Applied Polymer Science - American Chemical Society

8 Condensation Polymerization and Polymerization Mechanisms I. K. M I L L E R and J. Z I M M E R M A N

Downloaded by EAST CAROLINA UNIV on June 25, 2014 | http://pubs.acs.org Publication Date: September 25, 1985 | doi: 10.1021/bk-1985-0285.ch008

Experimental Station, E. I. du Pont de Nemours & Company, Inc., Wilmington, D E 19898

Preparation Methods for Condensation Polymers Polyamides Polyesters Polyimides Polyurethanes Polyureas Polycarbonates Polyanhydrides Molecular-Weight D i s t r i b u t i o n of Condensation Polymers K i n e t i c s and Mechanism of Nylon 66 Polyamidation Amidation Effect of End-Group Imbalance on Molecular Weight Effect of Monofunctional Monomers on Molecular Weight Kinetics Thermal Degradation Branching and C r o s s - l i n k i n g

The term "condensation polymers" was i n t r o d u c e d by W. H. C a r o t h e r s in h i s e a r l y work on the preparation of polyesters and polyamides to distinguish this class of polymers from vinyl polymers made by addition reactions. Condensation polymers were defined as polymeric molecules that may be converted by h y d r o l y s i s , or its equivalent, i n t o monomers that d i f f e r from the s t r u c t u r a l units by one molecule of H2O, HCl, NH3, etc. This broad d e f i n i t i o n includes many polymers made by ring-opening or addition reactions, for example, lactone and l a c t a m polymers, p o l y u r e t h a n e s , p o l y u r e a s , p o l y i m i d e s , and p o l y h y d r a z i d e s , as well as polymers made by t r u e condensation reactions. Although the formation of a low molecular weight polyester by condensation of ethylene g l y c o l and succinic acid was reported as e a r l y as 1863 (1) and low molecular weight ε-aminocaproic acid

0097 6156/ 85/0285-0159506.00/0 © 1985 American Chemical Society

In Applied Polymer Science; Tess, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.


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A P P L I E D POLYMER SCIENCE

polymers were prepared in 1899 (2), s y s t e m a t i c work on p o l y c o n densation reactions leading to l i n e a r high molecular weight polymers of practical utility d e r i v e s from the work begun i n 1929 by C a r o t h e r s and coworkers (3). In initial work, p o l y e s t e r s w i t h m o l e c u l a r weights ranging from 2500 to 5000 were prepared by the condensation of d i b a s i c a c i d s and glycols. As techniques were r e f i n e d , m o l e c u l a r weights i n excess of 10,000 were o b t a i n e d . Structure-property r e l a t i o n s h i p s developed i n e a r l y work soon showed that solubility c h a r a c t e r i s t i c s and low melting points made l i n e a r aliphatic polyesters u n s u i t a b l e for f i b e r applications. Similar studies of polyamidation showed that the desired properties could be obtained with polyamides. Many polyamides were prepared both from amino a c i d s and from diamines and d i a c i d s . The polyamide of hexamethylene diamine and adipic a c i d was s e l e c t e d f o r the first commercial development of a textile f i b e r because of an optimum balance of properties and the prospective manufacturing cost. The major research contributions leading to p r a c t i c a l technology for production of high molecular weight polymers by the r e l a t i v e l y s i m p l e condensation r e a c t i o n s were i n the areas of understanding both the mechanism of molecular weight increase and the requirements for attainment of high m o l e c u l a r weight. The dependence of m o l e c u l a r weight on the e x t e n t of the r e a c t i o n was shown by W. H. C a r o t h e r s , and the concept of nondependence of f u n c t i o n a l - g r o u p r e a c t i v i t y on molecular s i z e and d e r i v a t i o n of the molecular-weight d i s t r i b u t i o n f o r condensation polymers were shown by P. J . F l o r y . These c o n t r i b u t i o n s , as w e l l as the development of a n a l y t i c a l procedures for polymer end groups were e s s e n t i a l . In the f o l l o w i n g d i s c u s s i o n of the mechanism of condensation p o l y m e r i z a t i o n , the g e n e r a l requirements f o r p r o d u c t i o n of h i g h m o l e c u l a r weight polymers by condensation reactions rather than the d e t a i l s of the f u n c t i o n a l group r e a c t i o n s w i l l be emphasized. A d i s c u s s i o n of polyamidation w i l l be used to i l l u s t r a t e the p r i n c i p l e s which apply generally to polymers of t h i s c l a s s . Preparation Methods for Condensation Polymers Polyamides. Many methods have been r e p o r t e d f o r l a b o r a t o r y s y n t h e s i s of polyamides (4^. Commercial methods i n c l u d e m e l t p o l y c o n d e n s a t i o n , r i n g opening of l a c t a m s , and low-temperature s o l u t i o n polymerization. Direct Amidation. Formation of polyamides by condensation of amine and carboxyl groups i s i l l u s t r a t e d by (i) In t h i s reaction, an amino acid may be polymerized or a diamine may be polymerized with a d i a c i d . S i m i l a r condensation reactions occur w i t h d e r i v a t i v e s of the a c i d moiety. Examples i n c l u d e e s t e r aminolysis (Reaction 2) i n which phenyl or higher a l k y l esters are p r e f e r r e d , amide a m i n o l y s i s ( R e a c t i o n 3), and a c i d o l y s i s of a c y l d e r i v a t i v e s of diamines (Reaction 4).


