Chemical Reactions on Polymers - American Chemical Society

Chapter 26

Tri-n-butyltin Hydride Reduction of Poly(vinyl chloride) Kinetics of Dechlorination for 2,4-Dichloropentane and 2,4,6-Trichloroheptane 1

Fabian A. Jameison , Frederic C. Schilling, and Alan E. Tonelli Downloaded by CORNELL UNIV on August 17, 2016 | http://pubs.acs.org Publication Date: December 22, 1988 | doi: 10.1021/bk-1988-0364.ch026

AT&T Bell Laboratories, Murray Hill, NJ 07974 2,4-Dichloropentane (DCP) and 2,4,6-trichloroheptane (TCH) were reductively dechlorinated with tri-n-butyltin hydride ((n-Bu) SnH) directly in the NMR sample tube. C NMR spectra were recorded periodically to monitor the progress of DCP and TCH dechlorination. From these observations the following kinetic conclusions were drawn: i. meso (m) DCP was reduced 30% faster than racemic (r) DCP; ii. the Cl from DCP was removed 4 times faster than the Cl in 2-chloropentane or 2-chlorooctane; iii. the 4-Cl in mm-TCH is removed faster than the 4-Cl in mr-TCH which in turn is more reactive than the 4-Cl in the rr isomer; and iv. the 4-Cl in TCH is removed 1.5 times faster than the 2or 6-Cl's. Conclusions i. and ii. were previously observed at the diad level, at least qualitatively, in the (n-Bu) SnH reduction of poly(vinyl chloride) (PVC) to ethylene-vinyl chloride (E-V) copolymers. Using the kinetic information obtained from the reduction of DCP and TCH, an attempt was made to simulate the (n-Bu) SnH reduction of PVC to E-V copolymers. Comparison of the structures of the E-V copolymers simulated on the computer with those determined for (n-Bu) SnH reduced PVC by C NMR permits us to conclude that DCP and TCH are model compounds appropriate for studying the reductive dechlorination of PVC. 13

3

3

3

13

3

1 3

Starnes and Bovey ( l ) pioneered the method of C N M R analysis of reduced polyvinyl chloride) (PVC) to study the microstructure of P V C . Tri-n-butyltin hydride ((n-Bu) SnH) was found to completely dechlorinate P V C resulting in polyethylene (PE) whose microstructure (branching, end-groups, etc.) could be sensitively studied by C N M R . The present authors (2) subsequently produced a series of ethylene (E) - vinyl chloride (V) copolymers (E-V) by using less than the 3

1 3

1

Current address: Department of Chemistry, State University of New York at Stony Brook, Stony Brook, N Y 11794 0097-6156/88/0364-0356$06.00/0 © 1988 American Chemical Society

Benham and Kinstle; Chemical Reactions on Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

26.

Reduction of Poly (vinyl chloride)

JAMEISON ET AL.

357

stoichiometric amount of (n-Bu) SnH during the reductive dechlorination of P V C . Traditional means of obtaining E - V copolymers suffer from several shortcomings. Chlorination of P E (3) results in head-to-head (vicinal) and multiple (geminal) chlorination leading to structures which are not characteristic of E - V copolymers. Direct copolymerization of Ε and V monomers does not usually lead to random E - V copolymers covering the entire range of comonomer composition. Free-radical copolymerization (4,5) at low pressure yields E - V copolymers with V contents from 60 to 100 mol % . The 7-ray induced copolymerization (6) under high pressure yields E - V copolymers with increased amounts of E , but it appears difficult to achieve degrees of Ε incorporation greater than 60 mol % without producing blocky samples. The series of E - V copolymers obtained by partial reduction of P V C with (n-Bu) SnH were found (2) to have the same chain length as the starting P V C (~ 1000 repeat units). Their microstructures were determined by C N M R analysis (2) as indicated in Figure 1. The results of this analysis are presented in Table I in terms of comonomer diad and triad probabilities. A close examination of the data in Figure 1 and Table I leads to two interesting observations. First, as the amount of chlorine (CI) removed was increased, the ratio of racemic (r) to meso (m) W diads increased, and second the disappearance of W diads was greater than anticipated for the random removal of Cl's. Consequently, we concluded from the (n-Bu) SnH reduction of P V C to E - V copolymers and eventually to P E , that Cl's belonging to W diads are preferentially removed relative to isolated Cl's ( E V E ) and that m - W diads are reduced faster than r - W diads. Subsequent studies of the physical properties of this series of E - V copolymers obtained via the (n-Bu) SnH reduction of P V C have revealed that their properties, both in the solid state and in solution, are sensitive to their detailed microstructure (7-10). These observations prompted the present study concerning the mechanisms of the reductive dechlorination of P V C with (n-Bu) SnH. We have chosen the P V C diad and triad compounds 2,4dichloropentane (DCP) and 2,4,6-trichloroheptanefTCH) as subjects for our attempt to obtain quantitative kinetic data characterizing their (nBu) SnH reduction in the hope that they will serve as useful models for the reduction of P V C to E - V copolymers. Unlike the polymers ( P V C and E-V), D C P and T C H are low molecular weight liquids whose high resolution C N M R spectra can be recorded from their concentrated solutions in a matter of minutes. Thus, it is possible to monitor their (n-Bu) SnH reduction directly in the N M R tube and follow the kinetics of their dechlorination. Finally the kinetic data are compared to the microstructures of the E - V copolymers obtained by (n-Bu) SnH reduction of P V C to test the suitability of D C P and T C H as model compounds for P V C reduction. This is achieved by computer modeling the reduction of P V C to E - V copolymers with the aid of the kinetic parameters obtained from the study of D C P and T C H reduction, and then comparing the observed and modeled E - V microstructures. 3

