1H-NMR of the esterification of syndiotactic poly(methacrylic acid) with carbodiimides—II. Esterification with benzylalcohol and trifluoroethanol

European Polymer Jmwnal. Vol. 15. pp. 593 to 601

0014-3057 790601-0593502.(100

© Pergamon Press Ltd 1979. Printed in Great Britain

1H-NMR OF THE ESTERIFICATION OF SYNDIOTACTIC POLY(METHACRYLIC ACID) WITH CARBODIIMIDES--II ESTERIFICATION WITH BENZYLALCOHOL TRIFLUOROETHANOL

AND

E. KLESPER, D. STRASILLA and MARIA CHRISTINA BERG lnstitut fiir Makromolekulare Chemie, Universit~it Freiburg, D-7800 Freiburg, Germany, BRD

(Received 25 October 1978) Abstract--The partial esterification of syndiotactic poly(methacrylic acid) with benzyl alcohol or trifluoroethanol and dicyclohexylcarbodiimide as a condensing agent has been studied, evaluating triad and pentad probabilities by 1H-NMR. The mechanism of this esterification leads to a tendency toward alternation for esterified and unesterified monomer units along the chain and to a limiting conversion. Only a moderate approximation of triad and pentad probabilities by conditional probabilities of first and second order was possible. The esterification of poly(methacrylic acid) with trifluoroethanol has been carried out also in conc H2SO4 and leads to a random distribution of monomer units.

babilities of low M a r k o v order may be evaluated more fully.

INTRODUCTION The well k n o w n esterification of carboxylic acids with alcohols and carbodiimides [-1, 2] is applied to a polymer acid, viz. syndiotactic poly(methacrylic acid) (PMAA). Previously, the esterification of syndiotactic P M A A with m e t h a n o l and dicyclohexylcarbodiimide (DCC) has been studied by I H - N M R I - 3 , 4 ] and a brief report has also been given of the similar esterification of P M A A with benzyl alcohol or trifluoroethanol [4]. A more detailed ~ H - N M R investigation of the esterification with benzyl alcohol or trifluoroethanol and DCC is presented here. The previous c o m m u n i c a t i o n on the esterification of P M A A with m e t h a n o l and D C C has demonstrated the presence of glutaric acid type anhydride, which arises by reaction of P M A A with DCC. This cyclic anhydride is likely to be the intermediate in the esterification. The anhydride reacts, concurrently to its formation, with m e t h a n o l to yield one ester and one acid m o n o m e r unit for each anhydride ring. Because the rate of formation of cyclic anhydride by the action of D C C on P M A A is expected not to be greatly dependent on the type of alcohol in the reaction mixture, one may assume that the esterification.of P M A A with benzyl alcohol and trifluoroethanol has the same anhydride as a n intermediate as the esterification with methanol. Therefore, a limiting conversion may be expected and also statistics with a tendency towards alternation for the resulting copolymers. In addition, the convenient esterification of syndiotactic P M A A with trifluoroethanol in conc H2SO4 as solvent and catalyst is described. The trifluoroethylester holds some interest as a relatively active ester which may be useful for further reactions. Both trifluoroethanol and benzyl alcohol offer the possibility for evaluating pentad probabilities, besides triad probabilities, in their corresponding methacrylic acid copolymers. Therefore, the statistics of the methacrylic acid copolymers and its approximation by conditional pro-

