Addition and Condensation Polymerization Processes

47 The Polymerization of ß-Carboxymethyl Caprolactam HERBERT K. REIMSCHUESSEL

Downloaded by TUFTS UNIV on December 2, 2014 | http://pubs.acs.org Publication Date: June 1, 1969 | doi: 10.1021/ba-1969-0091.ch047

Corporate Research Laboratory, Allied Chemical Corp., P. O. Box 309, Morristown, N. J. 07960

The thermal polymerization of β-carboxymethyl caprolac tam results in a novel polyimide which has been identified as a poly(2,6-dioxo-1,4-piperidinediyl)trimethylene. The for mation of this structure is explained by a mechanism that consists in an initial isomerization of the caprolactam deriva tive to 3-(3-aminopropyl)glutaranhydride or its linear dimer and subsequent polymerization by condensation involving the terminal amino group and the anhydride moiety. Sug gested reaction schemes and corresponding kinetic equations are based upon the premise that the extent of polymerization is represented by the concentration of imide linkages. Re sults of rate studies carried out at 210°-290°C. support the proposed mechanism.

'^phe polymerization of β-carboxymethyl caprolactam has been of interest not only because it resulted in the formation of a novel and in teresting polymer structure but it demonstrated a new concept for synthesizing condensation polymers. For comparison, recall that the polymerization of the unsubstituted c-caprolactam results in an equilib rium in which the main product is a linear polyamide known as nylon 6. It is well known that its formation involves an initial ring opening and both condensation and stepwise addition reactions (2, 3, 7). The same is true also for the polymerization of known simple derivatives of c-capro lactam that may be derived by substituting a hydrogen atom in any of the five methylene groups by, for instance, an alkyl group. Regardless of which position from a through e a substituent may occupy, the equilib rium between monomer and polymer shifts toward the monomer as the size of the substituent increases (I). In all these cases the polymer formed is a polyamide. On the other hand, the polymerization of the A

717 In Addition and Condensation Polymerization Processes; Platzer, N.; Advances in Chemistry; American Chemical Society: Washington, DC, 1969.

718

ADDITION A N D CONDENSATION POLYMERIZATION

Table I.

Structures of Monomers and the Repeat Units of the Respective Polymers Monomer

^ , C H CH

/

\

N H

\


(CH ) 2

N H

M

_ ( C H

^ C H

CH

5

2

)

M

- C - N H -

- C H - ( C H

C = 0

2

m

N /

CH—CH

I

H

C = 0

CH —C 2

/

2

2

2

\

\

N

-(CH ) -CH 3

\^

CH

H c

- C - N H -

Ο

"

2

)

Ο < m < 4

^

\

2

R n= 4 -

2

CHf

)

2

Ο

(CH )„

K

II _ ( C H

CH2

CH

III

Ο

/C = 0

2

CH2

II

Polymer Repeat Unit

2

2

CH

PROCESSES

-

/ 2

C

II

COOH

β-carboxymethyl caprolactam results in a polyimide which we identi fied recently as a poly(2,6-dioxo-l,4-piperidinediyl)trimethylene [4, 5J. Structures of both the monomers and the repeat units of the respective polymers are shown in Table I for c-caprolactam (I), a polymerizable alkyl caprolactam (II), and β-carboxymethyl caprolactam (III). Comparing the structure of the monomer with that of the polymer as shown in Table I, we see that the polymerization of the β-carboxymethyl caprolactam must involve isomerization of the monomer ring system. This isomerization may be described by several possible proc esses, all of which are characterized by reaction between the amide and acid group of the β-carboxymethyl caprolactam. Based upon the results of our studies on the structure of this polymer (5) we may eliminate confidently those processes according to which the formation of the glutarimide moiety results either by intrachain cyclization or by transcyclization of certain intermediate polymer structures. The former would involve a polymer formed by a conventional ring opening polymerization:

In Addition and Condensation Polymerization Processes; Platzer, N.; Advances in Chemistry; American Chemical Society: Washington, DC, 1969.

47.

Ο

ο

—(CH ) —CH-CH -C-N-(CH ) 2

3

2

2

X

I

ΟΗ -ΟΟΓΟΗ" : 2

Δ

719

β-Carboxymethyl Caprolactam

REiMSCHUESSEL

Η

I . - · · - ml

H

Ο

-CH-^

X

I

CH -COrÔH ! 2

'

I

si*

I

CH-CO;OH

H

»

j

1


whereas the latter would require a polymer formed by a simple poly condensation:

To explain the formation of the isomerized structure, we may consider a process that is initiated by an intramolecular protonation of the amide group, proceeds with opening of the lactam ring, and results in the formation of 3-(3-aminopropyl)glutaranhydride according to:

Another possible mechanism may involve intermolecular proton transfer and reactions as depicted on p. 720. Polymerization could be envisaged by reaction of the anhydride group with the lactam moiety of the primary reaction product; since, however, amino end groups have been found in the polymer, transamidation as indicated in the above scheme must also occur. Although this mechanism cannot be eliminated completely from consideration, we do not feel that it plays a significant part in the conversion of the monomer.


