New Block Copolymers IV. Interfacial Polycondensation as a Novel Method for ABA

Block Copolymers

SUBHAS C. SHIT and SUKUMAR MAITI,* Polymer Materials Diuision, Materials Science Center, Indian Institute of Technology, Kharagpur 721302, India

INTRODUCTION

Generally ABA-type block copolymers are synthesized by anionic,' - 4 free radical, 5.6 and step-growth polymerization7** methods either in bulk or in solution. The interfacial polycondensation method is used for the preparation of (AB),-type mul- tiblock copolymer^,^ but not for triblock ABA copolymers. In the polycondensation process two types of prepolymers, A and B, are linked up by condensation reaction to produce the desired ABA block copolymer.

Recently, interfacial copolymerization of bisphenol-A polycarbonate (PO, bis- phenol-A-terminated polymethyl methacrylate (BTPMMA), and bisphenol-A-bis- chloroformate (BBF) was reported in which a mixture of ABA and AB block co- polymers was obtained.'O The mixture of the block copolymers was thoroughly extraded with acetone for removal of unreacted reactants. This method involves the use of the difunctional BBF which provides the linkage between the middle block and the preformed end bIocks.

We report on a novel and rapid method of synthesis of ABA block copolymers by interfacial polycondensation of terephthaloyl chloride and p-aminophenol in the presence of acid chloride terminated butadiene-acrylonitrile rubber without the for- mation of any homopolymer (i.e., polyesteramide) or the AB diblock copolymer.

EXPERIMENTAL

Materials

p-Aminophenol was recrystallized from distilled water and finally from toluene, m.p. 186°C. Terephthaloyl chloride was recrystallized from n-hexane, m.p. 80°C. Freshly distilled thionyl chloride was used. Benzene and n-hexane were purified by the usual procedure. All other reagents and solvents used were of pure grade. Car- boxy terminated butadiene acrylonitrile liquid rubber (CTBN, Hycar) was obtained from the B.F. Goodrich Company (USA) and was used without further purification. It has the following specifications: molecular weight 3320, equivalent per hundred grams of rubber (EPHR) 0.057.

Preparation of Acid Chloride of CTBN

In a reaction flask an appropriate amount of CTBN was dissolved in benzene (1 g of CTBN rubber in 20 mL of benzene) and treated with excess thionyl chloride (1:15 mol/mol) at 90°C. Next, the excess benzene and thionyl chloride were removed

* To whom all correspondence may be addressed.

Journal of Polymer Science: Polymer Letters Edition, Vol. 24,383-387 (1986) 0 1986 John Wiley & Sons, Inc. CCC 1360-6384/86/080383-05$04.00

384 SHIT ANDMAITI

by flash evaporation and finally washed with dry n-hexane and dried under vacuum at room temperature.

Synthesis of Triblock Copolymer

A typical run for the synthesis of the block copolymer may be described as follows: In a reaction vessel having a high-speed agitator system, 0.0023-mol acid chloride of CTBN in 375 mL cyclohexanone was taken and 0.0076 mol paminophenol was added with high-speed stirring at 0°C. Next, an excess amount of triethylamine followed by 625 mL distilled water was added to the reaction vessel. Terephthaloyl chloride (0.0112 moll dissolved in 250 mL cyclohexanone was added gradually (for 15 min) to the highly agitated reaction mixture. The reaction mixture was poured in a large ex- of water and methanol mixture. The yellowish white precipitate formed was filtered, washed with n-hexane, and dried under vacuum at mom tem- perature.

Polymer Characterization The polymer was characterized by elemental (nitrogen) analysis. IR spectrum was

recorded by a Perkin-Elmer 237 spectrophotometer using chloroform and nujol mull. lH-NMR spectrum of the polymer was recorded by a Varian 390 model 90 MHz spectrometer using CDCl, as solvent and TMS as standard. The polymer was end- capped by using excess diethylamine in the reaction medium just before isolation of the polymer by precipitation. The composition of the block and the sequence length (degree ofpolymerization) were determined by NMR peak area analysis. Molecular weight (MA of the polymer was determined by a Knauer vapor pressure osmometer aRer calibration with benzil in 1,pdioxane at 60°C and by the end group titration with methanolic KOH using phenolphthalein as an indicator. The inherent viscosity measurement was performed in 0.5% (w/v) solution of the polymer in 1,4dioxane at 30°C with a Ubbelohde suspended level viscometer.

RESULTS AND DISCUSSION The block copolymer was synthesized by a two-step process. First, acid chloride

of CTBN reads with excess paminophenol in cyclohexanone to produce hydrosty- terminated CTBN segment (0. Second, the formation of the end blocks is made by alternating polycondensation of paminophenol and terephthaloyl chloride - ti- ethylamine complex at both ends of middle block (C) in the water-cyclohexanone interfacial system in the presence of excess tiethylamine. It may be noted that the length of the polyesteramide segment at both ends of CTBN is not statistically equal, i.e., rn # n (Fig. 1). However, (rn + n) depends on the ratio of the readants used, and consequently the ratio of the two types of block in the tiblock copolymer may be controlled conveniently by altering the reactant feed ratio."

