Polymer Characterization - American Chemical Society

28 C-NMR Spectra of Cross-Linked Poly(styrene-co-chloromethylstyrene) Gels

13

WARREN T. FORD and T. BALAKRISΗΝAN 1

2

Downloaded by CHINESE UNIV OF HONG KONG on March 6, 2016 | http://pubs.acs.org Publication Date: June 1, 1983 | doi: 10.1021/ba-1983-0203.ch028

Oklahoma State University, Department of Chemistry, Stillwater, OK 74078 C-NMR

spin lattice relaxation times, line widths, nu clear Overhauser effects (NOE), and relative signal areas at 25.2 MHz in CDCl at 30 °C were measured for copolymers of styrene with 25 wt % chloromethylstyrenes cross-linked with 0-10% divinylbenzene (DVB). As cross-linking increases, T is almost invariant, line widths increase markedly, NOE ratios decrease signifi cantly, the aliphatic signal area decreases markedly, and the aromatic signal area remains constant up to 6% cross-linker and decreases with 10% cross-linker. The re sults are discussed in terms of distributions of correla tion times for polymer motions.

13

3

1

SOLUTIONS OF POLYSTYRENE

have b e e n investigated extensively by d y n a m i c Ή - a n d C - N M R spectroscopy to characterize rotational motions of the p o l y m e r chains. Relaxation times ΤΊ a n d T a n d nuclear Overhauser enhancement ( N O E ) factors of C require models w i t h broad distributions of correlation times to fit the data (1—6). S o l i d polystyrene also has b e e n investigated b y C - N M R spectroscopy b y u s i n g cross-polarization (CP) a n d magic angle s p i n n i n g ( M A S ) to f i n d rotating frame correlation times ( r ) , C P relaxation times ( r ) , a n d N O E values that also are interpreted as due to broad distributions of motional frequencies (7, 8). W e report here C - N M R relaxation mea surements on styrene copolymer gels that are i n s o l u b l e because of d i v i n y l b e n z e n e ( D V B ) cross-linking, but are h i g h l y s w o l l e n b y C D C 1 to give motional freedom more l i k e that of l i q u i d s than that of solids. In previous C - N M R investigations of p o l y m e r gels, h i g h resolution spectra were obtained i n good solvents without recourse to C P , d i p o 1 3

2

1 3

1 3

lp

CH

1 3

3

1 3

1 2

To whom correspondence should be addressed. On leave, 1980-81, from University of Madras, India.

0065-2393/83/0203-0475$06.00/0 © 1983 American Chemical Society

Craver; Polymer Characterization Advances in Chemistry; American Chemical Society: Washington, DC, 1983.

476

POLYMER CHARACTERIZATION

lar d e c o u p l i n g , a n d M A S (9-11). D o s k o c i l o v a and co-workers (12-14) performed M A S H - and C - N M R experiments on a solvent-swollen cross-linked polystyrene a n d interpreted the spectra i n terms of narrow l i n e w i d t h signals, t y p i c a l of polymers i n solution, superposed o n dipolar broadened signals, t y p i c a l of glassy polymers. C r o s s - l i n k e d polystyrenes c o m m o n l y are used as p o l y m e r i c supports for synthesis a n d catalysis, i o n exchange resins, a n d size e x c l u sion chromatography c o l u m n packings. Interest i n polymer-supported synthesis and catalysis p r o m p t e d us to study the motional behavior of the cross-linked s t y r e n e - c h l o r o m e t h y l s t y r e n e copolymer gels w e h a d prepared for catalyst supports. These samples cover a range of crossl i n k i n g from 0.5 to 10% b y w e i g h t d i v i n y l b e n z e n e ( D V B ) . T h e results are the first systematic data o n h o w degree of cross-linking of gels affects their C - N M R spectra.


