MMA for ring opening polymerization of cyclic esters

EUROPEAN POLYMER JOURNAL

European Polymer Journal 40 (2004) 2143–2152

www.elsevier.com/locate/europolj

In situ formation of Sm(III) initiator from SmI2/MMA for ring opening polymerization of cyclic esters Seema Agarwal

*

Fachbereich Chemie, Kernchemie und Makromolekulare Chemie und Wissenschaftliches Zentrum f€ur Materialwissenschaften, Institut f€ ur Physikalische Chemie, Philipps-Universit€at Marburg, Hans-Meerwein Strasse, D-35032, Marburg, Germany Received 5 February 2004; received in revised form 20 April 2004; accepted 13 May 2004 Available online 2 July 2004

Abstract Sm(III) complex was generated in situ by one-electron transfer reaction of SmI2 with MMA. The resulting Sm(III) complex was tried as ring opening polymerization (ROP) initiator for cyclic esters like e-caprolactone (CL). The polymers were obtained in high yield with low polydispersity index (1.1–1.2). The system was also used for the synthesis of block copolymers. Structural characterization of the resulting polymers was done using 1D (1 H, 13 C, 19 F) and 2D (1 H–13 C heteronuclear multiple quantum correlation (HMQC) and heteronuclear multiple-bond correlation (HMBC)) NMR spectroscopic techniques.
2004 Elsevier Ltd. All rights reserved. Keywords: ROP; SmI2 ; Bisinitiation; Cyclic esters

1. Introduction The ring opening polymerization (ROP) of cyclic esters is the major pathway for the synthesis of corresponding polyesters. In recent years the utilization of organometallic lanthanide complexes, for ROPs has gained a lot of attention [1]. Catalyst design with different ligands (by changing the stereoelectronic environment and steric requirements around the metal), leading to polyesters with controlled molecular and physical characteristics, is the aim of the current research in this field [2–4]. Yasuda et al. [5,6] have explored the living polymerization of various lactones like d-valerolactone (VL) and e-caprolactone (CL) by single component organolanthanides like SmMe (C5 Me5 )2 THF or [SmH(C5 Me5 )2 ]2 giving corresponding polymers with polydispersity index in the range 1.05–1.10. On the other hand, divalent organolanthanides like (C5 Me5 )2 -

*

Tel.: +49-6421-2825755; fax: +49-6421-2825785. E-mail address: [email protected] (S. Agarwal).

Sm, (C13 H9 )Sm(THF)2 and (C9 H7 )2 Sm(THF)x can initiate the polymerization of lactones, but the resulting polymers had rather broad molecular weight distributions and show bimodal pattern [4]. Evans et al. reported SmI2 (THF)2 to be a ROP initiator of CL at reflux but got polycaprolactones (PCLs) with multimodal GPC curves [7]. Enhanced reactivity was found for the reduction of 1-haloalkanes (halo ¼ bromo, iodo) and diaminoalkanes by reaction of SmI2 in the presence of Sm metal, which prompted us to use a Sm/SmI2 system for ROP of lactones as a part of our interest in studying the effect of ligands around Sm metal in controlling ROP reaction of cyclic esters [8]. Although it is found to be room temperature ROP initiator for CL and VL, it catalyses simultaneous degradation of the resulting PCL/PVL, thereby providing limited control over the molecular weight and polydispersity of the obtained polymers. The electron transfer reactions between Sm(II) compounds like Cp*2 Sm [(C5 Me5 )2 Sm] or Cp*2 Sm(THF)2 and polymerizable/non-polymerizable unsaturated substrates are well known [9–11]. In our previous studies we have utilized one-electron transfer property of SmI2 for

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2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2004.05.006

