Cationic Polymerization and Copolymerization of Trioxane - Advances

23 Cationic Polymerization and Copolymerization of Trioxane

Downloaded by UNIV OF ROCHESTER on September 2, 2017 | http://pubs.acs.org Publication Date: June 1, 1969 | doi: 10.1021/ba-1969-0091.ch023

High Resolution NMR Investigation of Polymerization Mechanism CATHERINE S. HSIA CHEN and A. DI EDWARDO Celanese Research Co., Box 1000, Summit, N. J.

The

polymerization

resolution

nuclear

of trioxane has been studied magnetic

zation the concentration steady state, independent

resonance.

of open-chain polymers reaches a of catalyst (BF3

tration. In copolymerization ing

of the ethylene

oxide occurs

2

immediately

on adding

completely while the bulk of

trioxane still remains. A surprisingly

high concentration of

builds up during copolymerization,

a maximum immediately Although

• Bu O) concen

with ethylene oxide, ring open

catalyst and is copolymerized formaldehyde

by high

In homopolymeri-

before formation of solid

the maximum formaldehyde

with ethylene oxide and/or

reaching polymer.

concentration

varies

catalyst concentration, the con

centration of open-chain polymer remains relatively constant regardless of monomer and/or

catalyst

concentration.

Τ η the cationic polymerization and copolymerization of trioxane i n the *** melt or i n solution, an "induction period" usually exists, during w h i c h no

solid polymer is formed and the reaction medium remains clear.

Nevertheless, reactions are known to occur during this period. B y using BF3

or an etherate as catalyst, i n homopolymerization, Kern and Jaacks

(I)

reported the formation of formaldehyde via depolymerization of

polyoxymethylene cations.

Θ

Θ

M A A ^ O - C H H r O - C H , -> Λ Λ Λ ( _ 0 — C H Ô - ^ Τ Τ Γ Ο — C H o + C H 0 2

359

Platzer; Addition and Condensation Polymerization Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1969.

360

ADDITION

AND

CONDENSATION

POLYMERIZATION

PROCESSES

M i k i , Higashimura, and Okamura (2) reported the formation of tetraoxane from polyoxymethylene cations by a 'TDack-biting" mechanism:


Φ ~0—CH2—O—CH2—O—CH2—O—CH2—O— CH2

—O—CH 2 —Ο

CHo

I

I

CH2 \

Ο

—

Θ ~ 0 - C H

2

•

+

CH2 I Ο

/

Ο—CH2

CH2 I Ο I

CH2

I

CH2

In the copolymerization with ethylene oxide, Price and M c A n d r e w (3) reported the formation of 1,3-dioxolane, presumably also by a back biting mechanism. Chromatography has been the most commonly used analytical tool for following these reactions. It appeared that high resolu tion nuclear magnetic resonance ( N M R ) spectroscopy should be a good analytical method for monitoring the dynamic polymerization during the clear period before solidification. This paper describes the results of in-situ N M R investigations of homopolymerization of trioxane and its copolymerization with ethylene oxide i n the melt at 6 5 ° C , the polymeri zation mechanisms based on these results, and the limitation of N M R as an analytical method for trioxane polymerization.

Experimental

Materials. Trioxane (Celanese) was purified by refluxing over metallic sodium followed by distillation (b.p., 1 1 4 . 3 ° C ) . Ampoules of ethylene oxide (Eastman white label) were opened immediately before use. The purity was established by mass spectrometry. Boron trifluoride dibutyl etherate (Eastman white label) was used without further purification. N M R Specifications and Calibrations. The N M R spectra were ob tained on a Varian high resolution N M R spectrometer, model A-60-A. The temperature was controlled with a Varian V-6040 variable heater unit. The instrument was tuned to a maximum resolution (0.3 c.p.s.) under reaction conditions (using the monomers) with no catalyst present. A l l spectra were recorded at 6 5 ° C . ( ± 1 ° C ) . A n accurate chemical shift for the trioxane proton resonance (vs. T M S ) was obtained by recording a spectrum at low gain. D u r i n g the actual homo- and copolymerizations, high spectrum amplitudes were used so that new proton in-growth signals could be observed easily. The chemical shifts of these new proton signals


23.

