Ring-Opening Polymerization - American Chemical Society

17 Polymerization and Copolymerization of N-Alkylaziridines

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on April 4, 2016 | http://pubs.acs.org Publication Date: August 16, 1985 | doi: 10.1021/bk-1985-0286.ch017

E. J. G O E T H A L S , M . VAN D E V E L D E , G. ECKHAUT, and G. BOUQUET Institute of Organic Chemistry, Rijksuniversiteit-Gent, Krijgslaan 281 (S-4bis), 9000 Gent, Belgium

A number of N-alkylaziridines have been found to give cationic ring -opening polymerizations with a high l i v i n g character, i . e . the ratios of the rate constants for propagation to those of termination ( k / k ) are high.(l) Provided a fast and quantitative i n i t i a t i o n , these polymerizations lead to the corresponding well defined polyamines with predictable molecular weight and low d i s p e r s i t y . This high l i v i n g character was observed with such monomers which contain a bulky N-substituent (such as tert.butyl) or which carry an additional methyl group on the carbon atoms of the a z i r i d i n e r i n g . The high l i v i n g character was ascribed to the " s t e r i c deactivation" (2) of the amino functions i n the polymer chain by the N- and C-substituents which markedly decrease the nucleophilic r e a c t i v i t y of these amino functions towards the e l e c t r o p h i l i c aziridinium ions which are the active species for the propagation. In the present communication some new results i n the study of these highly l i v i n g polymerizations are reported. In the f i r s t part, the influence of the presence of carbon-substituents on the " r e a c t i v i t y " of the a z i r i d i n e monomers w i l l be discussed. In the second part, the p o s s i b i l i t y of sequential polymerization to form block copolymers i s described. F i n a l l y some preliminary results on the copolymerization of these a z i r i d i n e s with other heterocyclic monomers w i l l be presented. p

t

Results and Discussion The influence of carbon substitution on the polymerization behavior of N-alkylaziridines. The polymerization of N-alkylaziridines i s generally characterized by a fast propagation and a fast termination reaction which consists of a nucleophilic attack of a polymer amino function on the active species, the aziridinium ion. Therefore these polymerizations generally stop at limited conversions producing low molecular weight polymers. N-tert. butyl a z i r i d i n e (TBA) i s an exception to the rule which i s ascribed to the " s t e r i c deactivation" of the polymer amino functions of the bulky tert.butyl groups. If the a z i r i d i n e ring carries an additional substituent on one 0097-6156/85/0286-0219$06.00/0 © 1985 American Chemical Society

McGrath; Ring-Opening Polymerization ACS Symposium Series; American Chemical Society: Washington, DC, 1985.


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RING-OPENING POLYMERIZATION

of the carbon atoms, the polymerization behavior changes dramatically. Under analogous reaction conditions the rate of polymerization decreases by several orders of magnitude but the rate of termination decreases even more markedly so that the polymerization has a higher l i v i n g character. In the case of Csubstituted N-tert.butyl a z i r i d i n e s , the rate of polymerization i s even reduced to zero, i n other words these monomers cannot be polymerized. When two (methyl) substituents are placed on one of the carbon atoms, the a z i r i d i n e does not polymerize either. This behavior i s surprising, taking into account the high r e a c t i v i t y of three membered rings. A c l a s s i f i c a t i o n of a z i r i d i n e monomers according to their polymerization behavior i s given in Table I .

Table

I . C l a s s i f i c a t i o n of A z i r i d i n e Monomers.

Fast Polymerization

P^N-
-\

No

Polymerization

(MBA)

(MTBA)

Ph.

f^N—|- (TBA) j^N-^

(BA)

Ph

(ΡΕΜΑ)

""Ph

(CEMA)

1>+ N-\^

(PTBA)

(DMBA)

Ph

I

N-x

(PEA) \-Ph

(CEA) \-CN

It thus appears that an a z i r i d i n e i s non-polymerizable i f i t contains a gem.disubstituted carbon atom or one substituted carbon atom and a bulky N-substituent ( t e r t . b u t y l ) . The reasons behind this behavior must be sought i n the s t e r i c structure of the two


17.

GOETHALS ET AL.