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S. Agarwal / European Polymer Journal 40 (2004) 2143–2152

lic samarium (III) compounds, generated by one-electron transfer from samarocene species Cp*2 Sm or Cp*2 Sm(THF)2 to unsaturated compounds through the intermediate formation of radical anions (Scheme 2) for polymerization of methyl methacrylate (MMA) [14–17]. Preformed bifunctional organosamarium initiators of the type Cp*2 Sm–R–SmCp*2 (Fig. 1) have also been used by Novak et al. for the synthesis of ‘link functionalized’ polycaprolactones (PCLs) [15]. Allyl complexes 1 and 2 produced PCL with bimodal molecular weight distribution and there was no systematic correlation of theoretical molecular weight and the obtained one with different allyl groups. Samarium halides like SmI2 are stable, easy to prepare and handle. Therefore, we tried to use in situ generated Sm(III) bisinitiator from methyl methacrylate and diiodosamarium (SmI2 ) in THF for ring opening polymerization of cyclic esters.

PCL

R

O

O Sm(III)

O

SmI2 H

20 oC, THF

CL

R

R

R

R O

O Sm(III)

PCL

Scheme 1.

the synthesis of hydroxy functionalized poly(p-xylylene)s (PPXs) starting from aromatic dialdehydes and their further grafting with e-CL and L -lactide (LA) [12,13]. As model reactions, in situ generated Sm(III) pinacolates have been utilized as ROP initiators for cyclic esters, thereby generating polyaliphatic esters linked through pinacolate units (Scheme 1). Novak et al. have utilized pre-formed (Fig. 1) and in situ generated bimetal-

Cp*2Sm

Cp*2Sm

SmCp*2

Cp*2Sm SmCp*2 SmCp*2

1 2

3

N

Cp*2Sm N

Cp*2Sm

N

SmCp*2

N

N SmCp*2

N

4 5 Fig. 1. Sm(III) bisinitiators for MMA and CL polymerization.

L

O

Sm COOCH3 L

L Sm or

L

O Sm

L

L

L

2

Sm 2

L .

O

O

O O O O

Sm(II)

(6)

Scheme 2.

L Sm L

S. Agarwal / European Polymer Journal 40 (2004) 2143–2152

e-CL is used as a representative example for cyclic esters. Living character of the growing polymer chains is shown by sequential polymerization of CL and VL. Structural characterization of the resulting polymers is done using 2D 1 H–13 C HMQC (heteronuclear multiple quantum correlation) and HMBC (heteronuclear multiple-bond correlation) NMR techniques.

2. Experimental 2.1. Materials and instrumentation THF was purified by distillation over potassium under nitrogen. CL (Aldrich) was dried over CaH2 for 2 days, distilled under reduced pressure, and degassed by freeze–thaw method (two times). Methyl methacrylate (MMA) (Aldrich) was dried over CaH2 and distilled under reduced pressure. SmI2 was prepared as 0.1 M solution in THF by reacting 10 mmol of Sm (Aldrich, 40 mesh) with 5 mmol of diiodoethane [20]. The solution is filtered under argon and filtered solution is further used for polymerization reactions. SmI2 solution was stored under argon and used maximum for one week’s time. After this it was discarded and prepared again. Molecular weight of the polymers was determined by GPC using Knauer system equipped with 2 columns PSS-SDV (linear, 10 ll, 60 · 0.8), a differential refractive index detector and a UV photometer using THF as eluent at a flow rate of 0.83 ml/min. 1 H (400.13 MHz) and 13 C (100.21 MHz), NMR spectra were recorded on a Bruker DRX-400 spectrometer. Neat CDCl3 was used, and 1 H and 13 C were referenced to residual solvent signals. 1 H–13 C correlation experiments were performed on a Bruker DRX-500 spectrometer, with a 5 mm multinuclear gradient probe and using gs-HMQC and gs-HMBC pulse sequences. The HMQC experiment was optimized for C–H coupling of 140 Hz, with decoupling applied during acquisition; while the HMBC experiment was optimized for coupling of 8 Hz, with decoupling during acquisition. 2D NMR data were acquired with 2048 points in t2 , and the number of increments for t1 was 256. 4 and 8 scans were used for HMQC and HMBC experiments respectively, and dummy scans of 4 was used for both the

