Cationic Polymerization - American Chemical Society

Chapter 6

Stability of Propagating Species in Living Cationic Polymerization of Isobutylene 1

1,3,4

1

2

Daniela Held , Béla Iván , Axel H. E. Müller , Feike de Jong , and Teun Graafland Downloaded by AUBURN UNIV on November 17, 2016 | http://pubs.acs.org Publication Date: May 1, 1997 | doi: 10.1021/bk-1997-0665.ch006

2

1

Institute of Physical Chemistry, University of Mainz, Welderweg 15, D-55099 Mainz, Germany Shell Research and Development Centre, Postbus 3033, NL-1003 AA, Amsterdam, Netherlands

2

The stability of living polyisobutylene chains (PIB) obtained by di- and monofunctional initiators in conjunction with T i C l coinitiator was investigated under monomer starved conditions (i. e. after 100 % monomer conversion) in the absence and presence of different additives, such as Ν,Ν-dimethylacetamide (DMA), 2,6-di-tertbutylpyridine (DtBP), pyridine (Py) and 2,4-dimethylpyridine (DMPy), in CH Cl /hexane (40:60 v/v) mixture at -78 °C. Only negligible amounts of chain ends with expected double bonds were formed as verified by H NMR, and all the additives, with the exception of DtBP, resulted in constant molecular weights for a period of four hours. However, chain coupling occurred in the presence of DtBP. On the basis of our experimental findings this effect is interpreted by proton abstraction in a reaction between DtBP and propagating chains leading to external double bonds which further react with active chain ends. Molecular weight distribution data indicate that there are differences among the examined nucleophilic compounds in their mode of action during living polymerization of isobutylene. 4

2

2

1

There have been significant developments in the field of living carbocationic polymerization (LCCP) during the past few years (see Refs. 7-5 for recent reviews). LCCP offers process control for the synthesis of a wide variety of novel materials with potential commercial interest. These include new macromonomers, telechelics, polymers with pendant functional groups (liquid crystalline homo- and copolymers, non-linear optical materials etc.), star-shaped macromolecules, block and graft copolymers (thermoplastic elastomers, amphiphilic blocks), cyclic Permanent address: Central Research Institute for Chemistry of the Hungarian Academy of Sciences, Pusztaszeri u. 59-67, P.O. Box 17, H-1525 Budapest, Hungary Corresponding author © 1997 American Chemical Society

Faust and Shaffer; Cationic Polymerization ACS Symposium Series; American Chemical Society: Washington, DC, 1997.

63

CATIONIC POLYMERIZATION

64

macromolecules and specialty networks (polyurethanes, polyepoxides, ionomers and amphiphilic networks as potential new biomaterials) (1-5). LCCP is a process in which active (propagating) chains are in equilibrium with dormant (inactive) species (1,6-8). For instance, halogen-terminated chains (P-X) are in equilibrium with the active species (Ρ ) propagating by monomer addition in living polymerization of olefins, such as isobutylene and styrene and its derivatives, in the presence of Lewis acids (MtX ), e. g. BC1 , TiCl , SnX (X = CI, Br) etc. Φ

n

Downloaded by AUBURN UNIV on November 17, 2016 | http://pubs.acs.org Publication Date: May 1, 1997 | doi: 10.1021/bk-1997-0665.ch006