CHEN

A N D DI

EDWARDO

361

Trioxane


CH2

5.17 5.07 PPM

(8)

Figure 1. Homopolymerization of trioxane in bulk at 65°C.

were determined using the weak 1 3 C H resonance peak of trioxane (J = 186 c.p.s. ) since in most cases the trioxane proton resonance was appreciably off scale. Intermittently, the spectra were integrated to determine the concentration of different species in the dynamic system. The following model compounds were used to help interpret the new proton signals appearing during the polymerizations: diethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, and Carbowax 400. Saturated solutions of gaseous formaldehyde in trioxane and trioxane-ethylene oxide mixtures were also recorded to determine the chemical shift of formaldehyde monomer in these matrices. Polymerizations. Polymerizations were carried out in the spectrophotometer, and spectra were recorded at appropriate intervals until solidification took place. Under nitrogen atmosphere and at 65 ° C , polymerization mixtures were prepared in separate larger glass vessels. After mixing thoroughly, they were transferred to NMR tubes, flushed with nitrogen, and capped immediately. Before adding catalyst to the larger vessel, a sample was withdrawn and placed in the spectrometer for the


362

ADDITION

A N D CONDENSATION

POLYMERIZATION

PROCESSES

dual purpose of optimizing the resolution of the instrument and deter mining the ethylene oxide content. All glassware, including the NMR tubes, were base treated in an alcoholic K O H bath before final cleaning and drying to eliminate inadvertent polymer formation catalyzed by untreated glass surface. Results

and

Discussion


Homopolymerization. Figure 1 shows the spectra of the homopoly merization of trioxane with various concentrations of B F 3 · Bu 2 0 catalyst.

SOLID POLYMER 27 MM.

21 MIN.

.-g?to-cHt-c>trow
MIN. κ

J

w

Ο

§

JL

WW

O MIN^C M IS

I I I I 5.17 5.07 4.94 4.87

I 4.0

I 3.6

PPM ( » )

Figure

2. Copolymerization of trioxane with 1.5 mole % ethylene oxide in bulk at 65° C. BF3 · Bu20: 1.0 X 10s mole %



23.

CHEN

Figure

3.

A N D DI

EDWARDO

Copolymerization bulk aï^5°C.

363

ΤήΟΧαΠβ

of trioxane with 3.3 mole % ethylene BF · Bu 0: 1.0 X 10~ mole % 3

2

oxide in

3

The trioxane proton signal appears at 5.17 p.p.m. (vs. T M S ) . Since the trioxane protons are magnetically equivalent, they appear as a sharp single peak. As the polymerization proceeds, the appearance of a new proton signal occurs at 5.07 p.p.m. This signal appeared and remained relatively constant during the clear period of the polymerization regardless of catalyst concentration. N o change i n the spectra occurred until the polymer precipitated from solution, when broadening and, consequently, loss of resolution occurred; hence, meaningful spectra could not be obtained. Assignment of this new peak was attributed to the low molecular weight-soluble open-chain polymer—viz.,—f-CH 2 —0-)ir: As would be expected, an increase in catalyst concentration increased the


364

ADDITION

AND

CONDENSATION

1/

POLYMERIZATION

PROCESSES

«V*

SOLID POLYMER


4.5 min.

C3'H

χ

4-0*20*,,

.•KH2-ÇïÎ -0->2

3 min.

2 min,

5LA

CHC -H 2

0 min. I I I I

I 4.0

5.17 5.07

2

1

I &6

4*4

4.87

4(B)

Figure

4.