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experiments. A relaxation delay of 1 s was used for all 1D experiments and 2 s for all 2D experiments. Typical experiment time was about 1.5 and 3.0 h for HMQC and HMBC respectively. 2.2. A typical procedure for polymerization reactions In general, all polymerization reactions were carried out under dry conditions in argon atmosphere due to air and moisture sensitivity of Sm compounds. A typical procedure for in situ generation of Sm(III) initiator and subsequent ROP of CL is given here (run 4, Table 1). MMA (0.0015 mol) was added to SmI2 (15 ml, 0.1 M solution in THF) in a predried Schlenk tube under argon at room temperature. The mixture is stirred for 1 h during which a yellow powder is precipitated out. This suspension is allowed to come at 0 C. To this suspension at 0 C, CL (5 ml, 0.043 mol) is added using a syringe under argon. The mixture became clear on addition of CL. The polymerization is allowed to proceed for 2 h. After this time period, polymer is precipitated in methanol containing HCl (5% v/v). Purification of the polymer was done by dissolving in THF and reprecipitating in MeOH–HCl system.

3. Results and discussion One-electron transfer reactions of Sm(II) compounds giving Sm(III) (samarocene complexes) are very well known by this time. They proceed through the intermediate formation of radical anions as shown in Scheme 2 when stoichiometric amounts of Sm(II) compound and MMA are reacted. In the present work in situ generated Sm(III) species (6) (Scheme 2) was used for the ring opening polymerization (ROP) of CL at room temperature (25 C) and at 0 C. Before carrying out detailed polymerization studies, it was considered worth to study the effect of non-stoichiometry of Sm(II) compound and MMA on ROP of CL. Polymerizations were carried out using different molar ratios of SmI2 : MMA:CL (0.0015:0.001:0.0438, 0.0015:0.0015:0.0438, 0.0015:0.003:0.0438 and 0.0015:0.01:0.0438 mol) at 0 C. The polymers obtained with 0.0015, 0.003 and 0.01 mol of MMA showed almost the same molecular weights

Table 1 GPC data for CL polymerization using SmI2 (0.0015 mol)/MMA (0.0015 mol) system at 0 C for 2 h Run

CL (mol)

Mn (corrected)

Mn (theoretical)

Mw =Mn

Yield (%)

1 2 3 4 5 6

0.0175 0.0263 0.0438 0.0614 0.1052 0.2062

2739 3958 5692 7704 15,000 22,500

2659 4000 6657 9333 16,000 25,073

1.10 1.12 1.13 1.15 1.20 1.31

99 99 98 99 98 82

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S. Agarwal / European Polymer Journal 40 (2004) 2143–2152 Table 2 GPC data for CL polymerization using SmI2 (0.0015 mol)/ MMA (0.0015 mol) system at 25 C

2.0 8000 7000

1.8

6000

CL (mol)

Mn (corrected)

Mn (theoretical)

Mw =Mn

Yield (%)

1 2 3

0.0263 0.0438 0.0614

4608 6709 9099

4000 6657 9333

1.51 1.41 1.55

95 94 97

Mw / Mn

1.6

5000

Mn

Run

4000 1.4 3000 2000

1.2

1000 0

1.0 20

40

60

80

I

100

O

Sm

Yield %

I

PCL O O

O

MeO

Fig. 2. Variation of Mn and Mw =Mn with yield; run 4, Table 1. O O

CH3

I Sm I

CH3

PCL COOCH3

(6)

(B)

PCL COOCH3

Scheme 3.

weights were obtained using polystyrene standards at room temperature. Therefore, they were corrected to the absolute values using a correlation factor of 0.45 [18]. The theoretical molecular weight was calculated assuming the bisinitiaton. The polymers were obtained with low polydispersities (1.1–1.2) (Table 1, entries 1–5). At monomer:initiator ratio of 275:1, a significant difference in the theoretical molecular weight and obtained molecular weight was obtained with increase in polydispersity to about 1.3 (entry 6, Table 1) showing transesterification becoming significant at this point. Fig. 2 shows the variation of Mn with the yield for