Downloaded by CORNELL UNIV on August 17, 2016 | http://pubs.acs.org Publication Date: December 22, 1988 | doi: 10.1021/bk-1988-0364.ch026

3

1 3

3

3

3

3

1 3

3

3

E X P E R I M E N T A L M A T E R I A L S .

The 2-chloro-4-methylpentane, 2-chlorooctane, and 4-


358

CHEMICAL REACTIONS ON POLYMERS

(α) IOIOIOI

ΟΙΟΙΟΙΟ

VV =01010 VE =01000 EV =00010

EE = ( * * 0 0 0 * * - O I O O O ) / 2 * = 0 or I


J

EVE VVE EVV VVV VEV VEE EEV

L

EEE = [(0000000+0000001) -OOOOOIO]/2 OOOIO 01000

0101010

(b)

=0001000 =0101000 =0001010 =0101010 =0100010 =0100000 =0000010

0100010

L 0000000 000Ô001 01010

OOOQOIO 0I00000

OOOIO 01000

looogioi 0 100010

ι

ι

ι

ι

ι

I ι 60

ι

ι

ι

ι

ι

ι

ι ι

1 ι 50

ι

ι

ι

ι

ι

ppm

ι

ι

ι—I ι ι 40

ι—ι

;

ι

ι

ι

ι

1 ι ι 30

ι

ι

ι

J ι—L_J—L 20

vs(CH ) Si 3

4

1 3

Figure 1. 50.31 M H z C N M R spectra of P V C (a) and two partially reduced P V C ' s , E-V-84 (b) and E-V-21 (c). Please note the table of E - V microstructural designations in the upper right-hand corner of the Figure, where 0,1 = C H , CHC1 carbons. Resonances correspond to underlined carbons. The assignment of different stereosequences is given in reference 2. 2


26.

JAMEISON ET AL.

Reduction of Polyvinyl

chloride)