EXPERIMENTAL The preparation of syndiotactic PMAA has been described [3]. The esterification o1' the PMAA with benzyl alcohol and DCC to yield benzylmethacrylate-methacrylic acid (BMA-MAA) copolymers is carried out similarly as previously described for methanol [3]. PMAA (300 mg = 3.49 mmol) is dissolved in pyridine (6 ml) with heating, triethylamine (0.75 ml), and benzyl alcohol .(13 ml = 125 mmol) slowly added and heated to 50°. Then DCC (0.72g = 3.50retool)in pyridine (3ml) is added to start the reaction and the mixture kept at 50~ for the desired time. For a given P(A), the following reaction times were typical: P(A) = 0.13 in 2 h r and P(A) = 0.53 in 60hr where P(A) is the probability for an esterified monomer unit in the copolymer. The esterification is stopped by cooling to 0 ~', filtering off"the dicyclohexylurea and precipitating in diethylether-petroleum ether mixtures. If the reaction was to be stopped more quickly, particularly after short reaction times, the copolymers were directly precipitated in diethylether. The copolymers are redissolved in 5ml 1 N KOH with brief heating, diluted to 100ml with water and precipitated with the minimum amount of conc HCI. For copolymers of higher P(AJ, pyridine may be added to assist dissolution. For esterification of PMAA with 2,2,2-trifluoroethanol and DCC to yield trifluoroethylmethacrylate-methacrylic acid copolymers, 9 ml trifluoroethanol (125 mmol) instead of 13ml of benzyl alcohol is used. For a sl~ecified P(AJ the following reaction times were typical: P(A) = 0.32 in 10rain, P ( A ) = 0 . 5 9 in 6hr (A =TFEMA-unit): other reaction times can be taken from Fig. 13. After cooling and filtering, the polymers are precipitated in diethylether. Copolymers of P(A) > 0.5 cannot be precipitated in diethylether, but are precipitated in n-hexane. Reprecipitation by H20-KOH/HCI was performed as with the BMA-MAA copolymers. The reactions of PMAA with DCC without trifluoroethanol are carried out as previously described [3]. The conversions of the resulting methacrylic acid anhydridemethacrylic acid (MAH-MAA) copolymers with trifluoro593

594

E. KLESPER, D. STRASILLAand MARIA CHRISTINA BERG

ethanol to the corresponding TFEMA-MAA copolymers are carried out under the same conditions as the one step esterification with alcohol and DCC, but omitting DCC. The esterification of syndiotactic PMAA with conc H2SO + is carried out by dissolving PMAA (300mg = 3.49 mmol) in conc H2SO+ (95-97% w/w) (8 ml) at 30 ° and adding trifluoroethanol (3 ml = 41.6 mmol) slowly enough to keep the temperature below 32 °. For a specified P(A), the following reaction times at ambient temperatures are used: P(A) = 0.16, 1 hr; P(A) = 0.22, 2hr; P(A) = 0.32, 4hr; P(A) = 0.49, 8hr; P(A) = 0.58, 12hr; P(A) = 0.68, 20 hr; P(A)= 0.76, 96 hr; P(A)= 0.83, 144 hr. The reaction is stopped by pouring into diethylether (100 ml), Precipitated polymer is collected by centrifuging. Copolymers of P(A) > 0.50 become soluble in diethylether, and are precipitated by adding n-hexane to the diethyl ether. The copolymers are redissolved in H20 (100ml) at room temperature, using sufficient KOH to turn the solution strongly alkaline, diluted to 300ml, and precipitated by conc HCl. By refluxing for 30 min, the precipitates are converted into an easily filterable form. Copolymers of P(A) > 0.504).60 cannot be dissolved in the aqueous KOH; they were, however, treated in the same way, i.e. without dissolution, to remove adhering H2SO+. then dissolved in pyridine (30 ml), diluted with water (100 ml), and precipitated with HCI. Instead of trifiuoroethanol, methanol or ethanol were also used for the esterification of PMAA in conc H2SO 4 with the same reaction conditions. In all cases significant conversion was found and the statistics of the resulting copolymers was shown to be random. The 1H-NMR spectra are obtained as previously described [3]. BMA-MAA copolymers are also recorded with

- C H..,--

"