ADDITION A N D CONDENSATION POLYMERIZATION PROCESSES

720

c

I

S

Ο


H

Ο II c — c

Ο

/

\

H N—(C) —C> 2

Ν—(C) —C

3

/

\

\

/

C

3

\

/ c — c

c — c

ο

ο

Finally, interaction between two intramolecular protonated monomer molecules may result in either or both the 3-(3-aminopropyl) glutaranhydride or its linear dimer.


47.

HO

ο

/

\Ν

It

C

\F

Ο

-c I \

II

? \

c — c

c y H ^ y o - cil /c Ο

c -oApH /

2 H.)N—(C) —C \

Ο /

s

OH

c

721


REiMSCHUESSEL

c—c

II


ο

o c-

/

H N-(C) -C \ 2

3

ο

M M

—C

\

/

/ II

\

N-- ( C ) , — c

c-— C

11 11 c— c

\

c— c

/

II ο

ο

To derive a kinetic expression the monomer conversion may in a first approximation for any of the considered mechanisms be represented by the simple chemical reaction, Reaction 1: — N H C O — + —COOH -> H N — + >(CO), 2


(1)


722

The concentration of the monomer (M) is given by the relationship: M = [—NHCO—] = [—COOH] The growth of the polymer chains (polymerization) may be described by a condensation reaction between the anhydride and amine functions as depicted here for the formation of a linear dimer: Ο

II ^CH —C Downloaded by TUFTS UNIV on December 2, 2014 | http://pubs.acs.org Publication Date: June 1, 1969 | doi: 10.1021/ba-1969-0091.ch047

2

^

v

2 HUN— (CHo) —CH

Ο

3

*

CH.,—

II ο Il

ο

ο

II

Η.,Ν—(CH ) —CH 2

N—(CH ) —CH

S

2

C

H

, 8

—

^

C

+ H 0

3

H

.

2

,

-

~ " i l

More generally the reactions for the chain growth may be represented by the following scheme: 2S -> S + HoO 2

1

S + S -> S + H 0 2

3

1

2

S ^ S ^ S ^ + H.O S + S ~~* S n

m

n + m

+ H 0 2

Where S denotes a linear chain, and the subscript identifies the number of repeat units in this chain. If we make the usual assumption that the reactivity of the functional groups is independent of the size of the respective molecules, chain growth may be represented by Reaction 2. —NH

2

+

^(CO) 0

lH(CO) N— + H 0

2

2

2

(2)

The concentrations of polymer molecules, c, (chains) and imide linkages, I, are given by the relationships C = [—NH ] = 2

/ =

[rr(CO) Q]

[ —(CO) N—] = 2

2

[H 0] 2


47.

REiMSCHUESSEL


723

I is a measure of the extent of reaction and depends upon both the concentrations of the monomer (M) and the polymer molecules (c). Mathematically this may be expressed by (3)

I = I(M,c)

The derivative of this function with respect to time is then repre sented by Equation 4 di__d]_dM dl dc_ ~dt~dM ~W dc dt Downloaded by TUFTS UNIV on December 2, 2014 | http://pubs.acs.org Publication Date: June 1, 1969 | doi: 10.1021/ba-1969-0091.ch047

+

(

4

)

Independent of any mechanism that may be considered, the con centration of imide linkages is given by the relationship J= l - M - c= Ό- c provided the concentrations are expressed in moles per mole of monomer as we have chosen to do in this work, and where U = 1 — M is the mole fraction of monomer converted to polymer. Considering this relation ship, the rate of formation of imide linkages is then according to Equa tion 4 dl _ _ dM dc _ dU dc ~dt ~~ ~df ~ ~dt ~~ ~dx ~~ dt According to Reaction 1 the rate of monomer conversion is given as - ^

=

^

= ^(1-17)2

while according to the Reaction 2 the rate at which the concentration of the linear species changes is as follows -

d c

at

-kc*

Substitution into Equation 5 gives, for the rate of polymerization, •^• = * ( l - C / ) 1

2

+ *c 2

2

(6)

This equation is well suited to evaluate easily obtainable experimental data and will permit us to ascertain whether the formation of our polyimide can be explained by the proposed mechanism. Experimental Polymerizations were carried out at temperatures of 210°, 220°, 230°, 240°, 250°, 270°, and 290°C. For each series about 10 small (8-10 mm. i.d.) polymerization tubes were used. Pure β-carboxymethyl caprolactam was placed into these tubes, and air was removed by alternate application