359

Table I


Diad and Triad Probabilities for E-V Copolymers

Copolymer

Pvv

PVE " PEV

PEE

PEVE

PVVE " P E W

Pvvv

PVEV

E-V-85

.742

.124

.011

.015

.115

.619

.114

.011

0.0

E-V-84

.709

.134

.023

.025

.108

.615

.101

.019

.004

E-V-71

.470

.239

.052

.063

.175

.310

.175

.048

.008

E-V-62

.344

.278

.099

.116

.177

.177

.177

.075

.027

E-V-61

.343

.275

.107

.121

.173

.198

.141

.083

.029

E-V-60

.316

.285

.114

.141

.167

.154

.179

.077

.038

E-V-50

.200

.297

.205

.192

.133

.073

.166

.129

.045

E-V-46

.147

.309

.235

.205

.116

.037

.149

.140

.098

E-V-37

.087

.286

.342

.219

.078

.012

.115

.158

.183

E-V-35

.061

.278

.383

.224

.064

.015

.090

.168

.208

E-V-21

.014

.197

.593

.190

.016

0.0

.035

.153

.436

E-V-14

0.0

.127

.746

.104

0.0

0.0

.051

.123

.599

E-V-2

0.0

.025

.950

.021

0.0

0.0

0.0

.026

.926

PVEE •


PEEV

PEEE

360


chlorooctane were purchased from Wiley Organics and used as received. The 2,4-dichloropentane was obtained from Pfaltz & Bauer and also used as received. Tri-n-butyltin hydride (Alfa Division, Ventron Corp.) was vacuum distilled and stored under argon before use. The free radical initiator azobis(isobutyronitrile) (AIBN) used in the reduction was also purchased from Alfa Division. The 2,4,6-trichloroheptane was obtained from a new synthesis which involves the hydrohalogenation of 1,6heptene-4-diol and the chlorination of the resulting alcohol. A detailed description of this method can be found elsewhere (11). S A M P L E P R E P A R A T I O N . In a small vial 22.5 mg of A I B N was mixed with 1.7 ml of perdeuterobenzene and the mixture held at 0 C to permit dissolution of the A I B N . The chloroalkane and 0.2 ml of the N M R reference material hexamethyldisiloxane (HMDS) were placed in a 10 mm N M R tube. The AIBN/benzene solution was added to the N M R tube and placed under an argon atmosphere in a glove bag. The freshly distilled (n-Bu) SnH (1.0-1.7 ml) was transferred by syringe into the solution. Following a thorough mixing, the sample was degassed with argon for several minutes and sealed with paraffin film. The amount of chlorinated alkane varied between 0.2 and 0.4 ml (7.0-12.5% v / v ) and the (n-Bu) SnH added was equal to the molar concentration of chlorine atoms present. The sample was placed in the N M R spectrometer at 50 ° C and the C N M R spectra were recorded as the reduction proceeded.


0

3

3

1 3

M E A S U R E M E N T S . Initially, the reduction of 2-chloro-4methylpentane was carried out in order to ascertain the ideal temperature which would lead to complete reduction in about six hours. The progress of this reduction was followed by Ή N M R , recording a single scan every thirty minutes. It was found that at 5 0 C the reaction reaches 80% of completion after 5 hr. A l l subsequent reductions were carried out at this temperature. The 50.31 M H z C N M R spectra of the chlorinated alkanes were recorded on a Varian XL-200 N M R spectrometer. The temperature for all measurements was 5 0 C . It was necessary to record 10 scans at each sampling point as the reduction proceeded. A delay of 30 s was employed between each scan. In order to verify the quantitative nature of the N M R data, carbon-13 Tj data were recorded for all materials using the standard 1 8 0 ° - r - 9 0 ° inversion-recovery sequence. Relaxation data were obtained on (n-Bu) SnH, (n-Bu) SnCl, D C P , T C H , pentane, and heptane under the same solvent and temperature conditions used in the reduction experiments. In addition, relaxation measurements were carried out on partially reduced (70%) samples of D C P and T C H in order to obtain T data on 2-chloropentane, 2,4-dichloroheptane, 2,6-dichloroheptane, 4chloroheptane, and 2-chloroheptane. The results of these measurements are presented in Table II. In the N M R analysis of the chloroalkane reductions, we measured the intensity of carbon nuclei with T values such that a delay time of 30 s represents at least 3 T . The only exception to this is heptane where the shortest T is 12.3 s (delay = 2.5Tj). However, the error generated would be less than 10%, and, in addition, heptane concentration can also be obtained by product difference measurements in the T C H reduction. Measurements of the nuclear Overhauser enhancement (NOE) for carbon nuclei in the model compounds indicate uniform and full enhancements for those nuclei used in the quantitative measurements. Table II also contains the chemical N M R

0

13

0

3

3

x

t

2

t


26.

JAMEISON ET AL.


361

Table Π C NMR Spin Lattice Relaxation Times ( Τ ) and Chemical Shifts (6) }

6


H-Sn4C-C-C-C),

Cl-SnfC-C-C-C), CI I

m,r

8.30 27.41 30.24 13.82

6.0 8.5 7.3 8.1

a b c d

17.53 27.04 28.19 13.64

4.5 7.0 5.9 7.3

CI

25.50 24.49 55.38 54.24 50.86 50.51

ι

c-c-c-c-c a b c

CI

T, (s)

a b c d

(r) (m)

M (m) (r) (m)

6.7 6.4 15.7 15.8 8.9 8.8

c-c-c-c-c

a b c d e

25.41 57.58 42.82 20.06 13.56

9.0 21.8 13.3 14.8

C-C-C-C-C

a 14.12 b 22.62 c 34.46

10.8 24.6 24.4

a

b

c

d

e

25.34 a 24.21 24.11

,

CI I

m,r

CI CI ι m,r |

c-c-c-c-c-c-c a

b

e

d

C

/") m

r

3.5— 3.7

/ \ (mm) r

m

55.19 55.07 w. 54.14 53.93

(rr) (rnr) (rm) (mm)

8.1—8.4

49.02 48.84 48.33 47.81

(«) (mr) (rm) (mm)

4.4-* 4.6

58.32

(rr) (mr) (rm) (mm)

v4

d 57.44 56.39

8.2 8.1 8.0

Continued on next page


362


Table U (continued)

is C NMR Spin Lattice Relaxation Times (Tj) and Chemical Shifts (6)

25.59 23.50 , 55.56 54.41 49.30 48.55 . 60.51


a

_ , Cl

Cl

c

I · I C - C - C - C - C - C - C a b c d β f g m

Γ

ι

b

c

d

W (m)

b

c

d

•c-c-c-c-c-c

(m)

25.33

(r)

25.30

(m)

·80

(r) M

57

U

d Q| I

(r) (m)

g 14.07

I C - C - C - C - C - C - C a

(r) (m)

(r) (m) (r)

m

f

m, r

(m)

59.58 41.10 40.07 19.76 19.61

e

I

(r) (m) (r)

Ο

t

.OU

40.07 40.02 2

4

1

0

24.02

.

