Living and enantiomer-selective polymerization of allene initiated by Ni complex containing chiral phosphine

Hai Zhu,Shizhong Luo*,Zongquan Wu*

a College of Chemistry and Materials Science,The Key Laboratory of Functional Molecular Solids,Ministry of Education,Anhui Laboratory of Molecular-Based Materials,Center for Nano Science and Technology,Anhui Normal University,Wuhu 241000,China

b Department of Polymer Science and Engineering,School of Chemistry and Chemical Engineering,Hefei University of Technology and Anhui Key Laboratory of Advanced Catalytic Materials and Reaction Engineering,Hefei 230009,China

Key words:Chiral allene Helical polymer Chiral phosphine ligand Allylnickel(II)complex catalyst Enantiomer-selective polymerization

ABSTRACT A new allylnickel(II)complex([S(R)]-N-[(1S)-2-( diphenylphosphino)-1-phenylethyl]-2-methyl-2-propanesul fi namide)(2,2,2-tri fl uoroacetato-k O)(p-allyl)nickel(2)was designed and prepared by using chiral phosphine.2 was revealed to efficiently initiate the polymerization of L-and D-N-(1-(dodecylamino)-1-oxopropan-2-yl)-4-(propa-1,2-dien-1-yloxy)-benzamide(L-1 and D-1)in a living/controlled chain grow th manner.Polymerization kinetics of L-1 and D-1 indicated that L-1 preferentially polymerized over the antipode D-1 by a factor of 1.9.In block copolymerization of rac-1 using the poly-L-150 as the macroinitiator,the polymerization proceeded in enantiomer-selective manner.It was found that enantiomeric excess(ee)value of the recovered monomer increased with the monomer conversion and finally reached to the maximum of 34%.These resultssuggest this chiral phosphine complex exhibits enantiomer-selectivity for the polymerization of chiral allene derivative monomer.

Most of the biological macromolecules like polysaccharides[1],protein[2]and DNA[3]areoptically activeand someof them present characteristic functionalities such as catalysing metabolic reactions,molecular recognition,structure-related and stores biological information.Inspired by biological helices and their elaborate functions,chemistshavebeen challenged to develop Artificial helical oligomers and polymers for potential applications in materials science,such as ferroelectric liquid crystals,nonlinear optical materials,sensing specific molecules,the separation of enantiomers and asymmetric catalysis[4].Chiral compounds show specific structure-activity relationships,Meanwhile racemic mixtures and individualstereoisomerscan differ significantly in living systems[5].Therefore,enantiomer separation from racemic mixtures is an important process that can now be accomplished with chiral columns[6,7],membrane fi ltration[8],asymmetric catalysis[9],enantiomer-selective crystallization[10,11]and enantiomer-selective polymerization[12,13].Enantiomer-selective polymerization means that one enantiomer is preferentially polymerized to obtain optically active polymer and unreacted monomer.It isaresolution of kinetic and stereospecific[14].Enantiomer-selectivity can be achieved by optically active catalystsor initiators.Even the preferred helical structure of the polymer chain can also contribute to the enantiomer-selectivity during the polymerization[15].Allene is a special kind of monomer with interesting characteristic which allow s them to obtain polymers through the selective polymerization of either part(1,2-or 2,3-)of cumulative double bond[16,17].Although investigations about polyallene have been carried out in recent decades,the study of opticalactivity isstilllimited[18,19].The allylnikel(II)catalystsisa stableand highly chemoselectivecomplex,which has been used for living polymerization of allenes containing various functional groups[20–22].Due to the interaction between the complex and monomers at the active end of living chain,the coordination polymerization has a good performance on the enantiomer-selectivity[23].One of the strategies for enantiomerselective polymerization is coordination polymerization with a chiral ligand complex catalyst[24,25].We believe the present study providesa clue for choosing chiral ligand to explore the possibility of enantiomer-selectivity for chiral allene monomer.In order to adjust the environment of the coordination polymerization,the PPh3in the allylnickel(II)complex catalyst was replaced by chiral phosphine.

