Cationic Polymerization of Cyclic Amines

1 Cationic Polymerization of Cyclic Amines E. J. GOETHALS, E. H . SCHACHT, P. BRUGGEMAN, and P. BOSSAER

Downloaded by CORNELL UNIV on September 5, 2016 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0059.ch001

Institute of Organic Chemistry, Rijksuniversiteit Gent, B-9000 Ghent, Belgium

It is now generally accepted that the propagation reaction in the cationic ring-opening polymerization of cyclic amines is a nucleophilic attack of the monomer nitrogen on a strained cyclic ammonium salt which is the active species of the growing macromolecule. The driving force of the polymerization is the relief of strain associated with the ring-opening of the active chain end. The resultant polymer molecules, however, also contain nucleophilic amino functions and therefore the polymer competes with monomer to react with the active species. This results in the formation of a (linear or cyclic) non-strained and therefore non-reactive ammonium salt.

With secondary cyclic amines (R=H), the proton on the terminated ammonium salt as well as the proton on the active species can be transferred to other amino groups present in the mixture including monomer. Dimers and other low oligomers are therefore the initial products, and the final products are branched polymers containing a distribution of primary, secondary and tertiary amino functions (1, 2). With cyclic tertiary amines (R = alkyl), the formation of a non-strained ammonium salt is a real termination reaction. If the rate of this termination reaction is not negligably small compared with the rate of the propagation, in other words if the ratio kp/kt is not very high, the polymerization will stop before 1 Saegusa and Goethals; Ring-Opening Polymerization ACS Symposium Series; American Chemical Society: Washington, DC, 1977.

2

RING-OPENING

POLYMERIZATION

a l l monomer i s consumed and low molecular weight polymers w i l l be obtained. Therefore, i t i s i n t e r e s t i n g to know the f a c t o r s which i n f l u e n c e t h i s t e r m i n a t i o n . The main purpose of the i n v e s t i g a t i o n s presented i n t h i s paper i s to determine the r e l a t i o n between monomer s t r u c t u r e and the r a t i o k / k . The i n i t i a t o r f o r a l l p o l y m e r i z a t i o n s was t r i e t h y l o x o n i u m t e t r a f l u o r o b o r a t e . This substance r e a c t s very r a p i d l y w i t h amines so that i t may be assu med t h a t i n i t i a t i o n i s f a s t compared w i t h propagation. A l s o , the counter i o n B F ^ i s s t a b l e and has a low n u c l e o p h i l i c i t y so that t e r m i n a t i o n r e a c t i o n s w i t h counter i o n may be neglected. p


Methods f o r the Determination

t

of k / k f p

For f a s t p o l y m e r i z a t i o n s w i t h values of k / k up to 100, a method based on the maximal conversions obtained w i t h d i f f e r e n t i n i t i a t o r concentrations was used. The r a t e of p o l y m e r i z a t i o n R i s given by : p

Rp - -dm/dt = k

p

t

m[p£]

p

[l]

where m • monomer concn. and [pj] = concn. of growing c h a i n s . I f t e r m i n a t i o n i s a f i r s t order r e a c t i o n the r a t e R i s g i ven by : t

R

t - - Φ η Ί / d t - k [p+]

[2]

t

I f t e r m i n a t i o n occurs by r e a c t i o n of the growing chains w i t h any of the amino f u n c t i o n s of polymer the r a t e R i s given by the se cond order eqn. : t

R

t "

_ d

p

[ nl /

d t

k

P

( m

3

= t t nJ o

[]

where tOq = i n i t i a l monomer concn. D i v i d i n g eqn. [ l ] by eqn. [2] or [3] leads to : dm _ ^£ dbnJ t k

m

dm d[pj]

0 Γ

=

k

t

m (m -m) 0

I n t e g r a t i o n of these equations between the l i m i t s m = m , f c and m = mf, [p£] = 0 leads to : 0

Q

mo k l n - = J i f c m k f

0

[4]

t

f o r a f i r s t order t e r m i n a t i o n and to :

Saegusa and Goethals; Ring-Opening Polymerization ACS Symposium Series; American Chemical Society: Washington, DC, 1977.