P®__MtX®

active

3

PX + MtX

4

4

n

dormant

On the basis of this mechanistic scheme, a kinetic model has been recently developed by us to evaluate LCCP systems of alkyl vinyl ethers and decomposition of propagating species in such polymerizations (9). Most of the LCCP systems contain either a nucleophilic additive (also referred to as Lewis base or electron donor), such as esters, ethers, amines, amides etc. (see Refs. 1-5 for review), chelating compounds (10), or a tetraalkylammonium salt (11-15). In the current practice, six major classes of additives are used to mediate LCCP of isobutylene (IB): (1) inert nucleophiles (DMA, dimethylsulfoxide, N methyl-pyrrolidone, esters etc.) (see Refs. 1-5 for review), (2) strong bases, such as Py and DMPy (16,17), (3) DtBP proton trap (18,19), (4) chelating agents, such as 2,5pentandione and Ν,Ν,Ν',Ν'-tetramethylethylenediamine (10), (5) inhiders (i/iAibitor + retarder agent) (1), (6) tetra-w-butylammoniumchloride (13,14). The exact role of these compounds has not been completely revealed yet, and there are several interpretations on their beneficial effects for LCCP to occur: (i) by pushing the ionic equilibria through interaction of the nucleophilic additives or their complexes with carbocations towards less reactive cationic species which are in equilibrium with dormant terminated chains as shown in equation 1 (1), (ii) by serving as a proton trap exclusively, as in the case of DtBP proton trap (19), and (iii) by forming dormant onium-type chain ends, as in the case of ethers and thioethers (20), which are in equilibrium with reactive ion-pairs and free ions that propagate by "classical" carbocationic polymerization. On the basis of "classical" carbocationic polymerizations, decomposition of the propagating carbocationic species is expected to lead to proton expulsion and formation of a double bond at chain ends as in chain transfer processes. This process is viewed to result from hyperconjugation between the inherently instable carbocationic center and neighboring protons (1) leading to proton expulsion and formation of a π-bond. In polymerization of IB, this means the appearance of exo and endo olefinic endgroups by chain transfer (see equation 2). Polyisobutylene (PIB) chains with external double bond terminus are able to react with the polymeric


6. HELD ET AL.

Propagating Species in Polymerization of Isobutylene


carbocations leading to coupling of PIB molecules, i . e. to increase of molecular weight (MW). It is expected that internal double bond containing PIBs do not react further because of steric hindrance. Therefore, proton expulsion in LCCP of IB under monomer starved conditions (i. e. after complete monomer consumption) can be detected by chain end structure and M W determinations.

The stability of propagating species, that is their ability to participate only in the desired propagation and to avoid side reactions leading to permanently terminated polymer chains, is a critical factor not only for the fundamental understanding of particular living polymerization processes but it is important from synthetic and technological point of view as well. This chapter deals with the results of our recent systematic investigations carried out to obtain information on the effect of different nucleophilic additives on the stability of active (propagating) species in isobutylene (IB) polymerization under monomer starved conditions.

Results and Discussion The Effect of Different Additives on the Chain End Structure and on Molecular Weight Distributions of PIBs Obtained by Difunctional Initiator. On the basis of the above thoughts, IB polymerizations initiated by a difunctional initiator, l,3-di(2chloro-2-propyl)-5-ter/-butylbenzene (fBuDiCumCl), in conjunction with T i C l in the presence of different additives, such as DMA, D/BP, Py and DMPy, were allowed to run to complete monomer conversions (reached in less than five min); then samples were withdrawn at predetermined time intervals up to four hours as described in the Experimental Section. For all the additives, a yellow to red precipitate was observed on the addition of T i C l to the solution of initiator, monomer and additive. Since precipitates were not observed in IB polymerization in the absence of additives, the formation of a Lewis base - Lewis acid complex is assumed in line with earlier results (1,18). On addition of methanol, the complex dissolved and discolored immediately for DMA, D/BP and DMPy. In case of Py, the discoloration was very slow. At the reaction time of four hours, a second portion of monomer was charged to the polymerization system in order to check the activity of the chains toward further polymerization. 4

4

Surprisingly, significant amounts of double bond containing chain ends were not detected even after four hours of standing under monomer starved conditions, i. e. in polymerization systems with 100 % monomer conversion, neither in the absence


65

66


nor in the presence of the different additives. Data obtained by molecular weight determination with GPC indicate lower than theoretical initiating efficiencies at complete monomer conversion in all cases. However, as shown in Table 1, the molecular weights and polydispersities ( M / M ) of PIBs did not change in the presence of D M A , Py and DMPy after complete monomer conversion was reached. Negligible changes in molecular weight distribution (MWD) were obtained in the absence of additives and in the presence of D M A , and no changes occured in the M W D in the presence of Py and DMPy for four hours. However, as indicated in Table 1 and Figure 1, MWs of PIB initiated by iBuDiCumCl, a bifunctional initiator, significantly increased in the presence of DtBP. As shown in Figure 1, the GPC traces of PIBs obtained in the presence of D/BP after quenching at 5, 60 and 240 mins clearly exhibit that PIB chains with higher M W are formed even after 100 % monomer conversion. This means that chain coupling of PIB molecules formed by LCCP of IB occurs in the presence of DtBP. The extent of coupling depends on the concentration of Dd3P. Higher D/BP concentration leads to higher rates and extents of PIB chain coupling as data indicate in Table I.