Polymerization of N-Alkylaziridines

221


partners i n the propagation reaction: the a z i r i d i n e and the aziridinium ion. The aziridinium ion i s always s t e r i c a l l y hindered at both sides of the 3-membered ring regardless the presence of additional carbonsubstituents, although i t may be assumed that these additional substituents w i l l render a nucleophilic attack on the ring more difficult.

Η

Η

Η

For the monomers the s i t u a t i o n d i f f e r s according to the Csubstitution. In the C-unsubstituted monomers the s i t e of the electron pair, which becomes the new C-N bond during the progatation step, i s unshielded and i t may therefore be expected that these monomers show a high nucleophilic r e a c t i v i t y . C,N-Disubstituted a z i r i d i n e s on the other hand, can adopt two conformations due to the easy inversion of the terminal amine function:

According to d i f f e r e n t conformational studies(4,5), these compounds are present "only" as the conformer in which the methyl group i s c i s to the electronpair and trans to the N-substituent. In such a conformer both sides of the ring are shielded by a substituent and i t may be assumed that this causes a lowering of the nucleophilic r e a c t i v i t y . Now the question arises whether the (observed) low r e a c t i v i t y i s due to the presence of only one (less reactive) transconformer or i s the consequence of an equilibrium between a predominant non-reactive trans-conformer and the reactive c i s conformer. Alkylation of ΡΕΜΑ with methyl t r i f l a t e leads to more than 95% of one of the t h e o r e t i c a l l y two possible diastereoisomers as shown by the NMR spectrum (Figure 1) of the aziridinium salts which shows



222


large and a very small doublet for the C-methyl substituent. Although we don't know yet which isomer i s the predominant one, the fact that almost only one i s formed, proves that a l k y l a t i o n of the a z i r i d i n e by methyl t r i f l a t e takes place p r e f e r e n t i a l l y with one conformational state. The r a t i o of the two reaction products may r e f l e c t the r a t i o of the conformers of the s t a r t i n g material as was described e a r l i e r f o r another a z i r i d i n e (6). The predominant s a l t can be r e c r y s t a l l i z e d and i s now being analyzed by X-ray d i f f r a c t i o n i n order to know i t s configuration. DMBA i s an example of a non-polymerizing a z i r i d i n e , although i t i s readily alkylated by methyl t r i f l a t e :

This proves that an a z i r i d i n e which i s shielded at both sides of the ring i s s t i l l reactive towards unhindered electrophiles such as methyl t r i f l a t e . However, reaction between DMBA and i t s highly hindered aziridinium salt ( i . e . propagation) i s not possible. MTBA i s another non-polymerizing a z i r i d i n e . A l k y l a t i o n of MTBA with methyl t r i f l a t e leads to only one aziridinium ion since i t s NMR spectrum shows only one doublet for the C-methyl group (Figure 2). This i s a strong indication that MTBA i s conformationally homogeneous, i . e . that only the trans-form e x i s t s , and i t may therefore be assumed that the aziridinium ion has the structure i n which the N- and C-methyl groups are on the same side of the r i n g .



17.

GOETHALS ET AL.

8


δ

7

§

I

3

223

5

ί~6

Figure 1 . 360 MHz Η-NMR spectrum o f 1,2-dimethyl-l-(2 phenylethyl)aziridinium t r i f l a t e .

CH — CH-CH 2

H 2

°

3

CF SO 3

(CH3) C 3

CH

3

0

3

X

Figure 2 . 360 MHz H-NMR spectrum o f 1 , 2 - d i m e t h y l - l - t e r t . butyl aziridinium

triflate.


224


Ή

3

3 +

CF S0 CH 3

3

3

Ν

C(CH )

CH C(CH )