[p+] =

1. GOETHALS ET AL.

Cationic

Polymerization

of Cyclic

3

Amines

f o r a second order t e r m i n a t i o n where f = e f f i c i e n c y f a c t o r of i n i t i a t i o n ; c = i n i t i a t o r c o n c e n t r a t i o n , and m£ = monomer con c e n t r a t i o n a t the end of the p o l y m e r i z a t i o n . With t r i e t h y l o x o n i u m t e t r a f l u o r o b o r a t e , the i n i t i a t i o n r e a c t i o n i s f a s t compared w i t h propagation, so that f ^ 1. The type of t e r m i n a t i o n r e a c t i o n d e f i n e s the f u n c t i o n a l form o f m , m^ and c , i . e . eqn. [4] or [ 5 ] . Consequently the ( g r a f i c a l ) s o l u t i o n of these equations permits to d i s t i n g u i s h between f i r s t order and second order t e r m i n a t i o n r e a c t i o n and t o determine the values of k /k . For slow p o l y m e r i z a t i o n s , separate values f o r k and k can be d e r i v e d from time-conversion curves by u s i n g eq. [β] or eq. [7] depending on whether t e r m i n a t i o n occurs by a f i r s t order o r a second order r e a c t i o n . 0

Q

0

p

t


p

m i -

In k

In

In k mc

r

p

- k

t

t

[6]

t

- k / (m -m)dt ο t

t

0

[7]

Rp can be measured from the tangent a t the time-conversion curves and ^ ( m - m ) d t i s the area under a time-conversion curve up to time t when (mQ-m) i s used as the o r d i n a t e ( 3 ) . I f termina t i o n i s slow compared w i t h propagation, i n other words i f the r a t i o k p / k has a high v a l u e , the kp can be d e r i v e d d i r e c t l y from f i r s t order p l o t s of the p o l y m e r i z a t i o n . I n that case the t e r m i n a t i o n becomes s i g n i f i c a n t only a t n e a r l y q u a n t i t a t i v e conversions and k can then be d e r i v e d from experiments i n which "dying" p o l y mer s o l u t i o n s are used t o i n i t i a t e new p o l y m e r i z a t i o n s a t r e g u l a r i n t e r v a l s . These second p o l y m e r i z a t i o n s become slower as the i n i t i a t i n g s o l u t i o n s become o l d e r . F i r s t order p l o t s o f these new p o l y m e r i z a t i o n s g i v e s t r a i g h t l i n e s the slopes o f which are equal to kp[Pn]« Since kp i s known, t h i s method allows to measure the decrease o f [F£] i n the i n i t i a t i n g polymer s o l u t i o n and hence t o calculate k . t

)

0

t

t

t

R e s u l t s and D i s c u s s i o n . 1) N-Substituted Ethylenimines. These monomers polymerize very r a p i d l y a t 0°C and i t was not p o s s i b l e t o evaluate separate values f o r kp and k a t t h i s temperature. G e n e r a l l y the polyme r i z a t i o n s stop a t l i m i t e d conversions. I t was found t h a t eqn. [4] (and not eqn. [ 5 ] ) leads t o s t r a i g h t l i n e s the slopes o f which are equal t o f . k / k f Some examples are shown i n F i g u r e 1. Conse quently the t e r m i n a t i o n r e a c t i o n s occur according t o f i r s t order k i n e t i c s . This can be explained by assuming that the t e r m i n a t i o n r e a c t i o n i s predominantly i n t r a m o l e c u l a r which means that the t e r minated ammonium s a l t s are c y c l i c . This i s i n agreement w i t h the o b s e r v a t i o n that a number of N - s u b s t i t u t e d a z i r i d i n e s form not t

p


4

RING-OPENING POLYMERIZATION


only polymer but a l s o the c y c l i c dimer ( d i s u b s t i t u t e d p i p e r a z i n e ) or c y c l i c tetramer (4>5). These c y c l i c oligomers are formed by degradation of the polymer. This i s supported by the f a c t that these oligomers are formed mainly a f t e r the p o l y m e r i z a t i o n . Gel permeation chromatography a n a l y s i s of the mixture shows t h a t du r i n g the time oligomer i s formed, polymer c o n c e n t r a t i o n decreases whereas r e s i d u a l monomer c o n c e n t r a t i o n remains unchanged. I t thus seems that a r e a c t i o n s i m i l a r to t e r m i n a t i o n continues to oc cur at the terminated c h a i n ends; f o r example :