15 9' O 16

O

O 18

3

5

1

O 2''

4

2

6

O

5 6 O

4

O

17

O

3

O

n1

8

3

OH 4

2

A

7

15 O

O 16

3

18

5 O

O O

9 2'

4

6

O

PCL O O

17

10

14

13

(A)

OMe

and polydispersity indexes (Mn  5800 and polydispersity 1.11–1.13) as compared to higher molecular weight obtained with 0.001 mol of MMA (Mn ¼ 9000 and polydispersity 1.20). These molecular weights are close to the predicted molecular weights of PCL using 0.0015:0.0015 and 0.0015:0.0010 molar ratios of SmI2 :MMA respectively. Therefore, little excess of MMA during in situ generation of Sm(III) initiator has no effect on further ROP of cyclic esters. Further copolymerization studies are carried out with stoichiometric amounts of SmI2 and MMA. Various copolymerizations were tried using different ratios of CL to initiator. The results for polymerizations at 0 C are summarized in the Table 1. There was an increase in the molecular weight on increasing the monomer:initiator ratio and a good correlation was observed between the experimental and calculated molecular weight values for low monomer:initiator ratios. The obtained molecular

PCL

PCL

11 12

Scheme 4.

1

3 2

O

O

5 4

6

O

8

3

OH n1

2

4

7

B

S. Agarwal / European Polymer Journal 40 (2004) 2143–2152

2147

ppm 32.11, 51.26 and 62.20. Some other minor peaks were at ppm 24.49, 25.11, 33.63, 34.00, 173.47 and 173.67. The peaks at ppm 51.26 and 62.20 in 13 C NMR are assigned to the methoxy carbon of linking MMA unit (–C(O)OCH3 ) (15) and end –CH2 OH (7) group. This assignment is based on the observation of cross peaks A and B in 2D 1 H–13 C correlated HMQC NMR spectrum (Fig. 5). These cross peaks A and B originate from cross coupling of (–C(O)OCH3 ) (15) and –CH2 OH (7) protons at ppm 3.57 and 3.53 in 1 H NMR with those of 51.26 and 62.20 peaks in 13 C NMR. 2D HMBC NMR spectrum was used for other peak assignments in 13 C NMR spectrum. The peak at ppm 33.63 is assigned to the –CH2 CH2 OH i.e. carbon attached to protons 8 of end CL unit as this produces a cross peak (C) with peak at ppm 3.53 (–CH2 OH (7)) in 1 H NMR in 2D HMBC spectrum (Fig. 6). Further, the peak at ppm 173.47 showed a cross peak (D) with (–C(O)OCH3 ) (15) protons of linking MMA units (peak at ppm 3.57 1 H NMR) and hence can be unambiguously assigned to carbonyl group (16) of MMA units (Fig. 7). Also, the end OH groups were benzoylated with pentafluorobenzoyl chloride in order to confirm them unambiguously. For end group functionalization, the growing polymer chains after completion of reaction were allowed to react with excess of pentafluorobenzoyl chloride (1:2.5 molar ratio of CL:pentafluorobenzoyl chloride) for 1 h. The end functionalized polymer was then precipitated and purified as described in experimental section. The end capping of OH group was confirmed using 1 H, 13 C and 19 F NMR spectroscopic