a Bruker 360 MHz spectrometer in the FT-mode in pyridine. d 5 at 30° with 1% concentration of polymer, using the convolution difference method for increasing resolution. RESULTS AND DISCUSSION Figure 1 shows the 360 M H z I H - N M R spectra of three syndiotactic benzytmethacrylate-methacrylic acid (BMA-MAA) copolymers, prepared by partial esterification of syndiotactic P M A A with benzylalcohol and DCC. Conversion increases from spectral trace I to trace III. In contrast to methyl methacrylatemethacrylic acid ( M M A - M A A ) copolymers prepared with D C C [3], not only triads but also pentads are resolved. The assignments of triads and pentads and the chemical shifts of B M A - M A A copolymers have already been given for 220 M H z spectra, but the resolution of the present convoluted 360 M H z spectra is superior [5]. The assignment is indicated o n the spectral traces of Fig. I. Similarly to the M M A - M A A copolymers, the triads ABA and BAB are relatively frequent, while the triads AAA and BBB are infrequent, pointing to a tendency towards alternation (A -- BMA-unit, B = MAA-unit). For Bernoullian statistics, which serves as the border between statistics with a tendency toward alternation and those with block-like character, the peaks for ABA and BAB should at all P(A) possess only half the integral intensity as their neighbouring AAB + and ABB + peaks

-- CH 3 I ABBBB"

++/l//A

n/~smA I t

,I I1

(AJlI/!IA/J ;

BBABB 1 I/ [ I

~

AABAB+

ABA

!

Jlll. ++

II

I'l IVl,

°

Fig. 1. IH-NMR of ~t-CH3 and fl-CH2-regions of benzylmethacrylate-methacrylic acid (BMA-MAAI copolymers prepared by esterification of poly(methacrylic acid) with b.enzyl alcohol and dicyclohexylcarbodiimide (DCC). Traces I, II and Ill correspond to P(A) = 0.40; 0.50 and 0.58, respectively (40°,, 50% and 58% conversion). Spectra recorded at 360 MHz, 30°, 1% concentration of polymer, applying a convolution difference to increase resolution.

IH-NMR of syndiotactic poly(methacrylic acid) with carbodiimides--II

is compared with the corresponding Bernoullian pentad probabilities (dark circles with curves drawn through). Even the partially alternating pentad AAABA ÷ has a higher measured probability as would conform to randomness. Figure 3(b) shows the probabilities for ABBBA, i.e. f o r blocks with 3 B-units, decreasing to zero in the limit of conversion. Blocks of 2 B-units, i.e. ABBA, do not fully disappear as the behaviour of P(AABBA ÷) already shows. It is possible to attempt to approximate the triad statistics by conditional probabilities of the first order:

BB8= 0 ABB+= Ix ABA= D

0,8

B

0.6

595

/o

0./. P(X,/X2) - P(XIX2~) P(X O

0,2

ASA,

7,' 67 ga

(1)

X = A, B P(X~X2) an P(X1) being obtained by

P(XtX2) = P(XIX2A) + P(XIX2B)

(2)

P(X) = P(XA) + P(XB)

(3)

and '

I

0,8

'

I

'

0,6

I

0,4

'

I

'

0,2

P(A)

Fig. 2. Experimental probabilities of B-centred triads of BMA-MAA copolymers prepared with DCC (points) compared with Bernoullian triad probabilities (curves, marked ABA,, BBB,, and ABB~+).

respectively (the superscript + indicates that both forward and reverse forms, e.g. AAB and BAA, are involved). The tendency toward alternation is supported by the pentad intensities at P(A) = P(B), i.e. at 50% conversion (trace II). Thus the "fully alternating" pentad BABAB is of considerably higher integral intensity than the "partially alternating" pentad AABAA and also higher than 509/o of the partially alternating pentad AABAB ÷, while Bernoullian statistics would demand equality. In addition, pentad peaks of copolymers with P(A) :# P(B) may be considered, e.g. either trace I or trace III. The pentad ABABA should possess for Bernoullian statistics at all values of P(A), only half the intensity as AABAB +. The former pentad is however enhanced because of its "fully alternating" character. If the triad probabilities are evaluated from the 360 MHz IH-NMR spectra and plotted versus P(A), one obtains Fig. 2 for the B-centred triads. With higher P(A), i.e. with increasing conversion, the measured triad probabilities (points) deviate increasingly from the calculated Bernoullian triad probabilities P(XXXr) (drawn out curves), so that the deviations for the B-centred triads are larger than for the A-centred triads. It is also remarkable that there is large negative deviation for P(ABB +) and lack of deviation for P(AAB+). This demonstrates the low frequency of B-blocks longer than one unit, in keeping with a glutaric type cyclic anhydride as the reaction intermediate. The anhydride is formed from two neighbouring B-units and then reacts with alcohols. Thereby P(ABB +) is strongly diminished at the limit of conversion. This limit is found at P(A) ~ 0,60. Besides triad probabilities, pentad probabilities are also evaluated. In Fig. 3(a) the measured probability of the fully alternating pentad ABABA (open circles)