Table 1 g i v e s a survey of values of f . k / k f o r d i f f e r e n t Ns u b s t i t u t e d ethylenimines together w i t h the pKfc values of the mo nomers. I t i s c l e a r t h a t there i s no simple r e l a t i o n s h i p between k p / k and the pR^ v a l u e s . On the other hand, the trend f o r e t h y l , i s o p r o p y l and t e r t . b u t y l a z i r i d i n e s t r o n g l y i n d i c a t e s that s t e r i c hindrance p l a y s an important r o l e . p

t

t

Table 1:

Values of f . k p / k f o r the p o l y m e r i z a t i o n of N - s u b s t i t u ted e t h y l e n i m i n e s , Ç 2 (a) I V R CH t

H

V

2

N-substituent

f.k /k p

b

t

1

(l.mol- )

(R)

max.yield f o r m = 1 and c • 0.01 m o l . l Q

of monomer

0

-C H -CH(CH ) -C(CH ) ~ 2 6 5 —CH2CH2C gHij -CH CH CN 2

6 21

5

3

3

C H

C

2

2

2 15 100 55 12 55

3

85 14 82

H

2

(a) In CH C1 a t 0°C w i t h E t 0 B F ^ as (b) E f f i c i e n c y f a c t o r f £ 1. 2

2

3

6.09 6.23 5.44 7.24 6.75 8.67 initiator.


1.

Cationic

GOETHALS ET AL.

Polymerization

of Cyclic

5

Amines

Of a l l monomers l i s t e d i n Table 1 only t e r t . b u t y l a z i r i d i n e showed no evidence of a t e r m i n a t i o n r e a c t i o n . Even w i t h small i n i t i a t o r concentrations (e.g. 10"" 3 mol.l"^-) d i d the polymeriza t i o n go t o completion. The absence o f t e r m i n a t i o n was f u r t h e r i n d i c a t e d by k i n e t i c s t u d i e s a t low temperatures (-40 t o -20°). As shown i n F i g u r e 2, the f i r s t order p l o t s of the r e a c t i o n gave per f e c t s t r a i g h t l i n e s up to h i g h conversion. The v i s c o s i t y data given i n Table 2, show that the molecular weight o f the polymer can be c o n t r o l l e d by the m / c r a t i o . This i n d i c a t e s that a l s o t r a n s f e r r e a c t i o n s a r e unimportant and i s i n agreement w i t h the high l i v i n g c h a r a c t e r f o r t h i s p o l y m e r i z a t i o n . Downloaded by CORNELL UNIV on September 5, 2016 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0059.ch001

0

0

Table 2: I n t r i n s i c v i s c o s i t i e s of poly(t.BA.HCl), as a f u n c t i o n of mo/c . 0

m

(mol.l" )

o (mol.l" )

1.0 1.0 1.0 2.0 1.0 1.0 1.0 1.0

0.05 0.03 0.02 0.04 0.01 0.006 0.002 0.00Γ

20 33 50 50 100 167 500 1000

c

1

1

W

ο Co"

( a

> 1

(dl.g" ) 0.035 0.066 0.078 0.078 0.12 0.18 0.43 0.84

(a) i n aqueous 0.4 N.KC1 a t 25°C. The absence of t r a n s f e r r e a c t i o n s i s confirmed by the p o s s i b i l i t y t o produce block-copolymers o f t e r t . b u t y l a z i r i d i n e by u s i n g " l i v i n g " c a t i o n i c polymers as i n i t i a t o r . This has been achieved w i t h l i v i n g p o l y ( t e t r a h y d r o f u r a n e ) a t 0°C (6) and w i t h l i v i n g po l y s t y r e n e p e r c h l o r a t e , a t -60°C 07). I n both cases the formation of block-copolymers was demonstrated by the s o l u b i l i t y p r o p e r t i e s of the obtained polymers which were d i f f e r e n t from those o f homo p o l y - ( t e r t . b u t y l a z i r i d i n e ) , and by g e l permeation chromatography.