In this contribution,the living and enantiomer-selective polymerization of allene derivatives initiated by 2 was reported.First,2 wasprepared according to a previously reported method by our group with slightly modification[26].The living nature of the polymerization of L-1 with 2 was investigated.Polymerization in different initial feed ratios of L-1 to 2 were then performed(Scheme 1).The Mns and Mw/Mnvalues of obtained polymers were analyzed by size exclusion chromatography(SEC)with equivalent to polystyrene standards.As SEC traces show n in Fig.1a,all the obtained polymers exhibited a single-modal elution peak.The Mnincreased linearly with the conversion of L-1 and all the obtained polymersshowed narrow molecular weight distributionswith Mw/Mn<1.24(Fig.1b).Thus,it was con fi rmed that the polymerizations of L-1 using 2 as an initiator proceeded in a living/controlled manner.

To get a deeper insight into this new polymerization system,kinetic studies of L-1 and D-1 with 2 were carried out.The polymerization was conducted in the presence of an internal dimethyl terephthalate(DMT)as standard.The reactions were followed by1H NMRmeasurements of the aliquots taken out from the polymerization system at appropriate time intervals to follow the changes in the relative relationship of L-1 or D-1 with respect to DMTto calculate the monomer conversions.The conversion of L-1 and D-1 in polymerization was show n in Fig.2a,which clearly revealed that the polymerization rate of L-1 was faster than that of the enantiomeric antipode D-1.

As summarized in Fig.2b,the linear correlation between?ln([M]/[M]0)and the polymerization time revealed that the tw o reactions obeyed fi rst-order rate law.The consumption rate of L-1(kL-1)was faster than that of D-1(k[54_TD DIFF]D-1).The kL-1and kD-1were calculated as 3.80?10?5and 1.99?10?5s?1respectively.It was w orthwhile to note that the reaction rates were much slower than that of using PPh3complex catalyst[26].Kinetic studies showed that L-1 was preferentially polymerized over the antipode D-1 by a factor of 1.9.These results indicated that the chiral Ni complex affected the addition of chiral monomer to the grow ing end.It can be attributed to the coordination structure around the metal center with the chiral ligand,which provided a rigid and asymmetric environment making the reaction affected.The addition of steric congestion at the metal center slowed the polymer formation.The difference in chiral compatibility between catalyst and tw o enantiomers made their reaction rate different.The obvious difference between kL-1and kD-1provided the possibility for the enantiomer-selectivity of allene bearing chiral amide pendants.

Then,using Ni(II)-terminated poly-L-150as macroinitiator,poly-L-1100,poly(L-150-b-D-150)and poly(L-150-b-rac-150)were obtained via sequential addition of monomer L-1,D-1 and rac-1 respectively.As show n in Table 1,Ni(II)-terminated poly-L-150can initiate a living/controlled block copolymerization.

Next,the optical activity changes of polymers were studied.Amides are w idely used in the design and preparation of functional materials.Chirality and intermolecular hydrogen bonding are essential to the stable helical conformation with a preferred handedness.As show n in Fig.3a,chiral amide-based polyallenes presented stable helix structure in THF.

Fig.1.(a)SECchromatograms of poly-L-1m prepared in CH2Cl2 with different initial feed ratiosof L-1 to 2 at 298 K.(b)M n and M w/M n values as functionsof conversion of L-1 to prepare poly-L-150.

Fig.2.(a)Plotsof the conversion with the polymerization time of monomer L-1 and D-1 initiated by 2 in CH2Cl2 at 298 K.(b)First-order kinetic plots for the polymerization of L-1 and D-1 initiated by 2 in CH2Cl2 at 298 K.

Table 1 M n and M w/M n values of the copolymer sampling form the polymerization system of preparing poly(L-150-b-D-150)and poly(L-150-b-rac-150).

The molar circular dichroism of the poly-L-1ms with different Mnwas show n in Fig.3b.When Mnwas increased in the range of 5.3 k Da to 20.0 k Da,the corresponding molar circular dichroism gradually became larger and reached the maximum value of?24.7 Lmol?1cm?1.20.0 k Da was a critical value of Mn.When the Mnwas above the value,the m olar circular dichroism no longer varied.This indicated that the helical structure with preferred handedness of poly-L-150(Mn=20.0 k Da)gradually stabilized,and no longer changing as repeating unit elongated.That's w hy we choose poly-L-150as a macroinitiator.