(r) (m)

W

(m)

12.4

-5.7 -11.0 y.

6.4

- 36 3

6

a 14.07 b 19.96 c 41.01

12.4 8.9 6.3

a 25.40 b 58.25 c 40.71 d 26.60 e 31.64 f 22.80 g 14.20

5.7 14.6 7.2 8.5 9.3 10.5 12.3

a 14.20 b 23.01

12.3 13.2 12.3 12.4

b e d

Q| I

C - C - C - C - C - C - C a b c d e f g

C - C - C - C - C - C - C a b c d

c 32.24 d 29.35


26.

JAMEISON ET AL.

Reduction of Polyfvinyl chloride)

363

shift data for all compounds studied. The chemical shift data for the T C H sample agrees well with that of an earlier report where the shift assignment of each stereoisomer was established (12). The percent reduction was determined by comparing the amounts of (n-Bu) SnH and (n-Bu) SnCl at each measurement point. 3

3

K I N E T I C S O F (n-BU) SnH R E D U C T I O N O F D C P A N D 3

TCH.

DCP R E D U C T I O N . As illustrated below D C P (D) is sequentially transformed into 2-chloropentane (M) and then to pentane (P) during its reduction with (n-Bu) SnH. 3


D — > Μ —>

Ρ

The ratio of rate constants K=(k /k ) can be obtained (13) from the concentrations of D and M measured at various degrees of reduction χ according to M

D

(D

κ-ι

where the subscripts ο and χ indicate concentrations initially and after % reduction x. A n alternative means to determine the relative rates of reduction of M and D, ie. Κ = k /k , is afforded by comparing the simultaneous (nBu) SnH reductions of D C P and 2-chlorooctane (Μ') to pentane and octane (O), respectively. M

D

3

kn D —>

7

In this case K = W*x>

i s

•> Ρ

M

S'iven by (13)

In

M' 0

(2)

ln

Equation (2) can also be used to determine the relative rates of reduction of meso (m) and racemic (r) D C P (D , D ) where M' and D are replaced by D and D . m

m

r

r



364

! ^

D r

>

M

h L

>

P

T C H R E D U C T I O N . In the early stages of the reduction of T C H (T) with (n-Bu) SnH it is possible to compare the relative reactivities of the central (4) and terminal (2,6) chlorines. A t these levels of reduction only 2,6 and 2,4-dichloroheptanes (2,6-D and 2,4-D) are produced, as shown below. Downloaded by CORNELL UNIV on August 17, 2016 | http://pubs.acs.org Publication Date: December 22, 1988 | doi: 10.1021/bk-1988-0364.ch026

3

κ

Ε

Τ —>

2,4-D

We can establish the relative reactivities, k /k , of the central (C) and terminal (E) chlorines directly from the relative concentrations of the resulting dichloroheptanes. c

k

c

E

2,6-£

=

k

2,4-D

E

(

3

)

R E S U L T S A N D DISCUSSION K I N E T I C R E S U L T S F O R D C P A N D T C H . The portion of the 50.13 M H z C N M R spectra containing the methylene and methine carbon resonances of D C P and the resultant products of its (n-Bu) SnH reduction are presented in Figure 2 at several degrees of reduction. Comparison of the intensities of resonances possessing similar T i relaxation times (see above) permits a quantitative accounting of the amounts of each species (D,M,P) present at any degree of reduction. In Figure 3 the percentages of D (DCP), M (2-chloropentane), and Ρ (pentane) observed during the (n-Bu) SnH reduction of D C P are plotted against the degree of reduction x. Equation (1) is solved for K = k / k by least-squares fitting the calculated and observed values of the ratio (M /D ). The observed ratios (p /D ) are substituted into E q . 1 to obtain the calculated ratios (M /D ) corresponding to the assumed Κ = k /k and these are compared with the observed ratios (M /D ). This procedure yields Κ = k /k = 0.26, which means that D C P is ~ 4 times more easily reduced than 2-chloropentane. Comparison of the simultaneous reduction of D C P and 2-chlorooctane gave according to Equation (2) K = k >/k = 0.24, lending further support to the observation that chlorines belonging to a W diad are removed 4 times faster than an isolated chlorine in say an E V E triad. Furthermore, the observed rates of (n-Bu) SnH reductions of 2- and 4-chlorooctanes were 1 3

3

3

M

x

x

x

x

M

D

0

x

x

M

x

D

7

M

D

3


z >

26. J A M E I S O N E T A L .

Reduction of Polyfvinyl chloride)