The conditional probabilities P(X,/X2) are employed as usual to calculate triads

P(XI X 2 X 3) = P(X1) P ( X I / X 2 ) P ( X 2 / X 3)

(4)

The numerals appearing as subscripts are only to label the consecutive order and to identify monomer units. Calculation of sequences longer than dyads by conditional probabilities of the first order is accurately possible only with first order Markov chains I-6,7]. In Fig. 4 the experimental probabilities for the A-centred triads (open symbols) are fitted by curves. The conditional probabilities P(X1/X2) have been derived from the curves and utilized to recalculate the triad probabilities (closed symbols). Apparently the fit of the calculated probabilities of the A-centred triads is imperfect, as is the case for the fit of the B-centred triads (not shown). An attempt was made to improve the fit between measured and calculated sequence probabilities by employing second order conditional probabilities:

P(XlX2/X3 ) -

P(XIX2X3)

P(X1 X2)

(5)

usedeitheras P ( X I X 2 X 3 X 4 X 5 ) = P(X2X3X4) P ( X 3 X 2 / X I ) × P(X3X4/Xs)

(6)

or P ( X 1 X 2 X s X 4 X s ) = P(X2X3) P ( X 3 X 2 / X I ) >< P(X2X3/X4) P(X3X4/X5)

(7}

The second order conditional probabilities were obtained from the triad data as before. In Fig. 5, points of the measured probabilities for two pentads and the sum of two pentads [O], [A], [[]] may be compared with the curves calculated by conditional probabilities of the first order [ . . . . . ] and second order [ ]. The curves of the second order result

596

E. KLESPER, D. STRASILLA and MARIA CHRISTINA BERG

(a) 0.15

P(ABABA} : O , Q P(AAABA~ "--O , I I o or'=

0,10

0.05

I

I

I

I

I

I

(b)

0.15 P(ABBBA) i O P(AABBA+) -: O

0.10 o 0.05

I

0,7

I

I

I

I

I

0.6

0.5

0,4

0.3

0.2

P(A) Fig. 3. Experimental probabilities of selected pentads (a). Values for P(ABABA) [0] and P(AAABA +) [E3] compared with the corresponding Bernoullian values [O1 In]. Curves drawn through the Bernoullian values. (b) Values for P(ABBBA) [O] and P(AABBA +) [D] and curves fitted to them, to demonstrate behaviour of B-blocks with 3 or 2 B-units, when approaching the limit of conversion.

in a somewhat better fit than those of the first order, O n the whole, this is the case also for most of the other pentads. Although the scatter of the experimental data is large for some of the pentads, there still seem to be considerable, real deviations between

measured pentads probabilities and those calculated by second order conditional probabilities. The esterification of PMAA with trifluoroethanol and DCC leads also to a limiting conversion of P(A) ~ 0.60 and to a tendency toward alternation. If

AAA • BAB = o.=

0.3-

0.2

P(triads 0.1

i

I

|

I

I

06

0.5

0.4

0.3

0.2 P(A)

Fig. 4. Test for approximating measured triad probabilities by conditional probabilities of first order. Measured triad data [O,/x, D1 and curves fitted to them are compared with triad probabilities calculated by approximation [O, A, II].

tH-NMR of syndiotactic poly{methacrylicacid) with carbodiimides--II

',,

AAABA+ = o e ,,

k

035

-

.'~-

597

BAARA+J

',, o \ ^

=

BAABB ÷ = o,m

\

P(XX/X)-

[

)

---I ....