Condensation Polymerization and Mechanisms 161

8. M I L L E R A N D Z I M M E R M A N

C R

H2NR2NH2+ R 2 ° 2

3

C 0

>H4HNRNHCORC09

R

2 2

1

2

2

1

2

2

1

2

+ NH3

(2)

(3)

3

1


2

2

2

R CONHR NHCOR + H0 CR COoH R CO^HNR NHCOR CO^ 1

>0R + R OH

> H4HNR NHCOR CO>^-—NH

H2NR2NH2 + H NOCR CONH 2

3

2

OH + I ^ O ^ H

3

(4)

R e a c t i o n of A c i d C h l o r i d e s . Low-temperature p o l y c o n d e n s a t i o n of diamines and d i a c i d c h l o r i d e s i s an important route f o r p r e p a r i n g high-melting polyamides such as aromatic polyamides, which decompose or c r o s s - l i n k i f prepared by high-temperature m e l t r o u t e s . The r e a c t i o n may i n v o l v e an i n t e r f a c i a l r e a c t i o n between the d i a c i d c h l o r i d e i n a w a t e r - i m m i s c i b l e s o l v e n t w i t h an aqueous diamine s o l u t i o n , or the r e a c t i o n may be c a r r i e d out i n a homogeneous s o l u t i o n . The presence of a base i s u s u a l l y needed to remove HCl so t h a t p o l y m e r i z a t i o n i s complete. With weakly b a s i c aromatic diamines, an acid acceptor i s not always needed because HCl can be evaporated from the reaction mixture. The general reaction i s given by HfHNR NHCOR CO^-

H2NR2NH2 + C1C0R C0C1

1

2

C l + HCl

2

(5)

Polysulfonamides (Reaction 6) and polyphosphoramides ( R e a c t i o n 7) can be made by s i m i l a r reactions. H NR NH + C1S0 R S0 C1 2

1

2

2

2

H NR|NH + R P0C1 2

2

2

> fHNR NHS0 R S0 > + HCl

2

2

1

2

2

(6)

n

> ^HNR NHR PO^+ HCl 1

2

(7)

2

f

Other low-temperature polyamide syntheses include reaction of N.»N.-" ( a l k a n e d i o y l ) lactams ( R e a c t i o n 8) or N[,N -(alkanedioyl) diimides (Reaction 9) with diamines (5). f

,—C=0 0=C-, R I I R I—N-CO-R'-CO-N-J

+

H N-R"NH 2

2

,

£(C0-R -C0NH-R"-NH)3L

^N-C-R-C-N^

R +

J

+ 2 £°

(8)

LNH

,

H N-R -NH 2

2

^(CO-R-CONH-R'-NH)^- + 2 N H ^


(9)

162

A P P L I E D POLYMER SCIENCE

Ring-Opening P o l y m e r i z a t i o n . Polyamides can be formed by r i n g opening r e a c t i o n s as i n the p o l y m e r i z a t i o n of c a p r o l a c t a m . The p o l y m e r i z a t i o n may be c a r r i e d out at h i g h temperature w i t h water, amino a c i d , or amine carboxylate s a l t as i n i t i a t o r . The predominate ring-opening mechanism with these i n i t i a t o r s i s carboxyl-catalyzed a m i n o l y s i s . In the case of water i n i t i a t i o n , h y d r o l y s i s occurs ( R e a c t i o n 10) to form s m a l l amounts of the amino a c i d . The amino acid then reacts with lactam amide groups (Reaction 11).