Copolymerization in bulk at 65°C.

of tnoxane with 3.3 mole % ethylene BFS · Bu20: 2.2 X 10s mole %

oxide

rate i n attaining the steady-state concentration and decreased the clear period of polymerization. N o formaldehyde was detectable even with high spectrum amplitude. Kerr and Jaacks ( J ) reported an equilibrium concentration of 60 mmoles/liter for the homopolymerization of trioxane i n methylene chloride at 3 0 ° C . In our systems, the concentration of formaldehyde, if formed, is probably low and therefore w i l l not be de-


CHEN AND DI EDWARDO

365

ΤΗοχαΠβ


23.

""β.07 4 § 4

Figure 5.

PPM (8)

Copolymerization of trioxane with 7.2 mole % ethylene oxide in bulk at 65°C. BF · Bu 0: 2.2 X 10 mole % 3

2

s

tected by this N M R method. Tetraoxane was not detected i n our N M R study for the following reasons: (1) low concentrations, and (2) the chemical shift difference in proton resonance from that of trioxane would probably be negligible. Copolymerization with Ethylene Oxide. Figures 2-6 show the N M R spectra of copolymerization of trioxane and ethylene oxide where the


366

ADDITION AND CONDENSATION POLYMERIZATION PROCESSES

effects of catalyst concentration and of ethylene oxide concentration can be observed. Features common to all copolymerization investigations are: (1) Opening of the ethylene oxide ring occurred immediately upon adding catalyst. ( 2 ) The polymerization remained i n 'clear stage" for some time even after ethylene oxide was no longer detectable.


(3) In-growth of new proton signals appeared. The new proton signals and their assignments are summarized i n Table I.

SOLID POLYMER APPEARED

bJ

i

Figure 6.

Copolymerization of trioxane with 10 mole % ethylene oxide in in bulk at 65° C. BF3 · Bu20: 6.0 X 10~3 mole %


23.

CHEN

A N D DI

EDWARDO

367

ΤήΟΧαΠβ

Although no appreciable difference in the in-growth of new proton signals was observed as the catalyst concentration or the ethylene oxide concentration was varied, differences were observed in the relative inten sities of the proton signals—viz., the C H 2 0 resonance at 4.94 p.p.m. Table I. Assignment o£ New Proton Signals During the Copolymerization of Trioxane and Ethylene Oxide Assignment Downloaded by UNIV OF ROCHESTER on September 2, 2017 | http://pubs.acs.org Publication Date: June 1, 1969 | doi: 10.1021/ba-1969-0091.ch023

P.p.m. (Vs. TMS)

O-CfL

CH 2 5.17 5.07

Ο

^^O—CH£ -4-CH -0^ 2

4.94

CH 0

4.87

—CH 2 —O—CH 2 —CH 2 —O—

3.60-4.0a

2

—(- ÇH 2 —CH 2 —Ο—)— χ ^n ^ 3 CH9—CHo

2.65 A C H peak symmetric to the one on the left of the trioxane peak also appears in this chemical shift range. α

U

The concentration of open-chain polymers, similar to what was observed i n the homopolymerization, reached a steady state and could be ascertained easily. However, the concentration of C H 2 0 increased rapidly as the polymerization progressed, reaching a maximum immedi ately before precipitation of the polymers. A n attempt to measure the maximum formaldehyde concentration was made by integrating the spectra immediately before solidification. This was somewhat difficult since the polymer precipitates from solution rapidly. Therefore, quanti tative estimates may not represent the maximum concentrations of CH2O but are a close approximation. The steady-state concentration of open-chain polymers and maximum concentration buildup of C H 2 0 under various catalyst and ethylene oxide concentrations are summarized i n Table II. In the present investigation the reactivity and distribution of ethylene oxide observed agreed with previous results (3) using chemical analysis and chromatography. A significant finding was the buildup of surprisingly high C H 2 0 concentration during copolymerization i n contrast to homo-


368

ADDITION AND CONDENSATION POLYMERIZATION PROCESSES

polymerization. In separate experiments, it was ascertained that when gaseous C H 2 0 was bubbled into molten trioxane at 6 5 ° C , the maximum solubility of C H 2 0 , before formation of solid polyformaldehyde, was 2-3 mole % ; however, when ethylene oxide was present i n trioxane, the solubility of C H 2 0 increased sharply (see Figure 7). The formaldehyde


Table II. Effect of Catalyst and Ethylene Oxide Concentrations on Concentrations of Open-Chain Polymers and Formaldehyde Mole %

Ethylene

Catalyst (X

Oxide

1.5 3.3 3.3 7.2 10

lfr3)

Open-Chain Polymer (Steady-State)

3.7 3.5 3.2 3.4 3.0

1.0 1.0 2.2 2.2 6.0

(Max.