1

,%

030 \\

o

~,.\

0.05

~

o

I

I

I

I

1

0,6

0,5

0,4

0,3

0.2 P(A)

Fig. 5. Test for approximating measured pentad probabilities by conditional probabilities of first and second order. Measured pentad data [©, &, []] are compared with pentad probabilities calculated by conditional probabilities of first order [. . . . . ] or second order [ ]. Points [0, A, I ] for identification of curves.

however, the esterification with trifluoroethanol is carried out not with DCC but by dissolving the reactants in conc H2SO4, then the limiting conversion is higher, i.e. P(A) ~ 0.85, and the statistics are not alternating but random. The ~-CH3 region of the ~H-NMR spectra of trifluoroethylmethacrylate-methacrylic acid (TFEMA-MAA) copolymers prepared in conc H2SO4 is shown in Fig. 6. The conversion increases from trace I to III. Only triads can be evaluated, but the beginning of splitting into pentads is seen for the B-centred triads (A = TFEMA-unit, B = MAA-unit). The order of assignment of triads written on trace I is the same as for the BMA-MAA copolymers or for the MMA-MAA copolymers and is supported by the use of generally valid statistical relations [8]. The randomness of the TFEMA-MAA copolymers i s demonstrated in Fig. 7 for the B-centred triads. The experimental points fit the calculated Bernoullian curves rather well. A similar fit is obtained for the A-centred triads (not shown). Resolution of the triads of the TFEMA-MAA copolymers into pentads was tried by changing the NMR solvent from pyridine to quinoline. The random copolymers were utilized for this purpose because with this case all possible pentads can be expected with measurable intensities at some, even if different, regions of P(A). This cannot be expected if there is pronounced tendency toward alternation or block character. In Fig. 8, the ~-CH3 and /~-CHz-tH resonances of a random TFEMA-MAA copolymer of P(A)= 0.52 are shown for different compositions of pyridine (Py)/quinoline (Quin) as NMR solvent. Starting with pure pyridine, the splitting into Dentad Deaks increases with increasing tool% o1 qumohne, going lrom 25 to 50 and to 100% quinoline. However, 100% quinoline does not seem to be the optimum for quantitative evaluation because of the strong overlap of the ct-CH3 with the fl-CH2region at high quinoline contents and because the

pentads of neighbouring triads start to merge again. The former disadvantage could in principle be eliminated by deuterating the fl-CH2-group. The behaviour ABB +

BBB

I AAB÷

ABA A

AAA

/

I

2~0

I

17G

I

I~0

I 1.25

Dpm

I 1,00

Fig. 6. tH-NMR spectra of the ~t-CH3 region of trifluoroethylmethacrylate-methacrylic acid (TFEMA-MAA) copolymers prepared with conc H2SO4. Traces I, II and III correspond to P(A) = 0.36; 0.52 and 030, respectively. TFEMA-unit written as A, MAA-unit written as B.

598

E. KLESPER, D. STRASILLAand MARIA CHRISTINA BERG

1.o o P(BBB) A P(ABB*) n P(ABA)

0'81

/

0.6 P (triads).

o

0/,-

0,2=

0 1.o

'

i O.e

_

/~

'

I o.6

o,6

\

0 3 ' 0

P(A)

Fig. 7. Demonstration of randomness ' for TFEMA-MAA copolymers prepared with cone. HzSO4. Experimental triad probabilities (points) compared with calculated Bernoullian triad probabilities (curves).

of the chemical shifts on changing gradually from pyridine to quinoline is more clearly seen in Fig. 9. The triads are all split into pentads with about 40% quinoline but, with about 60% quinoline, the pentads of neighbouring triads start to overlap again. Even with 35% quinoline, the high field part of the fl-CH2resonance starts to overlap with the low field part of the ~t-CH3 resonance. Because of this behaviour, 40 mol% quinoline was chosen for quantitative evaluation of pentad probabilities. The pentad peaks 1-18 of Fig. 8 are assigned in Fig. 9 to specific pentads. The order of the assignment is the same as for the BMA-MAA copolymers in Fig. 1, and is supported by a self consistent set of shift rules, as shown by inspection of Fig. 9. It should be mentioned that a similar shift behaviour as in pyridine-quinoline solutions has been observed in pyridine-acridine solution. The shift effect of acridine is about twice that of quinoline on a mole per mole basis. Acridine is, however, less convenient since it is a solid. The results for evaluation of the experimental pentad probabilities for the random TFEMA-MAA copolymers prepared in cone H2SO4 are shown for the