(10)

(11) n Low-temperature p o l y m e r i z a t i o n of c a p r o l a c t a m can be accomp l i s h e d by an anionic mechanism i n which r i n g opening i s effected by a strong base, u s u a l l y with the addition of an a c y l a t i n g c o c a t a l y s t (e.g., a c e t i c anhydride) as i l l u s t r a t e d by Reaction 12.

O

NaH —> Ac 0

X^C=0 L i N / N - (C0(CH ) NH) _

H (12) NC ft Whether lactams can be polymerized to form high molecular weight l i n e a r polymers depends on r i n g s i z e and other s t r u c t u r a l factors. Ring s u b s t i t u e n t s , e s p e c i a l l y on the n i t r o g e n atom, i n h i b i t r i n g opening. Linear polymers are the favored form at e q u i l i b r i u m only when the r i n g - o p e n i n g r a t e i s much g r e a t e r than the c y c l i z a t i o n rate. (This subject i s reviewed i n References 6 and 7.) 2

2

5

n

1

M i s c e l l a n e o u s Routes. Polyamides have been prepared by o t h e r reactions, i n c l u d i n g addition of amines to a c t i v a t e d double bonds, p o l y m e r i z a t i o n of i s o c y a n a t e s , r e a c t i o n of formaldehyde w i t h d i n i t r i l e s , r e a c t i o n of d i c a r b o x y l i c a c i d s w i t h d i i s o c y a n a t e s , r e a c t i o n of c a r b o n s u b o x i d e w i t h d i a m i n e s , and r e a c t i o n of d i a z l a c t o n e s w i t h diamines. These r e a c t i o n s are reviewed i n Reference 4. P o l y e s t e r s . D i r e c t E s t e r i f i c a t i o n . P o l y e s t e r s can be made from hydroxy acids and from d i a c i d s and d i o l s by d i r e c t condensation: H0RC00H = i H40RC09

-OH + H 0 (13) n T h i s r e a c t i o n i s best s u i t e d f o r the p r e p a r a t i o n of low m o l e c u l a r weight p o l y e s t e r s (degree of p o l y m e r i z a t i o n , 5-50) of a l i p h a t i c hydroxy acids or diacids reacted with g l y c o l s having either primary or secondary hydroxyl groups. 9

z


8.

MILLER AND Z I M M E R M A N

Condensation Polymerization and Mechanisms

High m o l e c u l a r weight p o l y e s t e r s can be o b t a i n e d o n l y w i t h s p e c i a l techniques because of the d i f f i c u l t y of obtaining complete water removal without l o s s of monomers. The r e a c t i o n i s s e l f c a t a l y z e d by c a r b o x y l groups and can be c a t a l y z e d by other a c i d s , for example, _p_-toluenesulfonic a c i d and by compounds such as titanium alkoxides, d i a l k y l t i n oxides, and antimony pentafluoride. R e a c t i o n of A c i d C h l o r i d e s . P o l y e s t e r s can be formed by reaction of d i a c i d c h l o r i d e s with d i o l s : HORjOH + C1C0R C0C1 Downloaded by EAST CAROLINA UNIV on June 25, 2014 | http://pubs.acs.org Publication Date: September 25, 1985 | doi: 10.1021/bk-1985-0285.ch008

2

> H^ORjOCOj^CO} C l + HCl

the (14)

Polyesters of a l i p h a t i c d i o l s can be made by heating the reactants to remove H C l or by r e f l u x i n g the r e a c t a n t s i n an i n e r t s o l v e n t . W i t h d i h y d r i c p h e n o l s , i n the p r e s e n c e o f a s t r o n g b a s e , polymerization can be c a r r i e d out at or near room temperature e i t h e r i n homogeneous s o l u t i o n or by i n t e r f a c i a l r e a c t i o n u s i n g aqueous NaOH as the base. Ester Exchange. The most p r a c t i c a l methods for the preparation of high molecular weight polyesters i n v o l v e ester exchange reactions. The s i m p l e s t exchange method i n v o l v e s a c a t a l y z e d a l c o h o l y s i s r e a c t i o n between a d i o l and a d i c a r b o x y l i c a c i d e s t e r w i t h e l i m i n a t i o n of the a l c o h o l : HORjOH + CH3OCOR2CO2CH3

> H40R OCOR COOR 0^1

2

H + CH3OH

1

(15)