Scheme 1.The preparation of poly-L-1100,poly(L-150-b-D-150)and poly(L-150-b-rac-150).

Fig.3.(a)Molar circular dichroism and UV-visspectra of poly-L-1m measured in THFat 298 K.(b)The plot of molar circular dichroism of poly-L-1m measured in THFat 298 Kas a function of M n(c=0.03 g/L).

The samples taken from the copolymerization system of preparing poly(L-150-b-D-150)and poly(L-150-b-rac-150)were analyzed.For tw o copolymerization systems,the Mns of macroinitiator were both close to 20.0 k Da.They also had similar molar circular dichroism of?24.7 Lmol?1cm?1and helical structure.As show n in Fig.3b,it can be seen that the macroinitiator had a stable helical structure.The copolymerization with D-1 was studied.As Fig.4a show n,the molar circular dichroism of the block polymer decreased with increasing Mn,which indicated that the poly-D-1 block had a destructive effect on the helix structure of the copolymer.As for the approximate linear relationship in Fig.4b,the reason may be that the number of the repeating unit increased(increased UV absorption)while the helical structure of the copolymer changed(decreased optical activity)simultaneously.

The process of block polymerization of poly(L-150-b-rac-150)illustrated that the new Ni complex can achieve the enantiomerselectivity.Sampling from copolymerization system of poly(L-150-b-rac-150)to analyze the obtained copolymer and recovered monomer.A large amount of methanol was added to quench the sample.The formed precipitate was isolated by centrifugation to afford copolymer and supernatant.The ee values of the supernatant were determined by high performance liquid chromatography(HPLC)using columns with chiral stationary phase.As show n in Fig.5a,with the conversion of rac-1,the ee value continuously increased and finally reached the maximum value of 34%.It was con fi rmed that this polymerization was a kind of kinetic resolution,in which L-1 was preferentially polymerized over the antipode.For copolymers,the correspondence between the molar circular dichroism and Mnwas show n in Fig.5b,from which it can be observed that the change of molar circular dichroism was slightly in the fi rst stage of polymerization.Combined with the kinetic curve,the reason m ay be that L-1 preferentially polymerized over the antipode D-1 at the same concentration.When the Mnexceeded 28.1 k Da,the molar circular dichroism changed rapidly.This is due to the increase of relative concentration of D-[57_TD DIFF]1 which made the advantages of L-[57_TD DIFF]1 weakened.The change in concentration still could not offset the difference in kL-[58_TD DIFF]9IF]1and kD-1.Therefore,the ee value was continuously increased throughout the process.Meanwhile,there was still a considerable CDintensity at the end of the copolymerization.

Fig.5.(a)Plot of ee value of the supernatant as a function of conversion of rac-1 during the copolymerization of poly(L-150-b-rac-150).(b)Plot of molar circular dichroism of poly(L-150-b-rac-1o)sampled during the copolymerization process measured in THF at 298 K as a function of M n(c=0.03 g/L).

In summary,we have demonstrated the enantiomer-selective polymerization of chiral allene derivative monomer by using chiral phosphine allylnickel(II)complex catalyst.The polymerization rate of L-1 was 1.9 times that of D-1.During the polymerization of poly(L-150-b-rac-150),both the obtained copolymer and the recovered monomer exhibited optical activity.It illustrated that the subtle change in the metal coordination environment had a huge impact on the polymerization.The chiral environment from the ligand was effective for the enantiomeric-selectivity.We believe that higher selectivity w ill be achieved by matching the monomers with new ligand of the chimeric center.

Fig.4.(a)Molar circular dichroism and UV-vis spectra of poly(L-150-b-D-1n)measured in THFat 298 K.(b)The plot of molar circular dichroism of poly(L-150-b-D-1n)measured in THFat 298 K as a function of M n(c=0.03 g/L).

Acknow ledgments

This work was sponsored by the National Natural Science Foundation of China(Nos.21622402,51673057,and 21574036).Z.Q.Wu thanks the Thousand Young Talents Program of China and the Open Project of State Key Laboratory of Supramolecular Structure and Materials(No.sklssm201624)for Financial Support.

Appendix A.Supp lementary data

Supplementary data associated with this article can be found,in the online version,at https://doi.org/10.1016/j.cclet.2018.03.002.