-donating ability than the methyl group in IP, which results in higher activity in the polymerization of MY.For the OC polymerization catalyzed by PNP-Sc complex 1, the larger steric hindrance of OC monomer led to low activity and poor selectivity (cis-1,4-selectivity: 56%, trans-1,2selectivity: 44%) (Table 1, entry 11).With the ionic radius increasing to Lu and Y, activity was obviously improved, and cis-1,4-selectivity was raised to 76% -99% (Table 1, entries 12-13).However, cis-1,4selectivity of POC catalyzed by PNP-Gd complex 4 was decreased to 91% (Table 1, entry 14).It could be explained that the electron-donating methyl group and alkyl-vinyl group in OC lead to a higher electric density of the conjugated double bond of OC monomer.This is unfriendly to the coordination of OC to relatively low Lewis acid PNP-Gd complex 4 via cis-1,4-mode.Therefore, the lowest Lewis acid PNP-Sm complex 5 was inert for the polymerization of OC (   1, entries 16-25).PNP-Sc complex 1 was inert for the polymerization of 2-FPB and 2-MOPB meaning it was poisoned by polar CDS because of its high Lewis acidity and strongly coordinating to O and F atoms (Table 1, entries 16 and 21).By contrast, other PNP-Ln complexes 2-5 realized high cis-1,4-selectivity (97% -99%), moderate molecular weights (M n : 2.7 -6.6 10 4 g•mol -1 ), and narrow molecular weight distribution (M w /M n : 1.12 -1.52) in polar CDs polymerization (Table 1, entries 16 and 21).The narrow molecular weight distribution of all resultant poly(polar CD)s indicates that the larger steric hindrance of polar CDs monomers could effectively limit chain termination and chain transfer to achieve living fashion, which was proved by GPC plots (Figure A72    The copolymerization of 2-MOPB and IP was carried out in toluene at 25°C by using the PNP-Gd complex 4/[Ph 3 C][B(C 6 F 5 ) 4 ]/Al i Bu 3 ternary catalyst system (Table 2, entries 1-4).As a result, high cis-1,4-selective P(IP-co-2-MOPB)s (> 99%, for both 2-MOPB units and IP units), moderate molecular weight (M n : 2.7 -6.3 10 4 g•mol -1 ), and narrow molecular weight distribution (M w /M n : 1.12 -2.00) were prepared.The contenting of 2-MOPB was at a range of 14 mol% -70 mol% depending on the feed ratio of 2-MOPB/IP from 50/350 -300/100.Moreover, the high cis-1,4-selective P(IP-co-2-FPB)s (> 99%) with the contenting of 2-FPB at a range of 17 mol% -87 mol% were also obtained under the same conditions (Table 2, entries 5-9).Obviously, the activity of copolymerization of IP and 2-FPB was higher than copolymerization of IP and 2-MOPB, properly due to the weak coordination of fluorine atom to Gd metal center.In view of the strong coordination of the pyridine group to the Gd metal center, which further led to the deactivation of the catalyst, equimolar amounts of AlEt 3 with MPEP were first mixed as a protection reagent.The copolymerization of MPEP and IP was successfully processed despite low contenting of MPEP (2 mol% -4 mol%) and gradually decreased activity with gradually increased MPEP/IP molar ratios (50/1000 -200/1000) (Table 2, entries 10-12).High active copolymerization of MY and 2-FPB was unexpectedly achieved with high cis-1,4-selectivity (> 99%, for both 2-MOPB units and IP units), moderate molecular weight (M n : 3.2 -5.3 10 4 g•mol -1 ), and narrow molecular weight distribution (M w /M n : 1.34 -1.42) (Table 2, entries 13-17).Considering the best result of the high cis-1,4selective homopolymerization of OC by using PNP-Y complex 3, copolymerization of OC and IP was catalyzed by PNP-Y complex 3 instead of PNP-Gd complex 4. The activity of that copolymerization was sharply decreased with the increased OC/IP molar ratios (100/300 -300/100) (Table 2, entries 18-20).The contenting of IP units in P(OC-co-IP)s was at a range of 58 mol% -95 mol% even at the low OC/IP molar ratios as 100/300.This is probably due to the low activity of homopolymerization of OC by PNP-Y complex 3. Furthermore, the block copolymerization of IP and 2-FPB under a 150/150 ratio, using two batches, resulted in block copolymer P(IP-b-2-FPB) (Table 2, entry 21).This was based on the living nature of IP homopolymerization by PNP-Lu complex 2.
All the GPC curves of the above copolymers were single peak and narrow molecular weight distributions, which suggested the formation of copolymers instead of a mixture of both homopolymers (Figure A52-S70). 1 H and 13 C NMR spectrum of resulted copolymers revealed the excellent cis-1,4-