An excess of g l y c o l i s used to cause complete a l c o h o l removal and form c h a i n s w i t h h y d r o x y l end groups. High m o l e c u l a r weights are accomplished by s e l f - a l c o h o l y s i s of these molecules to remove the excess g l y c o l and approach a g l y c o l / d i a c i d r a t i o of 1 i n the f i n a l polymer. P o l y e s t e r i f i c a t i o n by a c i d o l y s i s ester exchange i s a l s o a useful synthetic method, for example, reaction of the diacetate ester of a d i h y d r i c phenol w i t h a d i c a r b o x y l i c a c i d i n the presence of a c a t a l y s t to eliminate acetic a c i d . Ring-Opening P o l y m e r i z a t i o n . Many lactones can be converted i n t o l i n e a r polymers by ring-opening reactions as i l l u s t r a t e d by Reaction 16. The reaction u s u a l l y requires a c a t a l y s t and can be c a r r i e d out i n b u l k or i n an i n e r t s o l v e n t . As i n the case of lactam polymerization, ring s i z e and other s t r u c t u r a l factors are important i n determining whether high-molecular-weight polymers can be obtained. A review of these factors, as w e l l as a discussion of other reactions for polyester formation, i s given i n Reference 8.

->

40RC0}

n


163

164

A P P L I E D P O L Y M E R SCIENCE

P o l y i m i d e s (8). P o l y i m i d e s are prepared by r e a c t i o n of a diamine with a t e t r a c a r b o x y l i c acid or i t s d e r i v a t i v e . Polyimides that melt without decomposition can be synthesized by heating a mixture of the diamine and t e t r a c a r b o x y l i c acid to eliminate water (Reaction 17) or by a s i m i l a r reaction using a d i a c i d - d i e s t e r to eliminate water and an a l c o h o l (Reaction 18).


(17)

A more general method i n v o l v e s the reaction of a diamine with a d i a n h y d r i d e i n a s o l v e n t at or near room temperature to form a p o l y ( a m i d e - a c i d ) t h a t i s subsequently dehydrated to form the polyimide (Reaction 19).

Polyurethanes (8). Polyurethanes are prepared by the reaction of a d i o l with a diisocyanate:

HOITOH + OCNR„NCO —

1

Z

>

i

OCNR^NC-)

1

I

n

(20)

C a t a l y s t s , i n c l u d i n g bases, metal complexes, and o r g a n o m e t a l l i e compounds, are u s u a l l y used. Polyureas (8). Many synthetic methods for preparation of polyureas are known. Examples of r e a c t i o n s t h a t can be used f o r the p r e p a r a t i o n of a v a r i e t y of p o l y u r e a s t r u c t u r e s i n c l u d e r e a c t i o n of diamines w i t h phosgene ( R e a c t i o n 21), w i t h urea ( R e a c t i o n 22) or with diisocyanates (Reaction 23).


8.



NaOH

H NRNH + C1C0C1 2

> fNHRNHCOJ

2

H NRNH + H NCONH 2

2

2

> fNHRNHCO-J + NH

2

n

H NR NH + OCNR NCO


2

2

2

+ NaCl

(21) 3

> fNHR NHCONHR NHCO^

2

1

2

(22) (23)

Polycarbonates (4). Polycarbonates may be prepared by a variety of synthetic methods. Procedures most commonly used include reaction of d i o l s w i t h phosgene ( R e a c t i o n 24), t r a n s e s t e r i f i c a t i o n of d i e s t e r s of c a r b o n i c a c i d w i t h dihydroxy compounds ( R e a c t i o n 25), and polymerization of c y c l i c carbonates (Reaction 26). 0

HOROH+COC12

>H{ORO^

0

OROH+HC1

(24)

0

HORjOH + R OCOR 2

2

> H^ORjC-)—0R + R OH 2

2

(25)

(26)

Polyanhydrides (4). E l i m i n a t i o n of water between carboxyl groups of a d i c a r b o x y l i c a c i d can l e a d to the formation of p o l y a n h y d r i d e s . The polymerization i s best accomplished by reacting the d i c a r b o x y l i c a c i d w i t h a c e t i c anhydride to form a mixed anhydride t h a t i s then heated under vacuum to e l i m i n a t e a c e t i c anhydride. The o v e r a l l reaction i s represented by Reaction 27. I

||

(CH CO) O 2

2

HOCRCOH

jj

J

> ^RCOC^

(27)