CZ

I

CH

3

CZ

m,r

— CH

I

365

—

2

CH

2

I —

CH

—

CH

3

3

CZ

I

28% CH

3

—

I

CH — 2

CH

2

— C H

— C H

2

3

3


M-3 M-2

P-3

60%

CH

3

— C H

I

55

'

45

ppm vs

'

2

2

— C H

2

— C H

2

—

CH

3

3

35

TMS

13

Figure 2. 50.31 M H z C N M R spectra of D C P (D) and its products (M and P) resulting from 0, 28, 60, and 81% reduction with (n-Bu) SnH. 3

X,

%

REDUCTION

Figure 3. Distribution of reactants (D, M ) and products (M, P) observed in the (n-Bu) SnH reduction of D C P . D = D C P , M = 2chloropentane and Ρ = pentane (see Figure 2). 3


366


identical within experimental error. This means that the reactivity of an isolated chlorine is independent of structural position or chain end effects. The observed ratio of m to r isomers, m /r remaining during the (n-Bu) SnH reduction of D C P (see Figure 2) are plotted in Figure 4. ratio of k /k = 1.3. Substituting Apparently m - W diads are 30% more reactive toward (n-Bu) SnH than are r - W diads. Our C N M R analysis (2) of the E - V copolymers obtained via the (n-Bu) SnH reduction of P V C led to k / k = 1 . 3 1 ±0.1 in excellent agreement with the kinetics observed for the removal of chlorines from m- and r - D C P . We also found no W diads in those E - V copolymers made by removing more than 80% of the chlorines from P V C . This observation is confirmed in the (n-Bu) SnH reduction of D C P where the chlorines in this P V C diad model compound were found to be 4 times easier to remove than the isolated chlorines in 2-chloropentane, 2-, and 4-chlorooctane. The C N M R spectra of T C H before and after 43% reduction with (n-Bu) SnH are shown in Figure 5. The shift assignments given in the ngure and those listed in Table II were obtaind by comparison to the chemical shift data of T C H , D C P , 2-, and 4-chlorooctanes. From the relative concentrations of 2,6- and 2,4-dichloroheptane (2,6-D and 2,4-D) observed in the early stages of T C H reduction with (n-Bu) SnH we determine according to Equation (3) that the reactivity of the central chlorine in T C H is 50% greater than the terminal chlorines, ie. k /k =1.5. We also find the reactivity of the central chlorine in T C H to depend on its stereoisomeric environment as follows: mm>mr or rm>rr. In Figure 6 we have plotted and compare the triad sequences observed in the reduction of T C H and P V C with (n-Bu) SnH. The curves numbered 0,1,2, and 3 correspond to triads containing 0 (EEE), 1 ( V E E + E E V + E V E ) , 2 ( W E + E W + V E V ) , and 3 Î V W ) chlorine atoms. There is agreement between the curves describing the products of reduction for T C H and P V C providing strong support for considering T C H an appropriate model compound for the (n-Bu) SnH reduction of P V C . This clearly implies that the (n-Bu) SnH reduction of P V C is independent of comonomer sequences longer than triads. The first column of Table III lists all possible C NMR distinguishable E - V triads whose central units are V . In the next column we present the same triad structures in binary notation (0=E, 1=V) with the central unit labeled as the site of (n-Bu) SnH attack and the terminal units as either — (preceeding site) or + (following site). The final column presents the relative reactivities of the central V ( l ) unit in each triad toward (n-Bu) SnH based on the kinetics of reduction determined for D C P and T C H . For the E W (Oil) triads, removal of the central chlorine atoms is expected to be 3.5 (r) and 4.6 (m) times faster than for the isolated chlorine atom in the E V E (010) triad, because k /k = 4.0 and k /k = 1.3 for D C P . The central chlorines in V W (111) triads are 6.0 (mr or rm), 4.6 (rr), and 7.8 (mm) times more reactive toward (n-Bu) SnH than the chlorine in the E V E triad based on k /k = 4.0 and k J/k = 1.3 x

XJ

3

Dm

Df

3

1 3

3

m

r


3

1 3

3

3

c

E

3

3

3

1 3

3

3

D

M

Dn

3

D

for D C P and k /k c

E

M

D

= 1.5 for T C H .


Df

Df


26.

JAMEISON ET AL.


367

Figure 4. Ratio of the relative amounts of m and r isomers of D C P remaining after reduction by (n-Bu) SnH,as measured by the carbon-13 methylene (see Figure 2) and methyl resonances. 3



64

0%

62

—i

1 58

ppm

1

I 56

VS T M S

ι

ι

I 54

mm

•

mr

I

52

I

3

2

I —CH

CJl

CH

3

3

13

3

— CH

3

— CH2— CH 2

3

2

1

CH

I

2

2

2

2

5

2

4

5

— C H — CH

4

— CH — C H

I

C£

4

4

— CH — CH

m, r

4

2

2

2

2

2

—CH — CH

m,r — CH — C H — CH — C H

3

2

2

m, r

CH

3

— CH — C H

I

CJL

3

—CH — C H 2

I

2

3

3

m, r

1

CH

1

CH

I

est

6

2

—CH

7

7

— CH — CH

2

2

CH —CH 6

— CH

—

—CH — CH

3

3

3

3

3

— CH — C H

I

Figure 5. Methine carbon region of the 50.31 M H z C N M R spectra of T C H at 0 and 43% reduction with (n-Bu) SnH.

