POLYMER LETTERS EDITION VOL. 12, PP. 381-386 (1974)

SYNTHESIS AND CHARACTERIZATION OF ISOMERIC ALIPHATIC BLOCK COPOLYESTERS

Introduction

Recently (1-4) there has been much interest in the synthesis and properties of block copolymers, and among the most studied systems are those based on polystyrene-polybutadiene, organo-polydimethylsiloxane, and polystyrene-poly- ethylene oxide. A common feature of these and most other block copolymer systems is the gross incompatibility and chemical dissimilarity of the compo- nents in the copolymer. The phase separated morphologies which arise from this incompatibility are responsible for the unique solid-state properties (4,5) of block copolymers and for their unusually high melt viscosities (4,6,7).

In contrast, the block copolymers synthesized in the present study are com- posed of block segments which are very similar to one another, and phase sep- aration is significantly influenced by crystallization of one of the components. More specifically, these materials are multiblock copolymers based on segments of poly(hexamethy1ene sebacate) and poly(2-methyl-2-ethyl- 1,3-propylene seba- cate). Because these polyesters are isomeric, the blocks have identical chemi- cal compositions, but slightly different chemical structures. These structural differences make poly(hexamethy1ene sebacate) highly crystalline with a melt- ing point near 70°C and poly(2-methyl-2-ethyl- 1,3-propylene sebacate) com- pletely amorphous with a T, near -69°C (8). A previous paper discussed the homopolymerization and random copolymerization of these two isomers (8). In this letter we will discuss the synthesis and characterization of these block copolyesters, and subsequent reports will discuss many of their physical prop- erties.

Experimental

Homopolymer Synthesis The synthesis and characterization of the homopolymers used to prepare

the block copolymers have been described (8). Briefly, the polyesters were synthesized by condensing sebacic acid with a small excess of diol using p- toluenesulfonic acid as catalyst. Linear poly(hexamethy1ene sebacate) or poly (HMS)

381

0 1974 by John Wiley & Sons, Inc.

382 POLYMER LETTERS EDITION

used in this study has a M, equal 3 160, a MWD equal to 1.52, and is a crys- talline solid.

Poly(2-methyl-2-ethyl-l,3-propylene sebacate) or poly (MEPS)

C2 H5 0 0 I II It

I CH3

+O-CH* -C-CH2 -O-C-(CH2)8 -C+

Poly (MEPS)

is an amorphous polymer with a Block Copolymer Synthesis

(HMS) and poly (MEPS) with a difunctional coupling agent. Several different coupling agents including phosgene, dichlorodimethylsilane, toluene diisocyan- ate, and hexamethylene diisocyanate were found to be effective in producing block copolymers, but hexamethylene diisocyanate was used most frequently.

A typical procedure for preparing the isomeric block copolyesters is de- scribed below. A 100-ml resin kettle equipped with a stirrer, condenser, nitro- gen gas inlet, and thermometer with a temperature controller was used to make the copolyesters. Poly (HMS) (22.5 g, 7.1 X eq.) and poly (MEPS) (7.5 g, 3.0 X eq.) were charged into the kettle and heated to 135OC with stirring under a nitrogen blanket. Hexamethylene diisocyanate (1.68 g, 10.1 X

eq.) was then added to the fluid polymeric mixture, and the temperature was maintained at 135OC. Within 10 min the viscosity began to noticeably in- crease, and eventually the viscosity was such that the polymeric mass began to envelope and climb the stirrer. The coupling reaction was terminated after 1 hr, and the copolymer was dissolved in about 300 ml of benzene and precipi- tated with stirring into 3 liters of methanol. The precipitate was collected by filtration, washed thoroughly with methanol, and dried in vacuo. A yield of 26.5 g or 89% was realized from this coupling reaction. The block copolyester was found by means of NMR to contain 79% poly (HMS) and 21% poly (MEPS), and it had an intrinsic viscosity in chloroform of 0.90 dl/g at 25°C. Polymer Characterization

Copolymer compositions, i.e., the diol isomer ratios, were determined with an accuracy of *2% by a previously described procedure using proton magnetic resonance spectroscopy (8).

Intrinsic viscosities and gel permeation chromatograms were measured using chloroform as solvent at 25°C. Cannon-Ubbelohde viscometers were used for the former measurements and a Waters Model 200 instrument with four linear lo5 8, Styragel columns and a flow rate of 1 ml/min. was used for the chroma- tography work. Monodisperse polystyrenes from Pressure Chemical Company were used to calibrate the columns.