3

3

3

θ

3

The exclusive trans configuration of the MTBA monomer makes this monomer s t i l l reactive towards s t e r i c a l l y unhindered e l e c t r o p h i l i c reagents such as methyl t r i f l a t e but prevents i t s reaction with s t e r i c a l l y hindered electrophiles such as a highly substituted aziridinium ion ( i . e . the propagation reaction). It thus can be concluded that a l l a z i r i d i n e s , regardless their degree of substitution, are able to react with a s t e r i c a l l y unhindered e l e c t r o p h i l e such as methyl t r i f l a t e . Their r e a c t i v i t y towards s t e r i c a l l y hindered e l e c t r o p h i l e s , however, i s dramatically influenced by the presence of substituents. It seems that an unshielded face of the a z i r i d i n e ring i s necessary for a reaction to occur. Consequently gem. disubstituted a z i r i d i n e s and those 1,2disubstituted a z i r i d i n e s which exist only i n the trans form, cannot homopolymerize. Block-Copolymers by Sequential Monomer Additions. The rate constant for propagation of TBA at 0°C i n THF i s 0.18 l.mol" s e c " . The polymer i s "temporarily l i v i n g " . The rate constant for propagation of ΡΕΜΑ at 25°C i n CH C1 i s 0.013 l.mol" sec" and this polymer i s also "temporarily l i v i n g " . If ΡΕΜΑ i s added to a solution of l i v i n g poly-TBA, a block copolymer poly-TBA-poly-PEMA with narrow d i s p e r s i t y i s formed i n quantitative y i e l d as i s shown by GPC analysis of the end product (Figure 3). This proves that the i n i t i a t i o n of the ΡΕΜΑ polymerization by the tert.butylaziridinium end group i s quantitative and not slow compared with the ΡΕΜΑ propagation, i n other words: k^ * ^22* 1

1

2

1

1

2

2

k,

ΡΕΜΑ

poly-TBA-poly-PEMA


17.

GOETHALS ET AL.


225

This i s not unexpected since we know that the TBA aziridinium ion has a high r e a c t i v i t y towards a l l kinds of n u c l e o p h i l i c s . Surprisingly, the addition of TBA to a solution of l i v i n g polyPEMA also leads to the quantitative formation of a block copolymer as i s shown by GPC analysis (Figure 4). This proves that the rate constant of the i n i t i a t i o n for the TBA polymerization by the ΡΕΜΑ aziridinium ion i s of the same order of magnitude as the homopropagation constant for TBA or k2i « ^11


:

CH, ^21 -> poly-PEMA^N'^^N^I

poly-ΡΕΜΑ'—N, I

Ph I Ph ^11

TBA

poly-PEMA-poly-TBA

Consequently, the ΡΕΜΑ aziridinium ion and the TBA aziridinium ion must have similar r e a c t i v i t i e s towards the TBA monomer. This i s a strong evidence that the main reason for the much slower polymerization of ΡΕΜΑ compared with TBA must be the lower r e a c t i v i t y of the ΡΕΜΑ monomer rather than a lower r e a c t i v i t y of the ΡΕΜΑ aziridinium ion. The v a l i d i t y of this statement i s now further investigated by copolymerization experiments. Copolymerization of N-alkylaziridines with β-propiolactone. 3 propiolactone (PL) reacts with t e r t i a r y amines to form the corresponding zwitterion (7^). I f an N-substituted a z i r i d i n e i s used, a zwitterion containing an aziridinium ion and a carboxylate ion i s formed. This zwitterion can i n i t i a t e a cationic polymerization of the a z i r i d i n e or an anionic polymerization of the lactone or undergo a coupling reaction with another (monomeric or polymeric) zwitterion.

T B A ^ / ^

^

\

\

N S

JPL

N^coo-\.coo

\ coupling s

e

X

-\-

X


226



υV

Elution volume

»-

Figure 3 . GPC a n a l y s i s o f poly-TBA-poly ΡΕΜΑ

block-

copolymer (A) and o f the poly-TBA used as the macromolecular

initiator(B).

E l u t i o n volume

-

F i g u r e 4 . GPC a n a l y s i s o f poly-PEMA-poly TBA blockcopolymer (A) and o f the poly-ΡΕΜΑ used as the macromolecular i n i t i a t o r (B).



17.

GOETHALS ET AL.