-AAB"

-GH.j

ABA -CH E

Py:Quin

100 - 0

SO

S0

2

0 : t00 3:0

~s

=',o

~;s

m-

1;o

Fig. 8. IH-NMR spectra of =-CHs and p-CH2-regions of a random TFEMA-MAA copolymer of P(A) --- 0.52, recorded from different mixtures of pyridine (Py)-quinoline (Quin) given in mol% as the NMR-solvent (traces I to IV). Pentad peaks numbered 1-18.

~H-NMR of syndiotactic poly(methacrylic acid) with carbodiimides--II

599

aA~A*

aASaA*

Fig. 9. ~H-NMR chemical shifts for triads and pentads in the ~ - C H 3 r e s o n a n c e of random TFEMAMAA copolymers recorded from different mixtures of pyridine (Py) and quinoline (Quin) in mol°A.

B-centred pentads in Fig. 10. Obviously, the measured the A-centred triads to Fig. 12. The experimental symprobabilities (points) fit the calculated Bernoullian bols [O, A, U]] are fitted by curves from which the curves rather well, if it is considered that, with in- triad probabilities are taken to calculate the P(X 1/X2) creasing overlap of peaks, determination of relative and thence the P(X1XzX3), the latter being shown peaks areas becomes increasingly more inaccurate. as crossed points [O, A, II]. The fit between The results for the A-centred pentads are analogous. measured and calculated data is less than hoped, Turning to the preparation of TFEMA-MAA co- although the B-centred triads yield a somewhat better polymers with DCC, both triad and pentad probabili- fit. An improvement could not be obtained for penties have been evaluated. For the B-centred triads, tads when using conditional probabilities of the the experimental data are seen in Fig. 11. There is second order. This was the case despite the detera strong tendency toward alternation, as shown by mination of the P(X1X2X3), P(XIX2) and inspection of the behaviour of P(ABA) and P(BBB). P(X1Xz/X3) from the triad data measured in pure Moreover, the limiting conversion at P(A) --- 0.60 and pyridine, in which solvent triad peaks are better the strong decrease in P(ABB +) when approaching resolved than in pyridine-quinoline. Although the this limit become obvious. The A-centred triads con- measured pentad data show considerable scatter and firm these results. Also, the experimental pentad proare likely to be subject also to systematic errors, it appears that the non-Markovian character of the statbabilities exhibit a strong tendency for preferring, for istics for both TFEMA-MAA and BMA-MAA coinstance the fully alternating sequences ABABA and BABABA over the values demanded by Bernoullian polymers is responsible for part of the deviations. statistics (not shown). Even the sequences possessing The proposed mechanism for the esterification of only partially alternating character, e.g. AABAB ÷ and PMAA with benzyl alcohol or trifluoroethanol and ABAAB +, are preferred if only to a lesser degree. DCC requires cyclic anhydride as an intermediate Approximating the experimental triad probabilities [3, 4]. To support this reaction path, copolymers preof the TFEMA-MAA copolymers prepared with DCC pared with benzyl alcohol have been isolated in a by conditional probabilities of first order leads for way so as not to hydrolyse cyclic anhydride which

E. KLESPER, D. STRASILLA a n d MARIA CHRISTINA BERG

600

might be present, i.e. by precipitation in dry ether and reprecipitation in dry DMSO ether. The same

020

BaaB~" ""

0,1S o

" ":'""" ......