= 2550 and a MWD = 2.03.

The block copolymers were prepared by coupling hydroxyl terminated poly

POLYMER LETTERS EDITION

TABLE I

383

Synthesis Data for Isomeric Block Copolymers ~~~~

Samp I e Poly HHS Poly 8EPS Coup I i ng Y i e l d % HflS i n Copolymer (grams) (grams) Agent (%)

A 22.5 7.5 (CHI) 6 (NCO) z 88.4 79 B 15.0 15.0 ( c H ~ ) ~ ( N C O ) ~ 91.4 56 C 3-8 6.2 COCI, 70.0 57 D 3.8 6.2 (CH3)ZSi C & Z 61.0 47

F 6.33 10.3 CsHu(NCO)z 90.4 39 C 7.5 22.5 (CHz) 6 (NCO) z 84.4 28

E 6.33 10.3 (CHZ) 6 2 87.0 41

Results and Discussion

Synthesis data for some representative block copolymers prepared in this study are shown in Table I. The copolymers (A-G) range in composition from 28 to 79% HMS and were made with several different coupling agents. The re- actions involving the diisocyanates were done in the melt at 135OC without added solvent, and both hexamethylene and toluene diisocyanates produced high yields of nearly equivalent block copolymers (e.g., E and F). Copolymers C and D were prepared using phosgene and dimethyldichlorosilane, respectively, in refluxing chlorobenzene (132°C) containing a small amount of pyridine. Al- though an intensive comparative study was not made, the diisocyanates were found to be the preferred coupling agents because they gave higher yields of less colored higher viscosity block copolymers than the phosgene or silyl cou- pling agents. Also, published reports (9-14), indicating that hexamethylene di- isocyanate has little or no influence in x-ray and transition temperature studies of chain extended polyesters were instrumental in our preferring this coupling agent.

The coupling reactions discussed above result in the poly (HMS) and poly (MEPS) chains being connected end on end into a longer block copolymer chain. Schematically this is shown below for a hexamethylene diisocyanate coupled block copolymer

H O O H H O I II II I I II I. -N-C-O-Polyester-O-C-N+CH2 )6 -N-C-0-Polyester-0

t t Poly (HMS) Poly (MEPS)

The individual poly (HMS) and poly (MEPS) blocks do not necessarily alternate along the copolyester chain as depicted above but, in fact, are more likely to be randomly arranged. The ratio of crystalline poly (HMS) to amorphous poly

3 84

TABLE I1

POLYMER LETTERS EDITION

Characterization Data for Isomeric Block Copolymers , -

I n t r ins i c nwoa Mn Samp I e % HMS In Copolymer Viscosity ( W M n f (g/moief

(dl /g)

Poly HMS 100 0.21 I .52 3,165 79

56

57

0.90 2.32 25,700

0.81 2.26 21,700

0.40 I .80 8,200

D 47 0.33 1.67 6,100

E 41 0.75 2.87 20,000

F 39 0.81

C 28 0.70

Poly MEPS 0 0.18

2.48 22,000

2.28 18,000

2.03 2,550

aMolecular weight distribution measured by GPC in chloroform at 25°C.

(MEPS) in the copolymer is varied systematically by changing the relative amount of the components in the coupling reaction.

It is worth pointing out the small but consistent differences in the compo- sitions of the charges and the compositions of the resultant copolymers in Table I. The proportion of HMS to MEPS is always higher in the copolymers than in the reaction mixtures used to make the copolymers (e.g., copolymer B, 56 versus 50%). This may reflect a difference in reactivity of the poly (HMS) and poly (MEPS) endgroups or it may reflect a systematic error introduced in- to the system by the molecular weight, end group, or NMR analysis. Since the observed differences in composition are not much larger than the accuracy of some of these analytical techniques, it is difficult to pinpoint the exact cause(s). However, copolymers with predetermined compositions can be synthesized in high yields by taking this factor into account.

Table I1 gives the characterization data for the isomeric block copolymers. Evidence for copolymerization is given by the intrinsic viscosity and molecular weight data. In general, the intrinsic viscosities of the copolymers are consid- erably higher (0.33 - 0.90 dl/g) than the values of 0.21 and 0.18 dl/g for the individual segments, poly (HMS), and poly (MEPS). The diisocyanate coupling reactions produced the highest viscosity copolymers, whereas the phosgene and dichlorodimethylsilane coupling reactions resulted in appreciably lower molecu- lar weight copolymers. These latter two coupling agents also produced the lowest yields (Table I) of block copolymers in this series.