Polymerization of Ν-A Ikylaziridines

227

The last reaction i s another example of a spontaneous "alternating" copolymerization between an e l e c t r o p h i l i c and a nu c l e o p h i l i c monomer as described by Saegusa (8). The structure of the resulting polymer i s determined by the rate constants of the d i f f e r e n t reactions and by the concentration of the two monomers. This reaction has been investigated with the monomer pair PL-TBA, i n d i f f e r e n t solvents at 50°C. The polymers formed at the beginning of the reaction contain a considerable excess of amino units which proves that the cationic a z i r i d i n e propagation i s more important than the anionic lactone propagation under the reaction conditions used. A special feature of this polymerization, however, i s that large amounts (up to 50%) of c y c l i c oligomers are formed, which i s obviously due to an intramolecular coupling reaction between a carboxylate ion and an aziridinium ion of oligomeric products. This i s c l e a r l y demonstrated by the GPC analysis of the reaction mixture as shown i n Figure 5. The oligomers could also be analyzed by gas chromatography and coupling with the mass-spectrometer allows one to determine their structures (Figure 6). Table I I gives a survey of the structure and the r e l a t i v e abundance of c y c l i c oligomers formed i n the copolymerization of TBA and PL. The masses a l l correspond to the general formula ^ L ^ with η • 2, 3 or 4 and m - 1 or 2 (A = amine u n i t , L = lactone u n i t ) . Since i t i s unlikely that zwitterionic species would be v o l a t i l e , i t must be accepted that these oligomers are c y c l i c .

Table I I . Structure and r e l a t i v e abundance of c y c l i c oligomers formed i n the spontaneous copolymerization of equimolar amounts of TBA and PL.

M

1

Oligomer s t r u c t u r e

3

2

rel.

intensity

342

|-ALAL-|

74

342

r-AALIq

10

369

pAAAL-j

100

441

r-AAALL-|

468

pAAAAIq

98

441

|-AALAL-|

36

7

M = molar mass o f oligomer determined by CI-MS. A = amine u n i t ,

L = lactone u n i t .

from GC-MS using e l e c t r o n impact i o n i z a t i o n .



228

RING-OPENING

POLYMERIZATION

-Elution volume-

F i g u r e 5 . GPC a n a l y s i s of the r e a c t i o n products o f 1

equimolar amounts (0.1 m o l . l " ) of TBA and PL a f t e r 8 h r s a t 50° i n a c e t o n i t r i l e .

JVJ Retention time

F i g u r e 6 . Gas chromatographic formed from TBA and PL.

a n a l y s i s o f the oligomers



17.

GOETHALS ET AL.

Polymerization of Ν-A Ikylaziridines

229

In Figure 6, peaks nr. 1 and 2 correspond to oligomers containing two TBA and two PL units. Although the mass spectra could not be used to distinguish between the ALAL and AALL structural isomers, it is reasonable to assume that the much larger peak nr. 1 corresponds to the alternating oligomer since the occurrence of two adjacent PL units by homopolymerization is not very probable due to the low polymerization rate of PL at 25°C. The most abundant oligomers are the -AAAL- and -AAAAL- compounds (peaks nr. 3 and 5) which are 13- and 16-membered rings respectively. These are formed by a ring closure of the zwitterions AAALθ and AAAAL^. This confirms the assumption that in this copolymerization the TBA monomer is consumed in a considerable proportion by cationic homopropagation. This investigation is now continued with other N-alkylaziri dines and with pivalolactone. Literature Cited 1. 2. 3.

Goethals, E. J.; Munir, Α.; Bossaer, P. Pure & Appl. Chem. 53, 1753 (1981). Goethals, E. J. Proceedings of the 28 IUPAC Macromol. Symp., Amherst, 1982, p. 204. Le Moigne, F . ; Sanchez, J . Y.; Abadie, M. J. Preprints of the 6 Intern. Symp. on Cationic Polymerization, Ghent, 1983, p. 87. Maat, L . ; Wulkan, R. W. Rec. Trav. Chim. 100, 204 (1981). Razumova, E. R.; Kostyanovskii. Izv. Akad. Nauk SSSR, Ser. Khim. 9, 2003 (1974). Bottini, A. T.; Dowden, B. F . ; Van Etten, R. L. J . Am. Chem. Soc. 87, 3250 (1965). Jaacks, V.; Mathes, N. Makromol. Chem. 131, 295 (1970). Saegusa T.; Kobayashi, S.; Kimura, Y. Pure & Appl. Chem. 48, 307 (1976). th

th

4. 5. 6. 7. 8.

RECEIVED March 27, 1985


Ring-Opening Polymerization - American Chemical Society

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