The presence of characteristic absorption bands at 3300 cm-l (=NH), 1735 cm-l (ester =C=O), 1680 cm-' (amide =C=O), 2249 cm-' (-C= N), and 1620 cm-l (=C= C=) in the IR spectrum of the polymer confirms the structure of the block copolymer. It is also found that the IR absorption band for 4% N decreases with the increase of (rn+n), i.e., the total segmental length of A blocks at both sides of B.ll

The ABA structure of the copolymer was also confirmed by 'H-NMR spectrum (Fig. 1). The ABA structure was proved by determining y and (rn + n) of the assumed structure of the polymer:

(repeat unit of aromatic polyesteramide) - (CTBN), m+n

NEW BLOCK COPOLYMERS IV 385

TAB

LE 1

Sn

ythe

sis

and

Cha

ract

eriz

atio

n of

AB

A B

lock

Cop

olym

er

Feed

com

posi

tion,

mol

x l

o3

CTB

N

Nitr

ogen

%

Cop

olym

er

Inhe

rent

-

dens

ity

visc

osity

ac

id

p-A

min

o T

erep

htha

loyl

C

opol

ymer

C

TBN

" -

chlo

ride

ph

enol

ch

lori

de

yiel

d %

%

C

alc.

Fo

und

m +

nb

M,'

M

nd

g/cm

3 dl

/g

2.3

7.6

11.2

85

68

4.8

4.9

8 55

12

4965

1.20

0.

23

"Cal

cula

ted

from

'H-N

MR

ana

lysi

s.

bCal

cula

ted

from

pea

k in

tegr

rtio

n of

'H-N

MR

of d

ieth

ylam

ine

term

inat

ed b

lock

cop

olym

er.

Poly

mer

mol

ecul

hA

'Fro

m v

apor

pre

ssur

e os

mu.

. +

T.

dFro

m e

nd g

roup

ana

lysi

s.

-'+t

(M,)

is fo

und

to b

e 55

11.

13

NEW BLOCK COPOLYMERS IV 387

from 'H-NMR integration. The signal which is the result of four methyl protons at 1.2 ppm in the diethylamine terminated copolymer (Fig. 1) has been used for de- termination of (rn + n) and y of the copolymer because of a better signal-to-noise ratio for characterization of the desired peak, and the chemical shift of aromatic protons is sufficiently distinct.12 The two broad, unsplit doublets at 7.0-7.3 ppm and at 8.0-8.4 ppm are due to up' protons and b,b' protons, respectively, in the monomer unit of the aromatic polyester amide. This indicates blockiness and the symmetry of the doublet points to the fact that the polyesteramide chain is ordered, i.e., ter- ephthaloyl and p-aminophenol moieties are alternately arranged in the polymer backbone of A segment of the block copolymer.

The yield and physical properties of the block copolymer are listed in Table I. The molecular weight of the copolymer determined by three independent methods (viz. 'H-NMR analysis, vapor pressure osmometry, and chemical and group analysis) agrees very well.

The absence of free homopolymer (i.e., polyesteramide) in the reaction product was evident from the complete solubility of the latter in chloroform, chlorobenzene, 1,4-dioxane, and W, whereas the former (polyesteramide) is only soluble in HMPA. The absence of polyesteramide in the reaction product may be explained on the basis of relative reactivity and solubility of the participating groups in the reaction me- dium and other factor~.'~-'' Detailed discussion on the mechanism of the reaction and analysis of the copolymer structure will be reported elsewhere."

The authors thank the Department of Science and Technology, New Delhi for partial support of this work under Grant No. HCS/DST/814/80 and the B.F. Goodrich Company, USA for a girt of CTBN rubber (Hycar Reactive Liquid Rubber) sample.

References

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10. Z. G. Gardlund and M. A. K. Bator, J. Polym. Scie. (Polym. Chem. Ed.) 21,1251 (1983). 11. S. Maiti and S. C. Shit, to be published. 12. H. R. Kricheldorf, J. Jonte, and M. Berl., Makromol. Chem. Suppl. 12,25 (1985). 13. P. W. Morgan, SPE (Soc. Plast. Eng.) J., 15,485 (1959). 14. 0. V. Smirnova, E. Khorvat, and I. P. Losev, Vysokomol. Soedin. Ser. A. , 12,424 (1970). 15. S. G. Entelise, G. P. Kondrateva, and N. M. Chirkov, Vysokomol. Soedin., 3,1044 (1961). 16. J. H. Bradford, P. J. Crawford, and A. N. Humbly, Tmns. Famday Soc., 64,1337 (1968). 17. F. Millich and C. Carraher, Jr., ZnteTfacial Synthesis I , Dekker, New York, 1977, p. 111.

1174 (1969).

(1977).

Received September 26, 1985 Accepted February 19, 1986