V7

0°C

BF/,

θ^ι * /S^O^N/VN^J

Ν I R

poly(THF-baziridine)

R

9

* w C H -CH C10? 2 ι 4

ν I R

-60°C >

20°C β 4

>poly(styrene-b-aziridine)



6


Figure 1. Determination of f · k /k , according to for the polymerization of 1-benzyl aziridine (Φ), ethyl)aziridine (O), and l- 2-phenylethyl)aziridine Cl at 0°C with triethyloxonium tetrafluoroborate m = 1.0 mol · r p

t

{

2

Equation 4, l-(2-cyano(*) in CH as initiator, 2

1

0

Timelmin)

Figure 2. First-order plots of the polymerization of J-tertbutyhziridine with triethyloxonium tetrafluoroborate at differ ent temperatures. (1) -40°C; c = 0.020 mol · I' . (2) -30°C; Co = 0.025 mol · l . (3) -20°C; c = 0.018 mol · IK m = 1.0 mol · l . 1

0

1

0

0

1


1

GOETHALS ET AL.

Cationic

Polymerization

of Cyclic

Amines

7

When i n i t i a t i o n i s q u a n t i t a t i v e and i n the absence of t r a n s f e r and t e r m i n a t i o n , the molecular weight a t q u a n t i t a t i v e conver s i o n i s given by Mmo/c (M = molecular weight of monomer). Ac c o r d i n g l y , the l a s t polymer i n Table 2 (with [η] = 0.84) has a molecular weight of 100,000. The absence of a t e r m i n a t i o n r e a c t i o n i n the p o l y m e r i z a t i o n of 1 - t e r t . b u t y l a z i r i d i n e i s explained by the high s t e r i c hindrance caused by the t e r t . b u t y l groups around the n i t r o g e n atoms o f the polymer c h a i n . As a consequence a n u c l e o p h i l i c a t t a c k by these n i t r o g e n atoms on the a c t i v e species i s no more p o s s i b l e . P o l y ( t e r t . b u t y l a z i r i d i n e ) i s a h i g h l y c r y s t a l l i n e polymer w i t h a m.p. of 142°C. Downloaded by CORNELL UNIV on September 5, 2016 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0059.ch001

0

2) N - s u b s t i t u t e d Propylenimines. These monomers polymerize r a t h e r s l o w l y a t temperatures between 0 and 20°C and they lead t o high conversions. As an example F i g u r e 3 shows time-conversion curves f o r d i f f e r e n t monomers a t 10°C. F i r s t order p l o t s o f these p o l y m e r i z a t i o n s g e n e r a l l y give s t r a i g h t l i n e s up t o high conver sions which i n d i c a t e s that t e r m i n a t i o n r e a c t i o n s are not impor tant d u r i n g the course of p o l y m e r i z a t i o n . Values of kp ( d e r i v e d from the f i r s t order p l o t s ) and of k (derived from second mono mer a d d i t i o n s as d e s c r i b e d above) a r e l i s t e d i n Table 3. For these p o l y m e r i z a t i o n s propagation i s c l e a r l y much slower than i n i t i a t i o n and t h e r e f o r e the e f f i c i e n c y f a c t o r f o r i n i t i a t i o n f , may be assumed to be equal t o 1. t

Table 3; Values of k and k f o r the p o l y m e r i z a t i o n of N - s u b s t i tuted propylenimines, CI^-CH^ ( ) p

t

a

I »-* C&2

N-substituent

2

6

χ 10

2

k

(l.mol ^ s e c " ) 5

—CK^CI^C^Hcj -CH CH CN 2

p

1

(R) -CH C H

k

2

1.27 1.55 2.5-1.7

t

χ 10

k /k

6

p

pK

t

1

(sec *)

(l.mol" )

11.6(b) 1.6 4.5

1100 < > 10,000 5,500-3,800

b

of

monomer 7.00 5.93 8.00

b

(a) I n CH C1 a t 10°C w i t h Et 0BF4 as i n i t i a t o r . (b) Values obtained f o r p o l y m e r i z a t i o n s w i t h m / c up t o 40. For higher r a t i o s k seems to i n c r e a s e markedly. 2

2

3

0

0

t

Comparison of the k / k values l i s t e d i n Tables 1 and 3 c l e a r l y shows t h a t the i n t r o d u c t i o n o f a methyl group i n 2 - p o s i t i o n of the a z i r i d i n e r i n g r e s u l t s i n a dramatic increase o f the l i v i n g character o f the p o l y m e r i z a t i o n s . With these monomers i t i s p o s s i b l e t o prepare l i n e a r polyamines w i t h a d e s i r e d molecular weight by u s i n g the a p p r o p r i a t e m /c r a t i o . I n t h i s way polymers w i t h molecular weights up to 20.000 were obtained. I f c i s f u r p

t

0

0

Q




Figure 3. Time-conversion curves for the polymerization of l-(2-cyanoethyl)-2-methyl aziridine (CEMA), l-(2-phenylethyl)-2-methyl aziridine (ΡΕΜΑ), and l-benzyl-2-methyl aziri dine (BMA) inCH Cl at 10°C with triethyloxonium tetra fluoroborate. c = 0.015 and m = 0.80 mol · l . 2