Molecular-Weight D i s t r i b u t i o n of Condensation Polymers Polymers made by s t e p w i s e i n t e r m o l e c u l a r r e a c t i o n between b i f u n c t i o n a l monomers have a c h a r a c t e r i s t i c m o l e c u l a r - w e i g h t d i s t r i b u t i o n . A q u a n t i t a t i v e e x p r e s s i o n of t h i s m o l e c u l a r - w e i g h t d i s t r i b u t i o n , f i r s t d e r i v e d by P. J . F l o r y (9), i s g i v e n by Equations 28-30. The e x t e n t of the r e a c t i o n p i s e q u a l to the f r a c t i o n of the o r i g i n a l functional groups that have reacted at any point i n the polymerization and can be c a l c u l a t e d by: p = (T - T)/T Q

Q


(28)

165

166


In Equation 28, T i s the i n i t i a l c o n c e n t r a t i o n o f f u n c t i o n a l groups, and T i s the concentration of unreacted functional groups. The mole or number f r a c t i o n of chains containing x repeat units i s g i v e n by Equation 29; the weight f r a c t i o n i s g i v e n by Equation 30. The curves for these d i s t r i b u t i o n s are given i n Figure 1. Q

n

x

= p*"

1

(1-p) 1

w = xp*" ( 1 - p )

(29) 2

(30)


x

The number average degree of polymerization for a polymer with a most probable molecular-weight d i s t r i b u t i o n i s given by the value of x at the maximum i n the w e i g h t - f r a c t i o n c u r v e . For v a l u e s of p approaching unity, the number average degree of polymerization (DP) i s given by Equation 31. DP„ = l / ( l - p )

(31)

Although the mechanism of condensation r e a c t i o n s i s g e n e r a l l y s i m p l e , the p r e p a r a t i o n of condensation polymers w i t h u s e f u l m o l e c u l a r weight can be very d i f f i c u l t and can r e q u i r e very p r e c i s e l y c o n t r o l l e d reaction conditions because of the very high reaction y i e l d s needed, as shown by Equation 31. T a b l e I shows the e f f e c t of the e x t e n t of r e a c t i o n p on the molecular weight of nylon 66. A degree of conversion of nearly 99% i s needed to obtain nylon 66 w i t h a number-average m o l e c u l a r weight of 10,000, which i s the minimum molecular weight for l i n e a r polymers that i s the most useful for p l a s t i c s , f i l m , and f i b e r uses. To achieve a molecular weight in excess of 20,000 a degree of conversion of about 99.4% i s needed. Even at t h i s high reaction y i e l d the polymer molecular weight i s low compared to m o l e c u l a r weights a t t a i n a b l e , f o r example, by f r e e r a d i c a l a d d i t i o n r e a c t i o n s . The need f o r a high r e a c t i o n y i e l d introduces other requirements for monomer purity, monomer balance, r e a c t i o n r a t e , and c o n t r o l of s i d e r e a c t i o n s that must be met f o r p r e p a r a t i o n of condensation polymers of high m o l e c u l a r weight. These requirements are i l l u s t r a t e d by a discussion of the mechanism and k i n e t i c s of p o l y m e r i z a t i o n of the polyamide o f hexamethylene diamine and adipic acid (nylon 66). K i n e t i c s and Mechanism of Nylon 66 Polyamidation Amidation. S y n t h e s i s of n y l o n 66 i n v o l v e s condensation o f amine w i t h c a r b o x y l groups i n a m e l t p o l y m e r i z a t i o n . The r e a c t i o n i s represented by Reaction 32.

0 N

—NH + —COOH 2

Hf| -NC- + H 0 2

(32)

The e q u i l i b r i u m constant for t h i s reaction i s given by Equation 33.

fC0NH4 [H 0] . 2

= K

fCOOH] fNH ] 2


(33)


8.



Figure 1. Most probable molecular weight d i s t r i b u t i o n for condensation polymers (p * 0.99).