724



of vacuum and nitrogen. The tubes were connected to a manifold and immersed in a constant temperature oil bath. A nitrogen atmosphere was maintained throughout the polymerization. At various time intervals tubes were removed, quenched, and the reaction products were analyzed with respect to conversion and solution viscosities. The experimental data are presented in Figures 1 and 2. Figure 1 shows the conversion U as a function of time for the various temperatures of polymerization. The conversion was determined by extracting the polymer sample with water. Since it was found that the water-soluble fraction was essentially pure β-carboxymethyl caprolactam, U was cal culated from the value of the water insoluble weight fraction.

240 °C

2I0°C

U 0-5h

100

200

300

400

MINUTES

Figure 1.

Experimental conversion-time curves

Figure 2 shows the viscosities of the polymer solutions as measured at 25° db 0.05 °C. in an Ubbelohde viscosimeter employing a concentration of 0.52 gram of polymer per 100 ml. of m-cresol. The viscosimeter had a flow time of more than 100 sec. for the pure solvent. From the solution viscosities number average degrees of polymeriza tion were calculated according to (6) : log F = 2.008 + 1.241 log η n


(6)

REIMSCHUESSEL

47.

β-Carhoxy methyl Caprolactam y

2.0

725

290'C

..5


/

•O—2I0*C

200

250

300

350

MINUTES

Figure 2.

Viscosities of polymer solutions as a function of time

The concentration of polymer molecules was obtained according to U

Evaluation of Experimental Data Inspection of the experimental conversion data as shown in Figure 1 revealed the existence of a distinct temperature-dependent induction period. To evaluate the measured conversion data, the respective term in Equation 6 was integrated using the boundary conditions U = 0 at t = t where U is the induction period h

Figure 3 shows the plots of 1/(1 — U) vs. time. From the straight lines that start from the origin of the coordinate system used, values for ki were estimated. It can be seen easily that the values forfciare the reciprocal of the respective induction times U To evaluate data concerning the chain growth, the second term in Equation 6 was integrated, and

was plotted vs. time. Most of the plots

in this case were curves such as shown in Figure 4, in which the data for polymerizations carried out at 210° and 220°C. were used. The shape of these curves indicates that at short reaction times the con centration of polymer chains (c) increased as the time increased until


726


PROCESSES

a maximum of chains was obtained; from this point on the number of chains decreased owing to condensation between end groups as the time increased. It is obvious that to estimate values for k only the linear (second) portion of these curves can be used. For the higher reaction temperatures the shape of the first portion of the curve could hardly be determined. In all cases, however, where a determination was possible we found that c was at a maximum at a time that corresponded to about 2% In this evaluation we therefore designated the concentration of chains at the time 2U with c . 2


m

Figure 5 shows the linear portions of plots of — vs. time for all poly merization temperatures used. The straight lines in Figure 5 are represented adequately by: c

- _ c where At = t — 2U and c fined above.

m

— =k At c 2

is a temperature-dependent quantity as de

MINUTES Figure 3.

(8)

w

Second-order rate plot of conversion data


47.


REiMSCHUESSEL

160

140


120

100

80

60

40

20

200

100

1

300

400

MINUTES Figure 4.

Flot of 1 /c vs. time for polymerization temperatures of 210° and 220°C.

The rate constantsfciand k were estimated from the plots in Figures 3 and 5. The values obtained were used to construct the Arrhenius plots shown in Figures 6 and 7. The following relationships were derived from these plots: 2

Jkj = 74.9 X 10 exp (-23800/RT) 7

k = 237.22 X 10 exp (-23800/RT) 7

2

(9) (10)

from which follows k /k = 3.167 2

x


(11)

728

ADDITION A N D CONDENSATION POLYMERIZATION PROCESSES ι

1290 *C

270 C


e

240 *C

220 X 2I0*C

Figure 5. Equation 12 for c

Second-order rate plots for chain growth

m

could be derived from Figure 8 showing a

plot of — vs. the reciprocal of the absolute temperature Cm 49.528 Χ 10 - 73.96 3

(12)

Using Equations 7 through 12 rates of imide formation were calcu lated according to Equation 6 and plotted vs. time in comparison with experimental rates obtained by graphical differentiation of curves ob tained from experimental data. The result is shown in Figure 9.



47.