w

n

Table I. M and M / M as a function of time in isobutylene polymerization by the fBuDiCumCl/TiCl initiating system in the absence and presence of different additives (see Experimental Section for polymerization conditions) n

w

n

4

J

M -10 (M /M )

Time

n

(min) 5 10 30 60 120 180 240 250

-

DMA

b

0

n

DMPy

DtBP

DtBP*

7.04

7.49

8.96

6.96

9.76

7.01

(2.53)

(1.29)

(1.16)

(1.43)

(1.54)

(1.31)

7.55

7.44

9.08

6.86

10.4

7.12

(2.40)

(1.34)

(1.15)

(1.49)

(1.52)

(1.26)

7.84

7.43

8.71

6.88

10.5

7.43

(2.37)

(1.32)

(1.17)

(1.44)

(1.74)

(1.31)

7.17

7.48

8.67

7.09

11.0

7.90

(2.74)

(1.33)

(1.18)

(1.46)

(1.75)

(1.30)

8.08

7.48

8.53

7.09

12.0

8.12

(1.46)

(1.71)

(1.35)

(2.35)

(1.31)

(1.21)

8.04

7.39

8.40

6.97

12.6

7.93

(1.46)

(1.73)

(1.38)

(2.40)

(1.34)

(1.21)

8.55

7.37

8.44

7.14

12.3

7.86

(1.77)

(1.36)

(2.30)

(1.32)

(1.22)

(1.45)

-

-

11.4

10.8

21.3

(1.49)

(1.70)

r

(1.39) a

W

Py

c

[/BuDiCumCiyPiBP] = 1 IB added to the remaining solution after 240 min: Δ[ΙΒ] = 0.89 M (Py), 0.99 M (DMPy), and 0.89 M (DtBP) Bimodal


6. HELD ET AL.



quenching after 250 min

ν e

Figure 1. GPC traces of PIB obtained by /BuDiCumCl in the presence of D/BP at different times. The GPC trace of PIB obtained by adding IB to the polymerization system after four hours reaction time is also displayed in Figure 1. According to this Figure, higher M W polymer was formed, i. e. polymer chain ends were still active after four hours in this LCCP system of isobutylene. Similar observations were obtained in the presence of other additives as shown by data in the last row in Table I. It is noteworthy that the MWDs are different in the presence of the different additives under identical conditions as indicated by the M / M data in Table I. This reflects that there are differences in their mode of action which affect the kinetics of LCCP of isobutylene. In other words, the mode of action of nucleophilic additives in LCCP of olefins is most likely a complex process, and the differences indicate that a simple interpretation, such as exclusive proton scavenging (19) by all the known additives, is not sufficient as explanation for the occurrence of LCCP. w

n

Chain End Stability of Monofunctional PIBs under Monomer Starved Conditions. The surprising chain coupling which occurred in the presence of D/BP was further studied by using monofunctional TMPC1 as initiator for LCCP of IB. Theoretically, a bifunctional initiator can act not only as an initiating entity but also as chain coupling agent. This effect can be completely avoided by using monofunctional initiator, i. e. if chain coupling would also occur in this case it would exclusively involve reactive chain ends. Figure 2 shows the GPC traces of PIBs prepared by 2-chloro-2,4,4-trimethylpentane (TMPC1) as initiator for LCCP of IB in


67


68


the presence of OtB? at 5, 60 and 240 mins reaction times, i. e. after 100 % monomer conversion. As this Figure indicates, similar coupling phenomenon is observed as with the bifunctional initiator. These experiments prove that coupling under monomer starved conditions occurs by a reaction between two PIB chains. It is also exhibited in Figure 2 that increase of M W can be obtained when a second charge of monomer was added after four hours of reaction time.

ι

1 28

r=n—ι—ι 29

30

1 31

1

'

ν32/ m l e

1—' 33

1—> 34

1 35

·

1

ι— —ι 36

37

Figure 2. GPC traces of PIB obtained by TMPCl in the presence of OtB? at different times.