TABLE I . Extent of Reaction vs. the Number-Average Molecular Wt. of Nylon 66

0.995 0.993 0.990 0.980

22,600 16,142 11,300 5,650


167


168

I f nylon 66 i s e q u i l i b r a t e d at 280 °C under steam at atmospheric pressure, the water content of the polymer melt i s about (L16% or 89 mol per 10° g of polymer, and a v a l u e of 3000 ( e q / 1 0 ° g) i s found for the product of the c o n c e n t r a t i o n s of uncondensed amine and c a r b o x y l groups. These r e s u l t s y i e l d a v a l u e of 260 f o r the e q u i l i b r i u m c o n s t a n t ( E q u a t i o n 33) because the amide group concentration i s about 8800 eq/10° g.


z

E f f e c t of End-Group Imbalance on M o l e c u l a r Weight. I f T i s the concentration of end groups i n a l i n e a r polymer, expressed i n u n i t s of equivalents per 10 g of polymer, the number of chains per 10 g i s T/2 and the number of c h a i n s per gram i s T/ (2 x 10 ). The r e c i p r o c a l of t h i s i s the number-average molecular weight (Equation 34). 6

M = (2 x 10 )/T

(34)

n

I f nylon 66 i s e q u i l i b r a t e d at a known water vapor pressure to achieve a fixed end group product, the amine and carboxyl end-group c o n c e n t r a t i o n s are determined by the end-group i m b a l a n c e . T h i s imbalance can r e s u l t from a l a c k of i n i t i a l e q u i v a l e n c e of hexan e d i o i c a c i d ( a d i p i c a c i d ) and diamine, from diamine l o s s , from monomer i m p u r i t i e s , or from polymer d e g r a d a t i o n by adipamide c y c l i z a t i o n . I f P = fCOOH] . fNH ] and D = fCOOH] - fNH ], the endgroup concentrations i n e q u i l i b r a t e d polymer are given by Equations 35 and 36. 2

2

fNH ] 2

=

1

-D + ( D + A p ) / _ _ _ _ _

2

r-C00Hl=

1

2

D + (D + 4 P ) / V

2

(35)

2

;

(36)

2 For polymers containing no monofunctional monomers, these end-group c o n c e n t r a t i o n s determine the number-average m o l e c u l a r weight (Equation 34), as indicated i n Equation 37.

M=

2 x 10° 37

< >

n

2

(D + 4 P )

1 / 2

F i g u r e 2 shows the e f f e c t of end-group imbalance D on the molecular weight of nylon 66 e q u i l i b r a t e d with steam at atmospheric pressure at 280 °C to y i e l d a product of end-group concentrations P = 3000 ( e q / 1 0 ° g) . From F i g u r e 2, the needed degree of p r e c i s e c o n t r o l of monomer balance can be deduced f o r attainment of h i g h molecular weight polyamide and for precise molecular-weight c o n t r o l . Monomer balance i s important a l s o i n the preparation of condensation polymers by nonequilibrium reactions, for example, polyamides from diacid c h l o r i d e s and diamines, polyesters from d i a c i d c h l o r i d e s and


8.




d i o l s , or p o l y u r e . p a thanes from diamines and d i i s o c y a n a t e s . To obtain a conversion of monomer functional groups of 99.5%, monomer imbalance cannot exceed 0.5%. Precise c o n t r o l of i n i t i a l monomer balance with many polyamide systems can be accomplished by measuring the pH of an aqueous s o l u t i o n of the diamine and d i a c i d . With r e a c t i o n systems not amenable to t h i s type of measurement, very p r e c i s e metering techniques for monomer addition must be used. Effect of Monofunctional Monomers on M o l e c u l a r Weight. The m o l e c u l a r weight of a polyamide (Equation 34) i s determined by the t o t a l concentration of polymer end groups. These end groups, i n addition to unreacted c a r b o x y l groups, C, and amine groups, A, may i n c l u d e nonfunctional or " s t a b i l i z e d " end groups, Eg, due to the presence of monofunctional i m p u r i t i e s i n monomers or to the a d d i t i o n of monofunctional acids or amines for m o l e c u l a r - w e i g h t c o n t r o l . The molecular weight of a polyamide containing nonfunctional end groups i s given by Equation 38. End-group concentrations are expressed i n units of equivalents per 10 g of polymer.