/ ~ /2

a ABBBA

0,10

procedure was followed with copolymers prepared with trifluoroethanol. The i.r. spectra of the copolymers show anhydride by the two typical C -- O bands at 1805 and 1760cm -1 [3,4]. It has already been shown that anhydride forms in much larger quantity and at a relatively high rate if no alcohol is present [3]. Thus it remains to be demonstrated qualitatively that the two alcohols are able to react with the anhydride at an initial rate which is comparable with that of the overall esterification, i.e. the one step reaction. Similarly it was to be shown that the initial rate of anhydride formation is comparable to the one step reaction. The rates in question are compared in Fig. 13. The one step reaction of PMAA with trifluorocthanol and DCC [O] is comparable in initial rate to the reaction of a MAH-MAA copolymer with trifluoroethanol [A]. The subsequent slowing of the reaction is expected since no anhydride is newly

\

/~

\\

O,OS

0|

~o

•

BABAB--1

•

AABAA if

,

P(pentadsl 0.05

,ABA.'~ I

~ - : -o"X - ~ -]~ / // )

/ 0,~

o = BBB

i

•

0.15

1.0

BBBAB~ . . . .

0.8

/

A = A B B*

. ~

A BBBAA.+ ABBAB" --

- ~/

\

"++" "/-X

[] = A B A

\

BB

O.6 P(triads)

~o

0J, 8.

0~)5

0,2

o:8

d,~

0:6

I

A,,,+,

1.0

0.2

~+,z~+,,+"'~+ 0.8

0.6

0,4

\

i

0.2

0

P(A)

Pl A1

Fig. 10. Pentad randomness for TFEMA-MAA copolymers prepared with conc. H2SO+. Experimental pentad probabilities (points)compared with calculated Bernoullian pentad probabilities (carves). Only B-centred pentads are shown,

Fig. 11. Experimental probabilities of B-centred triads of TFEMA-MAA copolymers prepared with DCC (points) compared with Bernoallian triad probabilities (curves marked ABA,, BBB,, and ABB+). Triad statistics exhibit a tendency toward alternation.

AAA = 0 , 0 BAB = rl,II

0.3

02 Pltriads)

[]

0.1

•

0,6

0,5

0.4

0.3

re(A)

0.2

Fig. 12. Test for approximating triad probabilities by conditional probabilities of first order. Experimental triad probabilities [O, A, D] are compared to triad probabilities approximated by conditional probabilities of first order ro, A, ii].

tH-NMR of syndiotactic poly(methacrylic acid) with carbodiimides--II

601

the reaction of P M A A with D C C but without the alcohol [ | ] has a rate comparable to that of the one step reaction, if it is considered that the P(B) ........ is actually plotted for this reaction, 50% of which can only be transformed to A-units. Acknowledgements--Financial support by the Deutsche Forschungsgemeinschaft is gratefully acknowledged. One of us (M.C.B) is obliged for a stipend from the lnternationales BiJro der Kernforschungsanlage JiJlich, BRD, and the Conselho Nacional de Desinvolvimento Tecnologico e Cientifico (CNPq), Brazil.

0,2-1

....

] 4 ~ ..~-- - - ~ , . . . . . . .

-~

~-- . . . . .

REFERENCES

1

2

3

4

5

6

7 8 tin~(h)

Fig. 13. Conversion of the reaction of PMAA with DCC and trifluoroethanol [O], PMAA with DCC but without trifluoroethanol [ | ], and copolymer of methacrylic anhydride and methacrylic acid with trifluoroethanol [A]. For the second reaction P(A) is not plotted, but the B-units which have reacted to form anhydride, i.e. P(B)...... d = 0.40. formed during the reaction, contrary to the case of the one step reaction. It appears possible also that

[.PJ.

156

I

I. F. Kurzer and K. Douraghi-Zadeh, Chem. Rev. 67, 107 (1967). 2. H. G. Khorana, Chem. Rev. 53, 145 (1953). 3. E. Klesper, D. Strasilla and M. C. Berg, Polymer, Submitted for publication. 4. E. Klesper and D. Strasilla, J. Polym. Sci., Polym. Lett. 15, 23 (1977). 5. D. Strasilla and E. Klesper, Makromolek. Chem. 175, 535 (1974). 6. K, Ito and Y. Yamashita, J. Polym. Sci. Part A 3, 2165 (1965). 7. H, L. Frisch, C. L. Mallows and F. A. Bovey, J. chem. Phys. 45, 1565 (1966). 8. E. Klesper, J. Polym. Sci.. Polym Lett. 6, 663 11968L