Under monomer starved conditions, molecular weight increase was not observed with PIBs prepared by TMPCl in the presence of D M A , Py and DMPy in accordance with experimental findings described in the previous section for polymers obtained by the difunctional initiator. These results also indicate that the equilibrating propagating species and terminated polymer chains as shown in equation 1 possess long stability in the presence of nucleophilic additives. The Mechanism of Chain Coupling in the Presence of OtB?. Experiments with TMPCl (see previous Section) proved that chain coupling occurs by a direct interaction between two PIB chains in the presence of OtB?. Regular proton expulsion by "classical" carbocations would theoretically lead to 25 % endo and 75 %



6. HELD ET AL.


exo olefinic chain ends. The exo olefins can react with polymeric carbocationic species leading to chain coupling. Due to steric hindrance the endo olefins would not take part in such reactions, and these double bonds could be detected in PIBs after quenching the polymerization systems. However, neither external nor internal olefins were detected by H N M R in PIBs obtained in the presence of OtB? after four hours of reaction time. This means that exo olefins leading to chain coupling were not formed by a classical chain transfer process. The concentration dependence of chain coupling in the presence of DtB? as shown by the experimental results in Table I indicates that OtB? is directly involved in the formation of external olefins. OtB? is a widely known proton trap, and it has been claimed that it interacts only with protons and does not react with other species, such as Lewis acids and carbocations (19,2123). This means that in contrast to other basic additives, which form strong complexes with Lewis acids, free DrBP exists in T i C l coinitiated LCCP of IB. It is postulated by us that this free OtB? abstracts a proton from a methyl group of the propagating chain ends leading exclusively to external double bonds as shown in equation 3. The resulting polymer is an isobutenyl ended PIB macromonomer which quickly reacts with the active cationic species yielding chain coupling. Due to steric hindrance proton abstraction cannot take place from the - C H - methylene group next to the carbocation by DtB?. Therefore endo olefins cannot be detected even after long reaction times. 4

2

CH.3 L

+

(3)

L

3

The coupling process and the structure of the resulting polymer were verified by dehydrochlorination experiments. As shown in Scheme 1, when only low extent of coupling occurs, tert-chlorine chain ends are formed and dehydrochlorination by


69


70

iBuOK yields — CH -C(CH )=CH chain ends (24) together with low concentrations of ~-CH=C(CH ) endo olefins (-3-8 % of total double bonds) (25-27). Indeed, as shown in Figure 3, after dehydrochlorination, the characteristic signals at 4.58 and 4.78 ppm appear in the *H N M R spectrum of PIB that was produced with a 5 min reaction time. A small peak can be also observed at 4.64, 4.74 and 5.1 ppm. The latter is attributedto the endo olefinic proton (26,27) while the signals at 4.64 and 4.74 ppm indicates the presence of pendant ~-CH -C(=CH )-CH -~ and most likely internal ~-CH=C(-CH )-CH -~ olefinic groups, respectively. Figure 4 exhibits the H N M R spectrum, after dehydrochlorination with /BuOK, of PIB that was produced with a 240 min reaction time. Comparison of Figures 3 and 4 clearly indicates that the relative ratios between the N M R signals of exo and pendant olefin groups significantly decreased. The majority of double bonds in the dehydrochlorinated PIB with 240 mins reaction time is in the form of pendant olefinic groups. As shown in Scheme 1, proton abstraction by D/BP yields exo olefins which react with cationic chain ends leading to chain coupling. Termination of the coupled PIB gives a pendant teri-chlorine group the dehydrochlorination of which by /BuOK results in the pendant double bond. 2

3

3

2

2

2

3

2

2

2


!

CH»

CH,

I

_.DtBP __

\J

-CHj-C-CHj-C

. CH - C - CH - 0® Ti Cl !.. lu proton abstraction 2

2

CH

CH

3

e

2

3

AH,

.OTiCl

"

C H

>

CH

3

quenching

MeOH - 2 TiCL

CH — i ® Ti Clf

1.

chain coupling

2

2

έΗ

3

2.

quenching

CH

CH

3

CH

3

2

2

2

ίπ

CH

CH

3

CH

2

V

2

CH,

CH

2

in,

δ = 4.58 ppm δ = 4.78 ppm H

CH

3

CH

2

2

3

dehydrochlorination

2

iH

CH H 3

3

CH -i-CH -(i-CH -i-CH

2

iH

tBuOK/THF

tBuOK/THF

5

3

2

il

3

CH -i-CH -C^

CH

3

CH -i-CH -i-CH -i-CH

CH -t-CH -i-CI 2

CH

3

MeOH - 2 TiCI

58

ppm

x5

7.00

6.00

5.00

4.00

3.00

2.00

δ ppm Figure 4. H N M R spectrum of monofunctional PIB with 240 mins reaction time after dehydrochlorination with iBuOK.