\

2

•

,

' °

A + C+ E

6

(38)

s

Figure 3, which shows the effect of nonfunctional end groups on the molecular weight of nylon 66,with equal amine and carboxyl endgroup concentrations at 50 eq/10 g, i l l u s t r a t e s the necessity for the absence of monofunctional i m p u r i t i e s i n condensation polymer intermediates i f maximum molecular weight i s to be obtained. K i n e t i c s . In aqueous s o l u t i o n , second-order k i n e t i c s (Equation 39) are found f o r condensation of s i m p l e c a r b o x y l i c a c i d s and amines (10). S t u d i e s of p o l y a m i d a t i o n when a 90% c o n v e r s i o n l e v e l i s reached i n d i c a t e second-order k i n e t i c s for t h i s reaction (11). At conversions above 90%, evidence suggests that a carboxyl-catalyzed t h i r d - o r d e r r e a c t i o n assumes i n c r e a s i n g importance and becomes predominant. -d fCOOH]

=

-d fNH ] 2

k dt

2

fCOOH]

fNH ]

(39)

2

dt

With an anhydrous melt at c o n v e r s i o n s above 90%, Equation 40 applies. A l t h o u g h v a r i o u s v a l u e s have been r e p o r t e d f o r the a c t i v a t i o n energy of p o l y a m i d a t i o n , a v a l u e of about 20 k c a l / m o l appears to be j u s t i f i e d (12). Values of 2.6 x 10"v(eq/10 g) • min (280 °C) and 3.5 x 10" /(eq/10 g) • min (290 °C) have been reported for the second-order r a t e constant f o r m e l t a m i d a t i o n ( k i n Equation 38). V a l u e s of 5 x 1 0 ~ ° / ( e q / 1 0 g) * min (280 °C) and 9 x 10 ° / ( e q / 1 0 ° g ) » min (290 °C) have been obtained f o r the t h i r d order rate constant ( k i n Equation 40). From these rate constants, the time r e q u i r e d for the very h i g h c o n v e r s i o n s needed f o r polyamidation can be approximated. D

4

6

2

D

2

2

3


169



T

* -200

1

r

. -100 0 100 D (eq./l0 g)

S 200

6

Figure 2 . Effect of end group imbalance on number average molecular weight.

T

t 0

i

1

1

r

i

i

i

10 20 30 40 MONOFUNCTIONAL MONOMER (m/10 g)

J

50

6

Figure 3. Effect of monofunctional monomer concentration on number average molecular weight of condensation polymer.


8.


MILLER AND ZIMMERMAN d fCOOH]

(40) k

fCOOH]

3

2

fNH ] 2


dt For a s i m p l i f i e d case i n which the e n t i r e p o l y m e r i z a t i o n r e a c t i o n i s assumed to occur at a constant temperature under c o n d i t i o n s t h a t e l i m i n a t e the r e v e r s e r e a c t i o n (see R e a c t i o n 32), the time needed to polymerize nylon 66 to 99% conversion i s given by Equation 41. T h i s equation y i e l d s a r e a c t i o n time of 17 min f o r p o l y m e r i z a t i o n at 280 °C of n y l o n 66 to 99% c o n v e r s i o n (DP = 100). This c a l c u l a t i o n emphasizes the need for condensation reactions with a f a s t r e a c t i o n r a t e i n view of the high r e a c t i o n y i e l d needed. R e a c t i o n s w i t h r a t e c o n s t a n t s s i g n i f i c a n t l y s m a l l e r than the constants for a l i p h a t i c amidation w i l l require long reaction times and, t h e r e f o r e , w i l l be i m p r a c t i c a l f o r polymer f o r m a t i o n , p a r t i c u l a r l y i f monomer side reactions or polymer degradation can occur under conditions of the polymerization. t99 =

2 k (8,800)

99 2k (880)

+

2

(41) 2

3

Thermal Degradation. A l i p h a t i c polyamides undergo chain cleavage r e a c t i o n s at p o l y m e r i z a t i o n and p r o c e s s i n g temperatures. Degradation of many polyamides i n the absence of oxygen at temperatures above 300 °C can occur by cleavage of the C-N bond to form a double bond and primary amide group as shown by Reaction 42. OH 111 CNCH CH 2

0 (I CNH

>

2

2

+ H C=CH

(42)

2

Polyadipamides are l e s s thermally stable than other a l i p h a t i c polyamides because they are subject to a c h a i n - c l e a v a g e r e a c t i o n that occurs at temperatures below 300 °C. This reaction i n v o l v e s adipamide c y c l i z a t i o n (Reaction 43). CO

A R-NHC0(CH ) C0NHR

>

2 4

RNHC0-CHCH + RNH 2

CH CH 2

(43)

2

2

For an a l i p h a t i c polyamide that degrades by amide cleavage, the net rate of polymerization at high conversion under vacuum (that i s , where the water concentration approaches zero) i s given by Equation 44 where k^ i s the rate constant for chain cleavage, and [A ] i s the amide group concentration. m