1

1 3

1 3

Experimental A l l C - N M R spectra were obtained with a Varian XL-100(15) spectrometer equipped with a Nicolet T T - 1 0 0 P F T unit at 25.2 M H z and ambient temperature (ca. 30 °C) under conventional high resolution conditions. Details of sample preparation and spectral conditions are published elsewhere (15). 13

Results A l l copolymers examined contained 25 w t % chloromethylstyrene, styrene, and 0 - 1 0 % D V B . Spectra were obtained as C D C 1 solutions or C D C l - s w o l l e n gels. C h l o r o f o r m is an excellent solvent for the copolymers, g i v i n g the largest volume expansion a n d the narrowest C - N M R l i n e widths of any solvent w e have found. T h e l i n e widths at half-height i n spectra of copolymer solutions (Table I) do not d e p e n d o n concentration u p to 25 w t % . T h e broader lines i n spectra of cross-linked polymers are due to the cross-links, not to viscosity changes, as can be seen b y a comparison of the data on the 2 5 % solution w i t h the data o n the 4% cross-linked g e l . C r o s s - l i n k i n g increases the breadth of the aromatic region of the spectra dramatically as the ortho-, meta-, a n d para-carbon resonances merge into a single broad b a n d that even overlaps w i t h the ipso-carbon resonance at ^ 4 % cross-linking. A l i p h a t i c carbon signals also become broader as cross-linking increases u n t i l w i t h 10% cross-linker no distinct peaks for the backbone methine and chloromethyl carbon atoms can b e seen. T h e aliphatic b a n d appears to broaden less than the aromatic b a n d as cross-linking increases, b u t the aliphatic signal area decreases markedly as cross-linking increases. Data for T and N O E ratios are i n T a b l e I I . A s cross-linking i n creases, there is a n apparent m i n i m u m i n T of the aromatic carbon 3

3

1 3

1

t


FORD AND BALAKRISHNAN

28.

Cross-Linked Polystyrene Gels

T a b l e I. L i n e W i d t h s i n C - N M R Spectra of C r o s s - L i n k e d Styrene/Chloromethylstyrenes i n C D C 1 1 3

3

Wt% Polymer in CDCl


Sample, %DVB

Line width, Hz a

3

10 19 25 11 10 17 26 30 33

0 (soluble) 0 0 0.5 1.0 2.0 4.0 6.0 10.0

o, m

Ρ

Methine

CH Cl

20.5 21.5 21 25.5 40 95

14 15 15 20

15 15 15.5 14 16 22.5 36 65

8 8 9 9 11.5 15.5 15 34

6

b 180 240 470

2

6

6

W t % p o l y m e r i n s o l v e n t - s w o l l e n b e a d s e x c l u d i n g s o l v e n t i n interstitial spaces. C o u l d not b e m e a s u r e d .

a

b

atoms at 4% D V B , but the largest and smallest T values i n the series are almost w i t h i n experimental error of one another. T h e aliphatic carbon T values and the chloromethyl carbon T values decrease slightly as the degree of cross-linking increases. Previous reports of polystyrene C Τ χ values show a small solvent dependence w i t h the longest Τχ values i n the least viscous solvents (16-23). O u r T values for soluble copolymers i n C D C 1 agree w i t h i n 10% w i t h earlier data on polystyrene solutions i n l o w viscosity solvents at 3 0 - 4 0 °C w h e n spectrometer frequency differences are taken into account. T h e N O E ratios i n T a b l e II for the soluble polymers agree w i t h literature values at 3 0 - 4 0 °C for atactic polystyrene a n d for r i n g - m e t h y l derivatives of x

x

t

1 3

x

3

T a b l e II. C T and N O E Values of C r o s s - L i n k e d Styrene/Chloromethylstyrenes i n C D C 1 1 3

t

3

Sample, %DVB 0 1.0 2.0 4.0 6.0 10.0 a

b

are c

T,, ms

NOE Ratio"

a

o, m

ρ

CH

CH

CH Cl

123 112 100

106 102 100

97 93 72 73 78

65 50 56 51 72

141 155 132 104 110

93 113 c

C

2

C

2

c

Aromatic

Aliphatic

2.14 2.10 1.57 1.58 1.40 1.55

2.25 2.25 1.94 1.85 1.88 1.92

E s t i m a t e d error l i m i t s are ± 1 0 % or ± 1 0 m s , w h i c h e v e r is greater. S i g n a l area f u l l y c o u p l e d + s i g n a l area gated d e c o u p l e d . E s t i m a t e d error l i m i t s ±0.2. Not determined.