sample 4 (Table 1). There was a linear increase in Mn with increase in yield. After completion of reaction, the contents were maintained at similar conditions for further 5 h. There was a slight change in Mn after this time (Mn ¼ 7000) but a noticeable increase in polydispersity was seen (1.32). Also, when polymerizations were carried out at room temperature (25 C), there was not much change in the molecular weights of the obtained polymers as compared to the theoretical values but again a noticeable increase in the polydispersity values was seen. This shows some amount of intermolecular transesterification occurring at longer reaction times and at room temperature (Table 2). In situ generated Sm(III) species can start the polymerization in two ways as shown in Scheme 3. Again, initiation step could have ring opening reaction of cyclic ester or can occur without ring opening as shown in Scheme 4. For simplicity, the polymer chain only on one side of the linking initiating unit is shown. Here an attempt has been made to find out the preferred route for CL polymerization using 1D and 2D NMR spectroscopic techniques. The representative 1 H NMR spectrum of the obtained low molecular weight polymer (PCL, run 1, Table 1) is shown in Fig. 3. Besides characteristic signals of PCL [21,22] some minor obvious signals were also seen at ppm 2.05, 3.53 and 3.57. The triplet at ppm 3.53 is assigned to the end –CH2 OH (7) group. Methoxy protons (–C(O)OCH3 ) (15) of linking MMA units were obtained as singlet at ppm 3.57. 13 C NMR peak assignments are given in Fig. 4. Besides PCL peaks [21,22], other prominent peaks observed were at

6+10 2

3+5+8+18

4+17

15 7

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5 ppm

1

Fig. 3. H NMR spectrum of PCL (run 1, Table 1) in CDCl3 .

2148

S. Agarwal / European Polymer Journal 40 (2004) 2143–2152

6 1 9

175

174

3+17 2+2'+11

16

173

5

4

172

18 8 35

30

25

S 7

150

Fig. 4.

15

100

13

50

0 ppm

C NMR spectrum of PCL (run 1, Table 1) in CDCl3 .

6+16 1 1

H

15 7

H

ppm

ppm

13

C

13

C

10

20

20

40

30 A

15 7

60 B

6+16

40

8 (C’) (C)

50

80 4

3

2

1 ppm

60 70

Fig. 5. A part of 2D HMQC NMR spectrum (run 1, Table 1): 1 H NMR 0–4.5 ppm; 13 C 10–85 ppm.

techniques. The product after benzoylation showed three prominent signals in 19 F NMR (Fig. 8) arising from three different types of F atoms of hexafluorophenyl ring [23]. 1 H NMR spectrum of end capped link fuctionalized PCL is shown in Fig. 9. In 1 H NMR spectrum, the triplet at ppm 3.53, which was assigned to protons 7 of –CH2 OH end group is disappeared, instead a new signal at ppm 4.33 appeared (–CH2 C(O)C6F5) (70 ). 13 C NMR spectrum also showed a change in chemical shift of –CH2 OH group towards lower field after end capping to ppm 66.41 (Fig. 10). The detailed structural characterization shows route B (Scheme 2) to be the preferred way of ROP of cyclic esters. Vinyl polymerization of MMA is also reported to follow the

80 4

3

2

1 ppm

Fig. 6. A part of 2D HMBC NMR spectrum (run 1, Table 1): 1 H NMR 0–4.5 ppm; 13 C 0–90 ppm.

route B (Scheme 3) using Sm(III) bisinitiators [14,16,19]. Also the ratio of intensities of peaks at ppm 4.33 (–CH2 C(O)C6F5) and 3.57 (–C(O)OCH3 ) from 1 H NMR spectrum (Fig. 9) are found to be in the ratio 4:5.92. This value is in excellent agreement with the expected value of 4:6 assuming bisinitiation. A careful examination of HMBC (Fig. 6) NMR spectrum showed the presence of a cross peak C0 obtained by correlation of peaks around ppm 4.01 and