1 60

rr

rr

rm

H-2


»

en

*

»w

>

ο

w

η

oo


26.

JAMEISON ET AL.


369

X, % R E D U C T I O N 1 3

Figure 6. Comonomer triad distributions observed by C N M R analysis during the (n-Bu) SnH reductions of T C H ( ) and P V C ( ). 3

Table ΠΙ Relative Reactivities of the Central Chlorines in E-V Triads

E-V Triad

-

reduction site

+

k (relative)

EEE

0

0

0

0.0

EVE

0

1

0

1.0

1

3.5

1

4.6

EW

0

EW

0

VW

1

WV

1

VW

1

1 1

r m

m -

Γ

j

r

Γ

m -

1

4.0 X 1.5 = 6.0

1

6.0 -r 1.3 = 4.6

m 1

6.0 X 1.3 = 7.8


370


C O M P U T E R SIMULATION OF T C H AND P V C

REDUCTION.

We begin with 100 T C H molecules reflecting the stereochemical composition of our unreduced T C H sample, ie. 52 (mr or rm), 28 (rr), and 20(mm) stereoisomers. A T C H molecule is selected by generating a random integer, I , where I < 101. If I < 53, then the T C H molecule chosen is a mr or rm isomer. If 52 < I < 81, then the T C H is rr, and if I > 80 the T C H selected is a mm isomer. Next we randomly choose either one or the other terminal units or the central unit of our selected T C H isomer and check to see if it is a V ( l ) unit or an E(0) unit. If a terminal V unit is chosen we check to see if the neighboring central unit is V or E. If the central unit is also V, then we determine whether this W diad is m or r. For r and m W diads the relative reactivities of the terminal V unit chlorine are 3.5 and 4.6, respectively (see Table III). If the central unit is E, then we assume the relative reactivity of the isolated terminal units in the V E E or V E V triads to be identical to the isolated central unit in the E V E triads. This assumption is supported by the identical rates of (n-Bu) SnH reduction observed here for the 2- and 4-chlorooctanes. Finally, we select a random number between 0.0 and 1.0. If it is smaller than the relative reactivity divided by the sum of the relative reactivities of all chlorines in the V W , EW or W E , VEV, V E E or EEV, and E V E isomers of T C H and partially reduced T C H (see Table III), then we remove the terminal chlorine (1—K)) and modify the relative reactivity of the central V unit in the selected T C H isomer, because its terminal neighbor has been changed from V to E. This procedure is repeated until the desired per cent reduction, x, is reached, where χ = 100 X (# of chlorines removed 300). Each of the 100 T C H molecules is then tested for the number and sequence of V units remaining at this current value of x. In Figure 7 we plot the per cent of T C H molecules containing 3, 2, 1, and 0 chlorines, or V units, determined from our simulation and compare them to the values observed for T C H at various degrees of (n-Bu) SnH reduction. Agreement between the simulated and observed reduction products of T C H based on the kinetics observed for both D C P and T C H is good. Simulation of the (n-Bu) SnH reduction of P V C is carried out in a manner similar to that described for T C H . Instead of beginning with 100 T C H molecules we take a 1000 repeat unit P V C chain that has been Monte Carlo generated to reproduce the stereosequence composition of the experimental sample of P V C used in the reduction to E-V copolymers (2), ie. a Bernoullian P V C with P =0.45. A t this point we have generated a P V C chain with a chain length and a stereochemical structure that matches our experimental starting sample of PVC. We select repeat units at random, and if they are unreduced V units a check of whether or not the units adjacent to the selected unit are Ε or V is made. Having determined the triad structure (both comonomer and stereosequence) of the repeat unit selected for reduction, we divide the relative reactivity of this E-V triad by the sum of relative reactivities for all V centered E-V triads as listed in Table III to obtain the probability of reduction. A random number between 0.0 and 1.0 is generated, and if it is smaller than the probability of reduction of the selected E-V triad, we remove the chlorine from the central V unit which becomes an Ε unit. If either of the terminal units of the E-V triad selected are V units, then we modify their relative reactivities to reflect changing the central unit from V to E. The degree of reduction χ is calculated from 100 X (# r

r

r

r


r

3

3

3

m


26.

JAMEISON ET AL.