478


polystyrene to ± 0 . 1 (17-23). T h e N O E ratios decrease as crossl i n k i n g increases, a n d the decrease is greater for the aromatic signals than for the aliphatic signals. Peak areas per carbon atom for the aliphatic a n d aromatic reso nances i n gated d e c o u p l e d spectra are compared w i t h the peak area per carbon atom of a p o l y e t h y l e n e g l y c o l ( P E G ) internal standard i n T a b l e I I I . Some of the gated d e c o u p l e d spectra are i n F i g u r e 1. T h e a r o m a t i c / P E G area ratio is effectively 1.0 for samples w i t h up to 6% cross-linking, but the a l i p h a t i c / P E G a n d aliphatic/aromatic area ratios decrease as c r o s s - l i n k i n g increases to 6%. B o t h aromatic and aliphatic peak areas are r e d u c e d at 10% cross-linking. T h e results were con f i r m e d for some samples b y comparisons of peak areas i n fully c o u p l e d spectra. A g r e e m e n t b e t w e e n the gated d e c o u p l e d and fully c o u p l e d aliphatic/aromatic area ratios is remarkably good. T h e Τι, l i n e w i d t h , a n d N O E data for soluble polystyrenes cannot be e x p l a i n e d b y a single rotational correlation time (r ) for motion of backbone carbon atoms, or by two correlation times for aromatic car b o n atoms due to p o l y m e r backbone motion and to l i b e r a t i o n of the aromatic rings (1—6). N e i t h e r can the cross-linked polystyrene C relaxation data be e x p l a i n e d b y a single r . Several methods are avail able for calculation of distributions of correlation times i n soluble polymers (1 - 6 ) , b u t we choose not to apply them and to discuss our results only qualitatively because none of the measurements truly represent the entire sample. T h i s point w i l l be elaborated on later. As cross-linking increases, T values of backbone a n d aromatic C n u c l e i change little, l i n e widths increase greatly, and N O E ratios de crease significantly. C r o s s - l i n k i n g causes l o w frequency motions i n the p o l y m e r that affect T a n d N O E processes but have little effect on T j (7). T h e small decreases i n T of the — C H C 1 group as cross-linking increases indicate that those T values are on the motional n a r r o w i n g side of the m i n i m u m value of T attained at r ~ 5 x 10" s i n a 2 5 . 2 - M H z C experiment. T h e internal rotational m o t i o n of the - C H C 1 group makes its T longer than that of any other protonated carbon atom i n the p o l y m e r i n a l l samples. T h e l i n e widths i n a C - N M R spectrum of s o l i d polystyrene ob tained w i t h dipolar d e c o u p l i n g a n d M A S (24) may be compared w i t h the l i n e widths i n the spectrum of a cross-linked styrene copolymer gel. I n the former case (Figure 5 of Reference 24), the aromatic and aliphatic l i n e widths are about 300 and 900 H z , yet i n the most nearly comparable spectrum i n our work, that of the 6% cross-linked p o l y m e r i n F i g u r e 1, the aromatic l i n e w i d t h is 240 H z w i t h an additional broad shoulder on its l o w f i e l d side, but the methine and C H C 1 signals i n the aliphatic region still clearly are seen and are only 65 and 28 H z w i d e . I f we assume that the l i n e widths i n the spectrum of s o l i d c

1 3

c

1 3

t

2

x

2

x

l9

9

c

1 3

2

x

1 3

2



f

e

d

c

b

a

0.84 0.86 0.76 0.50 0.45 0.20

0.96 1.12 1.06 0.93 1.04 0.48

0.91 0.84 0.77 0.60 0.58

c

/

c

c

C

/

c

c

d

C

C

0.89

b

Fully Coupled Spectra Aliphatic/ Aromatic

e

0.82

0.98 0.87 0.84 0.87 0.88 0.77 0.72 0.54 0.43 0.42

0.89

Aliphatic/ PEG

Aromatic/ PEG

S e e T a b l e I, footnote a. S i g n a l areas p e r c a r b o n a t o m . P E G , M W 1500, is u s e d as i n t e r n a l s t a n d a r d . Not determined. A v a l u e o f 0.51 i n a separate d e t e r m i n a t i o n w i t h shorter d e l a y b e t w e e n a c q u i s i t i o n s . A v a l u e o f 0.56 i n a separate d e t e r m i n a t i o n w i t h shorter d e l a y b e t w e e n a c q u i s i t i o n s . C o u l d not b e m e a s u r e d .