S. Agarwal / European Polymer Journal 40 (2004) 2143–2152

1

ization of caprolactone copolymers [21,22]. Also, it cannot be explained by structure A (Scheme 4) but could be produced by cross coupling of protons 10 at around ppm 4.0 with carbon 11 around ppm 33.9 from unopened CL ring. This may suggest structure B (Scheme 4) as the probable polymer structure but cannot be reconfirmed from other cross peaks as other peaks from unopened ring structure/opened linking CL unit are overlapping with PCL peaks and are expected to show overlapping cross peaks. Also carbonyl carbon 90 (from opened CL unit) attached to MMA linking unit is expected to appear significantly downfield from carbonyl carbon of PCL backbone. In the present work no such signal is observed. Actually, only three signals in the carbonyl carbon region between ppm 173.2–173.72 were observed. Two of them unambiguously go to carbonyl carbons of MMA linking unit and PCL backbone and third could be due to the carbonyl carbon 9 next to the unopened CL ring as shown in Fig. 4. Monometallic samarium alkyls are also reported to add to lactones without ring opening [5]. Since the polymers were obtained with low polydispersities and transesterifications were found to be negligible at low M:In ratios and for less polymerization time, gives an indication of living growing polymer chains at least under these conditions. As preliminary studies, living character of the growing polymer chains

H

ppm

13

C

155

165

175

1

16

(D) 185

195

4

3

1 ppm

2

Fig. 7. A part of 2D HMBC NMR spectrum (run 1, Table 1): 1 H NMR 0–4.5 ppm; 13 C 150–200 ppm.

around ppm 33.9 in 1 H NMR spectrum and 13 C NMR spectrum respectively. This correlation cannot be explained from PCL backbone structure i.e. 4 bonds correlation between –OCH2 and –C(O)CH2 – of PCL is not expected. We did not observe this correlation in our previous studies regarding microstructural character-

a PCL

O

1 O

3 2

O

5 4

6O

2149

5

3

O

n

2

4

7'

F

F

O F

F

1'

b F

c

b a

c

0

-50

Fig. 8.

19

-100

-150

F NMR spectrum of pentafluorobenzoyl end capped PCL.

ppm

2150

S. Agarwal / European Polymer Journal 40 (2004) 2143–2152 a F

F

PCL

O

1

3

O

5

5

3

O

6O

4

2

O n

2

4

7'

b

1'

F

c

O F

F

2.5

2.0

6+16

6+16

B

A

15 7 4.0

6.5

6.0

5.5

15

3.5

5.0

7'

4.5

4.0

3.5

3.0

1.5

1.0

0.5

0.0 ppm

Fig. 9. 1 H NMR spectrum of PCL: (A) a part of 1 H NMR of PCL (run 1, Table 1); (B) of pentafluorobenzoyl end capped PCL.

a F

PCL

O

1 O

3 2

O

5 4

6O

5

3

O

n

2

4

7'

F

1' O F

b F

c F

6+16

7'

90

80

Fig. 10.

70 13

15

60

50

40

30

20

C NMR spectrum of pentafluorobenzoyl end capped PCL.

10

0 ppm

S. Agarwal / European Polymer Journal 40 (2004) 2143–2152

2151

Table 3 Sequential polymerization of CL with VL/CL at 0 C using SmI2 (0.0015 mol)/MMA (0.0015 mol) Run

Mn PCL (first step)

Mw =Mn (first step)

VL/CL (mol) (second step)

Mn (corrected) PCL-b-PVL/PCL

Mw =Mn PCL-bPVL/PCL

1 2

3958 3980

1.12 1.15

0.0175 0.0200

6511 5233

1.15 1.09

*

Run 1 CL is the second monomer; run 2 VL is the second comonomer.

4

O 1

2

O

3

O

6 5

O

a

O

e

c

O

d

b

PVL

PCL

4

5

2

3

d

c

b

35

30

6

66

1

65

25

20

e

64

63

62

a

175.0174.5 174.0 173.5 173.0 172.5

150

Fig. 11.