J

ro

=

k

3

£NH ] 2

o fCOOH] 1

-

k [Am] D

(44)

dt A l i m i t i n g molecular weight i s obtained when the amidation and degradation terms of the equation are equal. To obtain the highest


172



p o s s i b l e m o l e c u l a r weight w i t h a condensation polymerization the rate constant for polymerization must be very large compared to the rate constant for any chain-cleavage reactions. Compounds that catalyze polyamidation but not degradation w i l l i n c r e a s e the a t t a i n a b l e m o l e c u l a r w e i g h t . C a t a l y s t s for p o l y a m i d a t i o n i n c l u d e b o r i c a c i d , hypophosphorous a c i d and i t s s a l t s , and phosphoric acid (13). Branching and C r o s s - l i n k i n g . Branching or c r o s s - l i n k i n g may occur i n condensation polymers e i t h e r from the presence of polyfunctional monomers or i n some cases from s i d e r e a c t i o n s t h a t occur d u r i n g polymer p r e p a r a t i o n or p r o c e s s i n g . The most important e f f e c t of p o l y f u n c t i o n a l i t y i s a b r o a d e n i n g o f the m o l e c u l a r - w e i g h t distribution. Equations 45 and 46 express the weight-average molecular weight for condensation polymers containing t r i f u n c t i o n a l monomers of concentration b or tetrafunctional monomers (equivalent to c r o s s - l i n k s ) of c o n c e n t r a t i o n i. The q u a n t i t y T i s the t o t a l polymer end-group concentration. Units for T, b, and I are eq/10°g. —

4 x 10

=

6

tJLZL

(45)

T - 3b \M

=

4

X

1

Q

6

/ / ^ (46)

T - 8£ At high molecular weight where T i s s m a l l , very s m a l l concent r a t i o n s of branches or c r o s s - l i n k s have a l a r g e e f f e c t on the weight-average m o l e c u l a r weight. A branch concentration equal to T/3 or a c r o s s - l i n k c o n c e n t r a t i o n e q u a l to T/8 w i l l produce an i n f i n i t e polymer network. Polymer melt v i s c o s i t y i s very s e n s i t i v e to branching or crossl i n k i n g because melt v i s c o s i t y i s an exponential function of weightaverage molecular weight. For t h i s reason, conditions that cause branching or c r o s s - l i n k i n g must be c a r e f u l l y avoided i n cases where high-molecular-weight polymers are to be melt polymerized or melt processed. L i t e r a t u r e Cited 1. 2. 3. 4. 5. 6. 7. 8. 9.

Lourenco, A. V. Ann. Chim. Phys. 1863, 3, 67, 293. G a b r i e l , S.; Maas, T. A. Ber. 1899, 32, 1266. C a r o t h e r s , W. H. in " H i g h P o l y m e r s Series"; Vol. 1; Interscience: New York, 1940. " E n c y c l o p e d i a of Polymer S c i e n c e and T e c h n o l o g y , " Vol. 10, Wiley: New York, 1969. Ogata, N . ; S a n u i , K.; Nohmi, T. J. Polym. Sci. 1974, 12, 1327. S m a l l , P. A. Trans. F a r . Soc. 1955, 51, 1717. D a i n t o n , F. S.; Ivin, R. J. Quart. Rev. 1958, 12, 82. Hall, H. K. J. Am. Chem. Soc. 1958, 80, 6404. I b i d . , 1960, 82, 1209. " E n c y c l o p e d i a of Polymer S c i e n c e and T e c h n o l o g y , " Vol. 11, Wiley: New York, 1969. F l o r y , P. J. J. Am. Chem. Soc. 1936, 58, 1877.


8. MILLER AND ZIMMERMAN 10. 11. 12.

Morawetz, H . ; O t a k i , P. S. J. Am. Chem. Soc. 1963, 85, 463. F l o r y , P. J. Chem. Rev. 1946, 39, 137. "Kinetics and Mechanisms of Polymerization," V o l . 3, Solomon, D. H., Ed.; Dekker: New York, 1972. C h a r l e s , J.; Colonge, J.; Descotes, G. Compt. Rend. 1963, 265, 3107. Wyness, K. G. J. Chem. Soc. 1958, 2934. F l o r y , P. J . (to Du Pont), U. S. Patent 2 244 192 (1941). Sum, W. M. (to Du Pont), U. S. Patent 3 173 898 (1965).


13.



Applied Polymer Science - American Chemical Society

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