71


72 Conclusions


In line with earlier findings (30-32), the plausible explanation of the results obtained in the presence of OtB? is abstraction of a proton by free DrBP from the methyl groups adjacent to the cationic center as shown in equation 3. Due to steric reasons this process leads exclusively to external double bonds and forms isobutenyl ended macromonomers which quickly react with the active cationic species. Although the other additives, such as Py and DMPy, are strong bases as well, free additives are most likely absent in the polymerization systems containing these compounds since they form strong complexes with Lewis acids. It could also be argued that proton expulsion might occur in all LCCP systems through the gegenion, and terminal double bonds and HC1 are formed. In the presence of additives forming complexes with the Lewis acids, complete hydrochlorination of double bonds takes place immediately, and olefinic chain ends cannot be detected. If spontaneous proton expulsion would occur, free OtB? as proton trap captures the protons, and the exo olefins from the ensuing double bonds react with the carbocationic species leading to chain coupling. One can argue that because sterically hindered non-reactive internal (endo) olefins were not detected, this explanation has no sufficient ground, and the existing data indicate direct proton abstraction by free OtB?. Therefore protonation of the double bond can be imagined only by assuming reversible proton trapping by OtB?, i . e. efficient protonationdeprotonation equilibrium for protonation of the internal double bonds by the proton released from the proton trap. It has also to be noted that in case of the existence of spontaneous proton expulsion followed by re-hydrochlorination, proton scavenging by the nucleophilic additives, or more precisely by the Lewis acid-Lewis base complexes, in LCCP (19) cannot be accepted as explanation for the effect of these compounds in such polymerization systems. However, these complexes prevent initiation by protic impurities, such as H 0 , indicating efficient proton scavenging. Therefore there is no reason to doubt that scavenging the much stronger protic acid, HC1, if it was formed, also occurs in the presence of all the nucleophilic additives, on the one hand. If proton trapping was rapidly reversible, it would also prevent coupling with the involvement of the exo olefins, on the other hand. Thus, on the basis of the currently available results one must conclude that spontaneous β-proton expulsion does not take place in the presence of additives mediating LCCP of IB, i. e. chain coupling is the result of direct, bimolecular abstraction of the β-protons from the carbocationic species in the investigated living polymerization systems. 2

In all the investigated polymerizations in the presence of additive, M W increased by adding a new charge of isobutylene after four hours of standing under monomer starved conditions indicating the living nature of these polymerizations even after such extremely long reaction times. These results indicate that the propagating species are unexpectedly stable in LCCP of isobutylene in the investigated polymerization systems. Comparison of our new experimental results also indicates that the different additives influence the polymerization process differently. This is clearly shown by M / M data in Table I. w

n


6. HELD ET AL.


73

Data obtained in this study also provide some basis for a critical evaluation of the different mechanistic schemes proposed for LCCP so far, i. e. (1) LCCP occurs by "classical" propagating species (free cations and contact ion pairs) (20), and (2) nondissociated species (stretched covalent bonds) are the active (propagating) species (I). One would expect detectable proton expulsion in the absence of monomer if the active chain ends were "classical" ion pairs and/or free cations. However, as results of our studies under monomer starved conditions indicate, the active sites show significant stability in LCCP of isobutylene. There are also different opinions on the mode of action of additives in LCCP of olefins: (1) the Lewis acid (MtX ) forms a strong complex with the nucleophilic additive and this complex with reduced nucleophilicity mediates LCCP (see Réf. 1 and references therein), (2) the only role of all the nucleophilic additives (DMA, Py, DMPy etc.) is proton scavenging similar to that of DtBP (19). If the sole role of nucleophilic additives was only proton scavenging, then exactly the same results should have been obtained under identical conditions with all the additives. D/BP suppresses chain transfer during polymerization (19), i . e. in the presence of monomer. However, it leads to M W increase in the absence of monomer due to proton abstraction from the chain ends.


n

Experimental Initiator (l,3-di(2-chloro-2-propyl)-5-^-butylbenzene, /BuDiCumCl) was synthesized as described (16). TiCl , Ν,Ν-dimethylacetamide (DMA), 2,6-di-tertbutylpyridine (D/BP), pyridine (Py) and 2,4-dimethylpyridine (DMPy) (all from Aldrich) were used as received. Solvents and IB were purified as described (28). LCCP was carried out by a simple conventional laboratory technique (28,29) in 40:60 v/v CH Cl -hexane mixture at -78 °C.; [iBuDiCumCl] = [additive]/2 = [TiCl ]/32 = 0.01 M , [IB] = 0.88 M . Samples were withdrawn at predetermined times and quenched with prechilled methanol. A second charge of IB was added to the polymerization mixture after 4 hours, and it was allowed to polymerize for additional 10 mins. After evaporation of the solvent mixture the polymer was dried in vacuo at room temperature. Molecular weight averages and M W D were determined with GPC calibrated with PIB standards. H N M R spectra were recorded in CDC1 on a Bruker AC-200 equipment. 4

2

2

0

4

0

l

3

Acknowledgments: This study was supported by Shell Research B V , Amsterdam. Literature Cited 1.