Polystyrene Uncross-linked copolymer 10% s o i n 19% soin 25% soin 0.5% D V B 1.0% D V B 2.0% D V B 4.0% D V B 6.0% D V B 10.0% D V B

a

Sample

0

Aliphatic/ Aromatic

13

Table III. Aromatic and Aliphatic C-Signal Areas in Cross-Linked Polystyrenes Gated Decoupled Spectra


480


200

16Q

ι—

1

1

1

120

PPm

80

1

1

1

40 τ

ι

"


6% DVB

Figure 1. Gated decoupled C-NMR spectra of polystyrenes containing 25 wt % chloromethylstyrenes and 0—6% DVB. Spectra of cross-linked samples were taken with a 10-s delay between acquisitions and contain PEG as internal standard. The CDCl and PEG peaks have been trun cated. The spectrum of soluble polymer was taken with a 1.0-s delay between acquisitions, and the sample contains no PEG. Peak assign ments are aromatic C 145.3 ppm; aromatic C 6, 127.9 ppm; aromatic C ,125.8 ppm; CH Cl, 46.4 ppm; backbone methylene, 40—47 ppm; and backbone methine, 40.4 ppm. 13

3

ly

4

235

2

polystyrene are due strictly to T processes (all dipolar couplings a n d chemical shift anisotropies were removed), then comparison w i t h spectra of cross-linked gels indicates that cross-linking limits the aro matic r i n g motions that affect aromatic C T values more than it limits backbone motions responsible for T values of a l l C n u c l e i . That assumption seems unreasonable. A better explanation for the relative differences i n l i n e widths b e t w e e n the spectra of s o l i d polystyrene and of polystyrene gels is that M A S f a i l e d to remove a l l of the dipolar broadening i n the aliphatic region of the spectrum of the s o l i d . 2

1 3

2

2

1 3


FORD AND BALAKRISHNAN

28.

Cross-Linked Polystyrene Gels

A 1 5 - M H z C P / M A S spectrum of our 10% cross-linked copolymer as a dry solid has l i n e widths of about 500 H z for the protonated aromatic C peak and for the aliphatic C peak w i t h 2600-Hz s p i n n i n g . T h e broader aromatic yet narrower aliphatic l i n e widths i n the spectrum of the cross-linked polymer, compared w i t h that of polysty rene i n Reference 21, also suggest that 2 - K H z M A S failed to remove all of the dipolar broadening i n the aliphatic region of polystyrene i n the p i o n e e r i n g experiments of Schaefer (24). T h e most remarkable effect of cross-linking on C - N M R spectra of the styrene copolymer gels is the decrease i n signal area of the aliphatic resonances relative to both the internal standard and the aromatic signal area. T h e same effect occurs i n gated d e c o u p l e d spectra that have a pulse repetition time of 11.5 s and i n completely c o u p l e d spectra that have a pulse repetition time of 0.75 or 1.5 s. Therefore, the signal area losses cannot be due to large increases i n Τ χ of part of the aliphatic carbon atoms. T h e relaxation time Τ χ increases w i t h correlation time w h e n r > 10 s. Previous investigations of f i brous proteins (25) and of hydrophilic polymer gels (JO) also found de creased intensity or absence of some signals i n C - N M R spectra. T h e most l i k e l y explanation for the disparity i n signal areas is that crossl i n k i n g limits motion of some of the backbone carbon n u c l e i so that they are dipolar coupled, a n d the resulting broad signals are not dis tinguishable from base l i n e i n our spectra. Effectively, the aliphatic/ P E G and a r o m a t i c / P E G signal area ratios indicate the fractions of aliphatic and aromatic carbon atoms that undergo reorientation at a frequency > 1 0 s" . L o w frequency motions (

Polymer Characterization - American Chemical Society

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