100 13

50

0 ppm

C NMR spectrum of PCL-b-PVL (run 2, Table 3).

was tested by sequential polymerization of the growing chains with the second cyclic monomer like CL or VL (Table 3). The copolymers synthesized were characterized by NMR and GPC techniques. The presence of both the comonomers in copolymers was confirmed by NMR spectroscopic technique. The representative 13 C NMR spectrum for run 2, Table 3 (CL–VL copolymer) is shown in Fig. 11. Also, there was an increase in the molecular weight of the polymers on sequential addition of the second comonomer (CL/VL) (Table 3). The nature of GPC curves (remained unimodal) with very low polydispersity index further characterizes the block copolymers formed (Fig. 12). There could be a possibility of formation of some random sequences by transesteri-

fication reactions during sequential polymerization of two monomers. That is expected to be more probable for synthesis of high molecular weight block copolymers. Further work regarding synthesis of high molecular weight block copolymers and their detailed microstructural characterization is in progress.

4. Conclusions The successful synthesis of PCL and its block copolymers with other aliphatic polyesters is achieved using in situ generated Sm(III) initiator. Present studies showed the use of a stable, easy to prepare and handle,

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S. Agarwal / European Polymer Journal 40 (2004) 2143–2152

5.0 4.5 4.0 3.5

Intensity

(b) 3.0

(a)

2.5 2.0 1.5 1.0 0.5 0.0 -0.5 3.0

3.2

3.4

3.6

3.8

4.0

4.2

4.4

4.6

4.8

5.0

Log M

Fig. 12. GPC traces: (a) homo PCL; (b) PCL-b-PVL.

Sm(III) initiator for controlled polymerization of CL with low molecular weight distributions (1.1–1.2). Replacement of MMA linking unit with the other unsaturated monomers could provide the opportunity for property tuning and functionalization of PCL.

Acknowledgement The author would like to thank Prof. Andreas Greiner for useful discussions during the work.

References [1] Agarwal S, Mast C, Deknicke K, Greiner A. Macromol Rapid Commun 2000;21:195, and references therein.

[2] Ravi P, Gr€ ob T, Dehnicke D, Greiner A. Macromolecules 2001;34:8649. [3] Xu L, Jiang L, Sun W, Shen Z, Ma S. Polym Bull 2002;49(1):17. [4] Nishiura M, Hou Z, Koizumi T, Immoto T, Wakatsuki Y. Macromolecules 1999;32:8245. [5] Yamashita M, Takemoto Y, Ihara E, Yasuda H. Macromolecules 1996;29:1798. [6] Yasuda H, Ihara E. Bull Chem Soc Jpn 1997;70:1745. [7] Evans WJ, Katsumata H. Macromolecules 1994:2330. [8] Agarwal S, Brandukowa-Szmikowski NE, Greiner A. Macromol Rapid Commun 1999;20:274. [9] Evans WJ, Rabe GW, Ziller JW. J Organomet Chem 1994;483:21. [10] Evans WJ, Keyer RA, Rabe GW, Drummond DK, Ziller JW. Organometallics 1993;192:205. [11] Evans EJ, Gonzales SL, Ziller JW. J Am Chem Soc 1994;116:2600. [12] Brandukowa NE, Agarwal S, Greiner A. Acta Polym 1999;50:35. [13] Agarwal S, Brandukowa NE, Greiner A. Polym Adv Technol 1999;10:528. [14] Boffa LS, Novak BM. J Mol Catal A: Chem 1998;133:123. [15] Boffa LS, Novak BM. Macromolecules 1997;30:3494. [16] Boffa LS, Novak BM. Macromolecules 1994;27:6993. [17] Boffa LS, Novak BM. Tetrahedron 1997;53(45):15367. [18] McLain SJ, Drysdale NE. Polym Prepr (Am Chem Soc, Div Polym Chem) 1992;33(1):174. [19] Gosho A, Nomura R, Tomita I, Endo T. Macromol Chem Phys 2001;202:1614. [20] Girard P, Namy JL, Kagan HB. J Am Chem Soc 1980;102: 2693. [21] Agarwal S, Naumann N, Xie X. Macromolecules 2002;35: 7713. [22] Agarwal S, Xie X. Macromolecules 2003;36:3545. [23] Hesse M, Meier H, Zeeh B. Spektroskopische Methoden in der Organische Chemie. Stuttgart: Georg Thieme Verlag; 1991. p. 194.