REiMSCHUESSEL


729

Solid lines represent the values calculated according to Equation 6 while the symbols are experimental values. This result shows that the polymerization of β-carboxymethyl caprolactam, involving both conver sion of monomer and chain growth, may be explained by the mechanisms considered here. Since the disappearance of monomer follows secondorder kinetics, the mechanism involving proton transfer between two intramolecular protonated monomer molecules seems to be the one that governs this reaction. We favor this mechanism over the others con sidered. Isomerization by an internal molecular rearrangement involving a single monomer molecule would also satisfy the experimental observa tions if one assumes that the reaction is acid catalyzed. In view of the complexity of the process which involves isomerization of one ring system into another, it is not surprising that application of our simplified reac tion schemes does not lead to a rigorous description of all phases of this 60r-

Figure 6.

Arrhenius plot for k^



730

PROCESSES

200

100 70 50


30

ιο κ 2

2

1.8

Figure 7.

1.9

2.0

2.1

Arrhenius plot for k

2

polymerization. Additional information is necessary to describe ade quately all phases of this process. The studies on this system, to date, have not indicated the existence of a temperature-dependent equilibrium similar to the one in which the polymerization of caprolactam results. Here essentially quantitative con version was obtained independent of the polymerization temperature used. The final degree of polymerization, however, depends strongly on the temperature of polymerization; it increased when the polymerization temperature was increased. This may be explained by the increased mo bility of the polymer molecules as a result of a temperature increase. The final degree of polymerization (P ) has been defined as the highest that could be obtained at a given temperature and remained con stant upon prolonged heating at this temperature. e

To derive a relationship according to which P may be calculated for a given reaction temperature, let us consider Equation 8 and apply it to e


47.

731


REiMSCHUESSEL

the situation characterized by the absence of further chain growth. For this case follows

Cr

(8a)

C

m

Since the final conversion U ^ 1, the term — in Equation 8a may be replaced by P and we may write e

C c

e

(13)

P - — = k,At c„, Downloaded by TUFTS UNIV on December 2, 2014 | http://pubs.acs.org Publication Date: June 1, 1969 | doi: 10.1021/ba-1969-0091.ch047

c

A plot of P

e

10.

c

1 — vs. k resulted in a straight line as shown in Figure 2

C m

The slope of this line is At from which values for t (the time at which further polymerization at a given temperature stops) may be computed easily. e

e

In combination with the expression found for c (Equation 12) Equa tion 14 has been derived from Figure 10. This equation represents the temperature dependence of P . m

e

P = 18.621 Χ 10Π e

e x p

(-

**»°) +

-

2 8

.

2 1

30k-

20

10

J

• •I

L

1.9

J

I

L _L_L J

2.0

I

I

I

I

L

2.1

f (°ΚΓ Figure 8.

Temperature dependence of c

n


(14)


732

PROCESSES

100


dt

, u

i.o

250 C e

50

Figure 9.

100

150

200

250

300

MINUTES Experimental and calculated rate data

Figure 10.

Temperature dependence of P

e


47.

REiMSCHUESSEL

733

β-Carhoxymethyl Caprolactam

A comparison of experimental data for P and calculated values ac cording to Equation 14 is given in Table II. e

Table II.

Experimental and Calculated Values of P


T, °C. 210 230 250 270 290

c

calc. 108 156 276 521 1139

exp. 90 156 282 492 1149

In conclusion, the results of our study indicate that the principal fea tures of the formation of poly(2,6-dioxo-l,4-piperidinediyl)trimethylene by thermal polymerization of β-carboxymethyl caprolactam consists in an initial isomerization of the caprolactam derivative to a reactive species and subsequent polymerization of the latter by condensation. The reactive intermediate is in all probability either or both the 3-(3-aminopropyl)glutaranhydride or its linear dimer. Both the conversion of the lactam by isomerization and the polycondensation follow second-order kinetics. The conversion of an asymmetrical seven-membered ring structure into a symmetrical six-membered one that is highly stabilized by having become part of a macromolecule is considered to be the driving force in this polymerization. Literature Cited (1) Cubbon, R. C. P., Makromol. Chem. 80, 44 (1964). (2) Hermans, P. H., Heikens, D., van Velden, P. F., J. Polymer Sci. 30, 81 (1958). (3) Kruissink, C. Α., van der Want, C. M., Stavermann, A. J., J. Polymer Sci. 30, 67 (1958). (4) Reimschuessel, Η. K., Polymer Letters 4, 953 (1966). (5) Reimschuessel, H. K., Roldan, L. G., Sibilia, J. P., J. Polymer Sci. Pt. A-2 6, 559-574 (1968). (6) Walsh, Ε. K., Reimschuessel, Η. K., unpublished data, 1967. (7) Wiloth, F., Z. physik. Chem. N.F. 11, 78 (1957). RECEIVED

April 1, 1968.


Addition and Condensation Polymerization Processes

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