2

0

0

1



1. GOETHALS ET AL.

Cationic

Polymerization

of Cyclic

Amines

9

ther decreased,the maximum y i e l d s become lower i n accordance w i t h eqn. [4] and consequently a l s o the molecular weights l e v e l o f f . The d i f f e r e n t behaviour as f a r as t e r m i n a t i o n i s concerned between the a z i r i d i n e monomers and t h e i r 2-methyl analogues i s a l s o w e l l i l l u s t r a t e d by the observation that a polymerized s o l u t i o n o f 1benzyl-2-methyl a z i r i d i n e (BMA) i s capable of i n i t i a t i n g the p o l y m e r i z a t i o n of 1-benzyl a z i r i d i n e (BA) but the reverse i s not pos s i b l e . This again proves that the former polymer ( i f not too o l d ) s t i l l contains a c t i v e chain-ends capable to i n i t i a t e the polymeri z a t i o n of BA, whereas poly-ΒΑ i s "dead" d i r e c t l y a f t e r the polyme rization. Even more s t r i k i n g i s the behaviour o f l-benzyl-2,2-dimethyl a z i r i d i n e (BDMA)

BA

BMA

BDMA

I t was not p o s s i b l e t o polymerise t h i s monomer a t temperatures be tween 0° and 120°C, i n bulk or i n s o l u t i o n . BDMA does r e a c t w i t h t r i e t h y l o x o n i u m t e t r a f l u o r o b o r a t e t o form the expected a z i r i d i n i u m s a l t b u t t h i s s a l t does not give a ring-opening r e a c t i o n w i t h BDMA monomer. I t i s however an e x c e l l e n t i n i t i a t o r f o r the polymeriza t i o n o f BA : + BDMA

A l s o , BDMA does copolymer!se w i t h BA although the amount o f BDMA incorporated i n the copolymer i s s m a l l . These observations lead to the c o n c l u s i o n that a propagation r e a c t i o n between a h i g h l y s u b s t i t u t e d a z i r i d i n i u m s a l t and a h i g h l y s u b s t i t u t e d a z i r i d i n e monomer i s not p o s s i b l e , b u t that r e a c t i o n between the h i g h l y s t e r i c a l l y s u b s t i t u t e d a z i r i d i n i u m w i t h a n o n - s t e r i c a l l y hindered mo nomer or v i c e v e r s a , i s p o s s i b l e . 3) A z e t i d i n e s . The p o l y m e r i z a t i o n s o f these monomers a r e c h a r a c t e r i z e d by low r a t e constants o f p o l y m e r i z a t i o n and by a h i g h l i v i n g c h a r a c t e r . Values o f kp and k f o r two a z e t i d i n e s a r e l i s t e d i n Table 4. 1,3,3-Trimethylazetidine i s a very s l u g g i s h t


10


monomer even a t 80°C. With t h i s monomer, the a c t i v e species f o r the p o l y m e r i z a t i o n can be seen i n the NMR spectrum ( 8 ) . The ob served s i g n a l s a r e i n complete agreement w i t h the a z e t i d i n i u m i o n s t r u c t u r e and they remain unchanged d u r i n g and a f t e r the polyme r i z a t i o n . A t 80°C, 90% conversion i s reached i n a few hours but no t e r m i n a t i o n could be observed a f t e r 10 days so that t h i s system may be considered as a l i v i n g p o l y m e r i z a t i o n ( 9 ) . 1-Methylazetidine i s more r e a c t i v e than the t r i m e t h y l d e r i v a t i v e but the is s t i l l s m a l l compared w i t h k and t h e r e f o r e , p o l y m e r i z a t i o n leads to h i g h conversions. p


Table 4:

Rate constants i n the p o l y m e r i z a t i o n o f a z e t i d i n e s . C H

Monomer

Cationic Polymerization of Cyclic Amines

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