The final column in Table I1 shows the number average molecular weight (M,) data for the block copolymers. These data were calculated from intrinsic viscosity measurements by assuming that the relationship shown in equation (1) holds for the block copolymers

[ 9 ~ = 7.25 x 10-~

POLYMER 385

0.06 c 60 100 200 400 600 1000 -

An Fig. 1. Log-Log plot of intrinsic viscosity vs. A,,, the number average chain

length from GPC: (0) poly (HMS); (X) poly (MEPS); and (0) the block copoly- mers.

Batzer ( 15) originally published this relationship for fractionated poly (HMS) in chloroform, and we have found it to adequately correlate our intrinsic vis- cosity and M,, (end group) data for several low molecular weight, but broader distribution poly (HMS) samples. The basis for the assumption that equation (1) is also valid for the block copolymers is shown in Figure 1. In this plot of log [q] vs. log A,,, where A,, is the number average chain length calculated from the (GPC) chromatograms, all the data for the poly (HMS), poly (MEPS), and block copolymers samples are fitted with a correlation coefficient of 0.98 to a single straight line having a slope of 0.83. Since A, is directly proportion- al to M,, the data in Figure 1 indicate that the block copolymers follow an [q] -M,, relationship very similar to that shown in equation 1.

linked copolymers have M, values between 18 and 26,000 g/mol. This corre- sponds to a total of about 7 to 9 polyester blocks in each copolymer chain. In the case of the silyl coupled copolymer, the product looks very much like a diblock copolymer whereas a triblock copolymer containing two poly (MEPS) segments best describes the phosgene coupled copolymer.

The isomeric multiblock copolyesters prepared in this study readily crystal- lize over the entire composition range, and they are much more elastomeric than either of the homopolyesters employed in their preparation or the iso- meric random copolymers described previously (8). The physical properties studies are continuing and will be reported later.

An interesting feature of the & data in Table I1 is that all the diisocyanate

The author gratefully acknowledges the assistance of W. Stauffer in the syn- thesis of the copolymers and is indebted to A. Goedde and D. Williams for the NMR spectra and to J. Pacco for the viscosity and GPC data.

3 86 POLYMER LETTERS EDITION

References

(1) D. C. Allport and W. H. Janes, Eds., “Block Copolymers,” Wiley, New York, 1973.

(2) G. E. Molau, Ed., “Colloidal and Morphological Behavior of Block and Graft Copolymers,” Plenum Press, New York, 197 1.

(3) S. L. Aggarwal, Ed., “Block Copolymers,” Plenum Press, New York, 1970.

(4) G. Holden, E. T. Bishop, and N. R. Legge, J. Polym. Sci., C, 26, 37 (1969).

(5) G. Kraus, C. W. Childers, and J. T. Gruver, J. Appl. Polym. Sci., 11, 1581 (1967).

(6) G. Kraus and J. T. Gruver, J. Appl. Polym. Sci., ll, 2121 (1967). (7) P. F. Erhardt, J . J. O’Malley, and R. G. Crystal, “Block Copolymers,”

(8) J. J. O’Malley and W. J. Stauffer, J. Polym. Sci., l2, 865 (1974). (9) A. Turner-Jones and C. W. Bunn, Acta Crystallogr., E, 105 (1962).

(10) A. S. Ueda, Y. Chatani, and H. Tadokoro, Polym. J., 2, 387 (1971).

S. L. Aggarwal, Ed., Plenum Press, New York, p. 195.

(1 1) D. H. Coffey and T. J. Meyrick, Rubber Chem. Technol., 3 0 , 283 (1957).

(12) E. F. Gubanov, A. G. Sinaiskii, N. P. Apukhtina, and B. Ya. Teitel’baum,

(13) M. H. Thiel and L. Mandekern, J. Polym. Sci., A-2, t3, 957 (1970). (14) K. Onder, R. H. Peters, and L. C. Spark, Polymer, l3, 133 (1972). (15) H. Batzer, Makromol. Chem., 5, 5 (1950); and as cited in “Polymer

Handbook,” J. Brandrup and E. H. Immergut, Eds., Interscience, New York, 1966.

Dokl. Akad. Nauk SSSR, 163, 1151 (1965); Chem. Abstr., 63, 164858 (1965).

James J. O’Malley

Webster Research Center Xerox Corporation Webster, New York 14380

Received January 31, 1974 Revised March 27, 1974