371

Reduction of Poly(vinyl chloride)

of chlorines removed -f 1000), and if it corresponds to the desired level of reduction we print out the numbers of each type of triad remaining in the E - V copolymer. This whole procedure is repeated for several P V C chains until the fraction of each E - V triad type at each degree of reduction remains constant when averaged over the generated set of chains. Figure 8 presents a comparison of observed (see Table I) and simulated E - V triad composition plotted against the degree of overall reduction by (n-Bu) SnH. The agreement is excellent being much improved over that found for T C H reduction. This is at least partially a consequence of the relative accuracy of the C N M R data used to obtain the E - V triad compositions resulting from the reduction of P V C , because the T C H data is gathered during reduction and is an average over the time required to accumulate C N M R spectra (~ 10 min.), while E - V data is obtained on static samples removed from the reduction flask. In Figure 9 we have plotted the ratios of r / m W diads observed by C N M R in E - V copolymers obtained by the (n-Bu) SnH reduction of P V C (2). They are compared to the r / m ratios resulting from our computer simulation of P V C reduction made possible by the observation of the kinetics of (n-Bu) SnH reduction of D C P and T C H . The agreement is good, and provides us with a knowledge of E - V stereosequence as a function of comonomer composition. The excellent agreement between the simulated and observed reduction of P V C with (n-Bu) SnH means that both D C P and T C H are appropriate model compounds for the study of P V C reduction. D C P is useful to obtain kinetic information on the relative reactivities of m- and r-diads and W and E V diads. Reduction of T C H yields the relative reactivities of the central and terminal chlorines in the V W triads. The physical properties (7-10) of our E - V copolymers are sensitive to their microstructures. Both solution (Kerr effect or electrical birefringence) and solid-state (crystallinity, glass-transitions, blend compatibility, etc.) properties depend on the detailed microstructures of E - V copolymers, such as comonomer and stereosequence distribution. C N M R analysis (2) of E - V copolymers yields microstructural information up to and including the comonomer triad level. However, properties such as crystallinity depend on E - V microstructure on a scale larger than comonomer triads. For example, the amount and stability of the crystals formed in E - V copolymers depend on the number and length of uninterrupted, all Ε unit runs. Our ability to computer-simulate the (n-Bu) SnH reduction of P V C permits us to obtain this information concerning the longer comonomer sequences in the resultant E - V copolymers. In Figure 10 we present the percentage of Ε units that are found in uninterrupted, all Ε unit runs as a function of the length of each run and the over all degree of reduction. These data were obtained in two ways: i.) simulation of the (n-Bu) SnH reduction of P V C and ii.) assuming random removal of CI during the reduction. The simulated data in Figure 10 make clear that the numbers and lengths of all Ε unit runs in E - V copolymers obtained from the reduction of P V C with (n-bu) SnH are significantly reduced compared to those resulting from random CI removal. Though not shown in Figure 10, the percentage of Ε units in all Ε unit runs . . . V E , V . . . with χ > 29 is 27 % for random CI removal and just 14 % for the (n-Bu) SnH reduced P V C . This observation has to be considered when discussing the crystalline 3

13


13

13

3

3

3

13

3

3

3

3




372

X,%

REDUCTION

Figure 7. A comparison of the observed (symbols) and simulated (solid lines) comonomer triad distributions in (n-Bu) SnH reduced T C H . 3

PVC

X,% R E D U C T I O N

Figure 8. A comparison of the observed (symbols) and simulated (solid lines) comonomer triad distributions in (n-Bu) SnH reduced P V C . 3


26.


JAMEISON ET AL.

373

4.0h


3.0

2.0

40

20

60

80

X, % R E D U C T I O N

Figure 9. A comparison of the observed (O) and simulated ratios of r / m W diads during the (n-Bu) SnH reduction of P V C . 3

15

\ 70% REDUCTION /\\ -Γ

\

;

^ V.

\

Λ

; 90% \ \ Λ REDUCTION V I

Λη

/ \ / !- \f

A

\

\

A

/\

/ v

/ \

0

2

ι

4

I 6

1 8

T^-4>C^J»' 10 12 14

V

ι\ / ι Ν ί ι Ν 16 18 2 0 22 24 26 28

X(...VE V...) X

Figure 10. Percentage of Ε units in uninterrupted, all Ε unit runs as a function of run length and degree of reduction. Solid lines correspond to the computer simulation of P V C reduction with (n-Bu) SnH and the dotted lines to the simulated results assuming random CI removal. 3


374

CHEMICAL REACTIONS O N POLYMERS

morphology of E - V copolymers obtained by reduction of PVC with (n-Bu) SnH. With the (n-Bu) SnH reduction of PVC successfully simulated via the kinetic studies of DCP and T C H reduction, it remains to explain the mechanisms (14) of this reductive dechlorination. We need only consider the mechanisms of the reduction of DCP and T C H , because we have demonstrated that the kinetics of their reduction are the same as those observed for PVC when also reduced by (n-Bu) SnH. 3