CH20 Measured)

9.0 29 12 28 28

concentrations were determined by N M R , and the lower-limit detection was 1 mole % . The increased solubility of C H 2 0 i n the copolymerization systems elucidate, at least partially, the lower rates of polymerization and lower molecular weights of polymers invariably obtained in copolymerization compared with homopolymerization. Since C H 2 0 is formed from the depolymerization of the propagation chains, and at equilibrium,

* oi y r a .[CH 2 0] [POM+] ^* oi y m.[POM + ] P

deP

Therefore, if C H 2 0 were soluble i n the reaction medium, it would be deactivated, and the equilibrium consequently displaced to the left. As a result, depolymerization is favored. Table II shows that an increase i n ethylene oxide concentration (cf., Rows 1 with 2 and 3 with 4) increased the maximum buildup of C H 2 0 concentration. A n increase i n catalyst concentration (cf., Row 2 with 3) decreased the maximum buildup of C H 2 0 concentration. The steady-state concentration of open-chain polymers nevertheless was insensitive to both ethylene oxide and catalyst concentrations. Based on the spectra obtained from copolymerizations (Figures 2-6), the presence of 1,3-dioxolane could not be ascertained. This again might be attributed to low concentrations which could not be detected by N M R . Parallel copolymerization experiments were carried out, vaporphase aliquots were analyzed during the polymerization by mass spectrometry, and the formation of 1,3-dioxolane was detected.



23.

CHEN AND DI EDWARDO

PPM Figure

7.

Solubility

of CH20

369

Ύήοχαηβ

(9)

in trioxane mixture

and

trioxane-ethylene

oxide

Conclusion

H i g h resolution N M R spectroscopy is advantageous and unique for studying the homo- and copolymerizations of trioxane. Reactions involv ing changes i n proton magnetic resonance which are not ascertained conveniently by other analytical means have been revealed during the "clear period" of polymerization usually regarded as the induction period. The appearance (and equilibrium concentration) of soluble linear polyoxymethylene chains has been determined for the homo- and copoly merizations. The extremely high buildup of formaldehyde concentration during copolymerization has been revealed, and more insight into the polymerization mechanisms has been gained. N M R , like other instrumental techniques, has limiting factors which preclude complete insight into changes occurring during a chemical reaction. One of the major disadvantages is its limiting sensitivity which precludes detection of chemical moieties at low concentrations. The formation of formaldehyde and/or tetraoxane reported for the homopolymerizations (1,2), and 1,3-dioxolane for the copolymerization could not be verified.


370

ADDITION

A N D CONDENSATION

POLYMERIZATION

PROCESSES

The combined techniques of N M R , gas chromatography, and mass spectrometry, together with polymerization kinetics, should be sufficient to understand the complicated mechanism of trioxane polymerization. Literature

Cited

(1) Kern, W., Jaacks, J., J. Polymer Sci. 48, 399 (1960). (2) Miki, T., Higashimura, T., Okamura, S., J. Polymer Sci. Pt. A-1, 5, 95 (1967). Downloaded by UNIV OF ROCHESTER on September 2, 2017 | http://pubs.acs.org Publication Date: June 1, 1969 | doi: 10.1021/ba-1969-0091.ch023

(3) Price, M. B., McAndrew, F. B., J. Makromol. Sci. Pt. A-1, 2, 231 (1967). RECEIVED

March 18, 1968.


Cationic Polymerization and Copolymerization of Trioxane - Advances

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