Mechanism study of polymerization of CDs by PNP-Ln/[Ph 3 C][B(C 6 F 5 ) 4 ]/Al i Bu 3 ternary catalyst systems
For the high cis-1,4-selective polymerization of non-polar CDs (including IP, MY, OC), the high coordination sites and low steric hindrance of rare-earth metal center allowed the cis-1,4-coordination mode non-polar CD monomers.The following cis-1,4-insertion formed the syn-η 3 -π-allylic intermediate.The coordination and insertion of the next non-polar CDs forced the lasted inserted syn-η 3 -π-allylic intermediate to form cis-1,4-selectivity (Figure A110a).As for the polymerization of polar CDs (including 2-MOPB and 2-FPB), the coordination of heteroatom groups was generally superior to the coordination of the vinyl group.Thanks to the electron-donating effect of PNP-ligand, the dynamic equilibrium between the coordination of heteroatom groups and the coordination of conjugated double bond permitted the following cis-1,4-coordination and insertion of polar CDs monomers.The syn-η 3 -πallylic intermediate is also formed.The coordination and insertion of the next polar CDs also forced the lasted inserted syn-η 3 -π-allylic intermediate to form cis-1,4-selectivity (Figure A110b).

Conclusion
In summary, a series of highly cis-1,4-selective (up to 99%) poly((polar) CDs) (PIP, PMY, POC, P(2-MOPB), P(2-FPB)) were achieved with high activities (up to 64.0 10 5 g•mol cat. -1 •h -1 ), high molecular weight (up to 13.2 10 4 g•mol -1 ), and narrow molecular weight distribution (< 2.0).It is achieved using the synthesized PNP-ligated rare-earth metal complexes (PNP-Ln, Ln= Sc, Lu, Y, Gd, Sm, 1-5) with [Ph 3 C][B(C 6 F 5 ) 4 ] and Al i Bu 3 .The electronic factors and steric factors of both catalysts active species and monomers determined the best match of catalysts and monomers, making a great difference in the regioselectivity and activity for CDs polymerization.To our knowledge, this is the first rare-earth metal catalyst system that achieves high cis-1,4-selectivity polymerization of non-polar CDs and polar CDs due to appropriate steric hindrance around catalysts active species and electronic donating groups in chelating-ligand.Furthermore, the various kinds of copolymers of (polar)dienes P(CD-co-CD)s (including P(IP-co-2-MOPB), P(IP-co-2-FPB), P(IP-co-MEPB), P(MY-co-2-FPB), and P(OC-co-IP)) with high cis-1,4-selectivity of both two comonomers units and board range of comonomers contenting were obtained by using PNP-Ln complexes 3-4.The reactivity ratio studies demonstrated that the microstructure of P(CD-co-CD)s tends to be random sequence distributions.Moreover, the block copolymer P(IP-b-2-FPB) with narrow molecular weight distribution was achieved.It can be deduced that the mechanical and physical properties of these highly cis-1,4-selective copolymers may be greatly different from traditional homopolymers of CDs.The work on the exploration of (co)polymerization of other kinds of CDs by these catalyst systems and the application of the P(CD)s and P(CD-co-CD)s is ongoing.

Appendices
The Supporting Information is available free of charge at 1 H NMR spectra of polar dienes; 1 H NMR spectra of complexes 1-5; 1 H, 13 C NMR spectra of resultant polymers; GPC and DSC curves of resultant polymers.
).The above results demonstrate that the coordination of heteroatoms to the metal center and the coordination of conjugated double bonds to the metal center were dynamic transitions based on the relatively lower Lewis acidity of the metal center.A series of highly cis-1,4-selective poly(CDs) with high activity and high molecular weight, and narrow molecular weight distribution were respectively obtained by PNP-Ln 1-5/[Ph 3 C][B(C 6 F 5 ) 4 ]/Al i Bu 3 ternary catalyst systems.The ionic radius of metal centers and substitute groups of CD monomers played an important role in the activity and regioselectivity of CDs polymerization.

Figure 1 .Table 2 .
Figure 1.(a) Plots of M n and M w /M n against the conversion of monomers.(b) GPC curves of polymers obtained from the polymerization of IP in different reaction times.Table 2. Highly cis-1,4-selective copolymerization of conjugated dienes by the complexes 2-4/[Ph 3 C][B(C 6 F 5 ) 4 ]/Al i Bu 3 ternary systems a .