Kennedy,

J. P.;

Macromolecular

Iván,

B.

Designed

Polymers

by

Carbocationic

Engineering: Theory and Practice, Hanser Publishers:

Munich, New York, 1992 2.

Iván, B.; Kennedy, J. P.

In Macromolecular

Design of Polymeric

Materials; Hatada, K; Kitayama, T.; Vogl, O., Eds.; Marcel Dekker: New York, 1996; pp. 51-84


74


3. 4. 5.

Iván, Β; Kennedy, J. P. Ind. J. Technol. 1993, 31, 183 Iván,

B. Makromol. Chem., Macromol. Symp. 1993, 75, 181

Sawamoto, M. Prog. Polym. Sci. 1991, 16, 111

6.

Iván,

B; Kennedy, J. P. Macromolecules 1990, 23, 2880

7.

Iván,

B. Makromol. Chem., Macromol. Symp. 1993, 67, 311

8.

Iván,

B. Macromol. Symp. 1994, 88, 201

9.

Iván,

B.;Müller,Α . Η . E. In Preprints of 11th IUPAC Symp. on Cationic


Polymerization and Related Processes; Borovets, July 5-8, 1993; p. 44 10.

Ref. 1, pp. 37, 64

11.

Ishihama, Y.; Sawamoto, M.; Higashimura, T. Polym. Bull. 1990, 23, 361

12.

Ishihama, Y.; Sawamoto, M.; Higashimura, T. Polym. Bull. 1990, 24, 201

13.

Pernecker, T.; Kennedy, J. P. Polym. Bull. 1991, 26, 305

14.

Pernecker, T.; Kennedy, J. P.;Iván,B. Macromolecules 1992, 25, 1642

15.

Lin, C. H.; Xiang, J. S.; Matyjaszewski, K. Macromolecules

16.

Storey, R. F.; Lee, Y. J. Macromol. Sci.-Pure Appl. Chem. 1992, A29,

17.

Storey, R. F.; Choate, K. R., Jr. Macromol. Symp. 1995, 95, 71

18.

Faust, R.;Iván,B.; Kennedy, J. P. J. Macromol. Sci.-Chem. 1991, A28, 1

19.

Gyor, M.; Wang, H.-C.; Faust, R. J. Macromol. Sci.-Pure Appl. Chem.

20.

Matyjaszewski, K.; Sigwalt, P. Polym. Int. 1994, 35, 1

21.

Brown, H.C.;Kanner, Β. V.J.Am. Chem. Soc. 1953, 75, 3865

22.

Brown, H. C.J.Chem. Soc. 1956, 1248

23.

Brown, H.C.;Kanner, Β. V. J. Am. Chem. Soc. 1966, 88, 986

24.

Kennedy, J. P.; Chang, V. S. C.; Smith, R. Α.; Iván, B. Polym. Bull.

25.

Kitayama, T.; Nishiura, T.; Hatada, K. Polym. Bull. 1991, 26, 513

26.

Lubnin, Α. V.; Kennedy, J. P.; Goodall B. L. Polym. Bull. 1993, 30, 19

27.

Feldthusen, J.; Iván, B.;Müller,Α. H. E. unpublished results

28.

Everland, H.; Kops, J.; Nielsen, Α.;Iván,B. Polym. Bull., 1993, 31, 159

29.

Feldthusen, J.;Iván,B.;Müller,Α. Η . E.; Kops, J. to be published

30.

Guhaniyogi, S.C.; Kennedy, J. P.; Ferry, W. M.J.Macromol. Sci.-Chem.

1993, 26,

2785 1017

1992, A29, 639

1979, 1, 575

1982, A18, 25 31.

Nuyken, O.; Park, S. D.; Walter, M. Polym. Bull., 1982, 8, 451

32.

Ref. 1, pp. 128-136


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