3

3

POSSIBLE M E C H A N I S M F O R T H E R E D U C T I O N O F P V C W I T H (n-BU} SnH. The reductions of alkyl halides with (n-Bu) SnH are known (15) to be free-radical chain reactions, where the (n-BuJ Sn* radical (R») abstracts the halogen (X) from the alkyl halide (ROC) creating an alkyl radical (R'*) and (n-Bu) SnX. In Figure 11 (a,b) the rr and mm isomers of T C H are depicted in their most probable conformations (tttt and gtgi) (16), with their central CPs about to be abstracted by the R». Attack at the central chlorine by R« is hindered (14) by both adjacent methine protons in the tttt conformer of rr-TCH, while only a single methine proton obstructs the radical attack of the central chlorine in the gtgt conformer of mm-TCH. We would expect, as is observed, the mm isomer to be more readily reduced than the rr isomer based solely on considerations of steric interactions. 3

3

3


3

rr ( t t t t )

(a)

CH

(b)

,CH

3

3

mm

(gtgt)

t Figure 11. (a,b)-The rr and mm isomers of T C H in the tttt and gtgt conformations.


26.


JAMEISON ET AL.

375

Since the (n-Bu) Sn* radical (R#) is nucleophilic (17), a partial negative charge must be produced at the methine carbon whose chlorine is being abstracted. The rate of this abstraction should clearly be enhanced by electron-withdrawing groups on R'« due to their stabilization of this charge by inductive effects. As observed, the removal of CI from E W (or W diad) is expected to be more facile than from E V E (or V E diad) as a result of a 7-halogen effect in the former structure. Thus, the enhancements in chlorine removal from W diads compared to E V diads and from m - W diads compared to r - W diads observed in the (n-Bu) SnH reduction of D C P , T C H , and P V C are consistent with the free-radical chain reaction mechanism. Inductive effects produced by neighboring 7-Cl's tend to favor the reduction of W diads relative to E V diads and steric interactions resulting from different preferred conformations in each isomer favor the removal of CI from mW diads relative to r - W diads. 3


3

LITERATURE CITED

1. Starnes Jr., W. H.; Schilling, F. C.; Plitz, I. M.; Cais, R. E.; Freed, D. J.; Hartless, R. L.; Bovey, F. A. Macromolecules 1983, 16, 790, and references cited therein. 2. Schilling, F. C.; Tonelli, A. E.; Valenciano, M. Macromolecules 1985 , 18, 356. 3. Keller, F.; Mugge, C. Faserforsch. Textiltech. 1976, 27, 347. 4. Misono, Α.; Vehida, Y.; Yamada, K. J. Polym. Sci., Part Β 1967, 5, 401, and Bull. Chem. Soc. Jpn. 1967, 40, 2366. 5. Misono, Α.; Uchida, Y.; Yamada, K.; Saeki, T. Bull. Chem. Soc. Jpn. 1968, 41, 2995. 6. Hagiwara, M.; Miura, T.; Kagiya, T. J. Polym. Sci., PartA-11969, 7, 513. 7. Tonelli, A. E.; Schilling, F. C.; Bowmer, T. N.; Valenciano, M. Polym. Prepr., Am. Chem. Soc., Div. Polym. Chem. 1983, 24 (2), 211; and Tonelli, A. E.; Valenciano, M.; Macromolecules 1986, 19, 2643. 8. Bowmer, T. N.; Tonelli, A. E. Polymer 1985, 26, 1195. 9. Bowmer, T. N.; Tonelli, A. E. Macromolecules 1986, 19, 498. 10. Bowmer, T. N.; Tonelli, A. E. J. Polym. Sci., Polym. Phys. Ed. 1986, 24, 1631; and ibid 1987, 25, 1153. 11. Schilling, F. C.; Schilling, M. L. Macromolecules Submitted. 12. Tonelli, A. E.; Schillling, F. C.; Starnes, Jr., W. H.; Shepherd, L.; Plitz, I. M. Macromolecules 1979, 12, 78. 13. Benson, S. W. "The Foundations of Chemical Kinetics"; McGraw -Hill:New York, 1960. 14. Starnes Jr., W. H.; Schilling, F. C.; Abbas, Κ. B.; Plitz, I. M.; Hartless, R. L.; Bovey, F. A. Macromolecules 1979, 12, 13. 15. Carlsson, D. J.; Ingold, K. U. J. Am. Chem. Soc., 1968, 90, 7047. 16. Flory, P. J.; Pickles Jr., C. J. J. Chem. Soc., Faraday Trans. 1973, 69, 2. 17. Grady, G. L.; Danyliw, T. J.; Rabideux, P. J. Organomet. Chem. 1977, 142, 67. RECEIVED September 23, 1987


Chemical Reactions on Polymers - American Chemical Society

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