print pageprinter friendly
Reports: ND754177-ND7: New Catalysts for Stereoselective Polymerization of Functional Alpha-Olefins

Lin Pu, University of Virginia

The research progress during the year of 2014-2015 is summarized below.

1.  Synthesis of the macrocyclic ligands 1 and 2

Two optically active chiral macrocycles 1 and 2 have been synthesized according to Scheme 1.

Scheme 1. 

2.  Polymerization using the chiral macrocycles

a.  Polymerization of N,N-dimethyl acrylamide

The macrocycle 1 was first treated with NaH in THF at 0 oC.  Then 1 equiv Yb(OTf)3 was added to prepare a macrocyclic Yb complex.  CuBr was added to prepare the bimetallic complex.  The resulting catalyst mixture was used to promote the polymerization of N,N-dimethyl acrylamide in the presence of the initiator ethyl a-bromoisobutyrate (Scheme 2).  The polymerization was completed in 4 h with 98% conversion as monitored by 1H NMR analysis.  The 1H NMR spectrum of the polymer showed an atactic that it contains approximately equal distribution of m and r units, that is an atactic polymer.. 

Scheme 2.

A different procedure was investigated for the polymerization.  The monomer was first mixed with a Lewis acid complex, and then the macrocycle 1, the initiator and CuBr were added.  The type and amount of the Lewis acid complex were varied and the results of the polymerizations are summarized in Table 1.  These results show that at 5 equiv, the use of the Lewis acid complexes such as Yb(OTf)3, Y(OTf)3, La(OTf)3 and Eu(OTf)3 gave 82 – 89% isotacticity (m%) and high conversions.  Using Zn(OTf)2 gave lower tacticity and conversion.  The conversion of the polymerizations and the tacticity of the resulting polymers were determined by 1H NMR analysis. 

Table 1. Influence of the amount and type of Lewis acid on the polymerization

Amount

(equiv)

 Isotacticity (%)/Conversion (%) obtained with various Lewis acids

Yb(OTf)3

 Y(OTf)3

 La(OTf)3

 Eu(OTf)3

 Zn(OTf)2
5

 89/87

 82/80

 88/98

 85/80

 57/53
3

 81/10

 78/66

 82/50

 84/66

 54/33
1

 N/A

 72/35

 N/A

 79/32

 51/24

A typical experimental procedure is given here.  N,N-Dimethylacrylamide (218 mg, 2.2 mmol), methanol (0.227 mL) and Yb(OTf)3 (68.2 mg, 0.11 mmol) were added to a Schlenk flask, and the mixture was stirred for 0.5 h.  Then, the macrocycle 1 (15 mg, 0.022 mmol) was added.  After the mixture was stirred for 0.5 h, ethyl α-bromoisobutyrate (8.6 mg, 0.044 mmol) was added and the reaction mixture underwent three freeze-pump-thaw cycles for degassing.  Then at room temperature, CuBr (15.8 mg, 0.11 mmol) was added and the reaction flask was taken inside a glove box and stirred for 24 h.  The polymer was dissolved in ethanol and precipitated in diethyl ether.  Then the polymer was isolated by centrifugation.  The process was repeated for three times to get the pure polymer in 70% yield.  The molecular weight of the polymer was determined as Mn = 6876 (PDI = 1.05) by MALDI mass analysis. 

The effect of the amount of CuBr on the polymerization was also studied.  As shown in Table 2, increasing the amount of CuBr from 2 equiv to 5 equiv increased the isotacticity from 75% to 85%, but further increasing CuBr decreased the isotacticity.

Table 2. Influence of the amount of CuBr on the polymerization

CuBr

(equiv)

 2

 5

 10
Yb(OTf)3

(equiv)

 5

 5

 5
Isotacticity

(%)

 75

 85

 80
The solvent effect on the polymerization was studied.  It was found that in THF the isotacticity was 82% but in methanol the isotacticity increased close to 90%.  Lowering the polymerization temperature to 0 oC decreased the isotacticity to 74%. 

We also tested the use of the macrocycle 2 for the polymerization under the same conditions as the use of 1.  It was found that when Yb(OTf)3 (5 equiv) was used, polymerization of N,N-dimethyl acrylamide (100 equiv) gave 83% isotacticity and 80% conversion.  Thus, macrocycle 1 is slightly better than 2 in this polymerization. 

In the absence of the macrocycle, no polymerization was observed even after 24 h.  When the macrocycle 1 was added, the polymerization was initiated immediately as the solution became viscous very quickly.  Thus, the macrocycle is necessary for the polymerization.  Without the Lewis acid complex such as Yb(OTf)3, the polymerization also cannot occur.  That is, the macrocycle without the coordination of the Lewis acid can inhibit the polymerization.  This might be attributed to the radical inhibition function of the phenol units of the macrocycle. 

b.  Polymerization of N-isopropyl acrylamide.

We have studied the polymerization of N-isopropyl acrylamide because the resulting poly(N-isopropyl acrylamide) is a very interesting temperature sensitive material.  Controlling its tacticity should be significant for its application.  Both macrocycles 1 and 2 were used for the ATRP polymerization of N-isopropyl acrylamide (Scheme 3).  The polymerization in the presence of 1equiv of 1 or 2, 5 equiv of Yb(OTf)3 and 100 equiv of N-isopropyl acrylamide was successful and the use of 1 led to better polymerization with the conversion of monomer reaching 88% after 24 h.  The isotacticity of the resulting polymer was found to be 90% by analyzing its 1H NMR spectrum.  When the amount of Yb(OTf)3 was reduced to 1 equiv, the polymerization gave lower conversion (78%) and lower isotacticity (67%).  In the presence of the macrocycle 2 and 5 equiv Yb(OTf)3, the isotacticity was found to be 84% with 60% conversion.  When 1 equiv Yb(OTf)3 was used, the isotacticity and conversion were found to be 60% and 57% respectively.    

Scheme 3.

3.  Summary

We have synthesized two chiral macrocyclic compounds and studied their use for the ATRP of acrylamides.  In the presence of a Lewis acid complex, these macrocycles have exhibited good tacticity control in the polymerization.  In the next project period, we will try to modify the structure of the chiral macrocycles in order to further enhance the stereoselectivity of the polymerization.  Efforts will also be devoted to understand the mechanism of the polymerization process.