An NMR Study on the Bulk Cationic Copolymerization of Trioxane with

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Chapter 18

A n N M R Study on the Bulk Cationic Copolymerization of Trioxane with 1,3-Dioxepane 1

1

1

Min-HuiCui ,Yuli Zhang , Mark Werner , Nan-Loh StevenP.Fenelli , and John A. Grates 2

1,*

Yang ,

2

1

Department of Chemistry, City University of New York, College of Staten Island, 2800 Victory Boulevard, Staten Island,NY10314 Celanese AG, 90 Morris Avenue, Summit,NJ07901 2

The mechanism of cationic copolymerization of trioxane with 1,3-dioxepane was investigated using in situ C NMR based on microsequence data established via P F G - H M Q C and P F G - H M B C two-dimensional N M R . Kinetic profiles of pentad sequences and butylene oxide (CH CH CH CH O) counit placement were obtained. The incorporation of 1,3-dioxepane into the copolymer sequences was found to involve its initial homopolymerization followed by redistribution of counits through chain transfers, e.g. transacetalization. Unlike the copolymerization of trioxane with dioxolane, the insertion of formaldehyde into 1,3-dioxepane to form a larger acetal ring prior to substantial copolymerization does not play a significant role. For the first time nonad sequences were observed directly by N M R analysis of the copolymer. 13

2

228

2

2

2

© 2003 American Chemical Society

229

Introduction


The cyclic acetal 1,3-dioxepane (DOP) can be copolymerized with T O X to form copolymers having properties comparable to other acetal resins:

2

\

/ ζ

£

4

initiator (TOX/DOP copolymer)

(13-dioxepane DOP)

(U^-trioxane TOX)

(M: CH 0; B: CH CH CH CH 0) 2

2

2

2

2

Only limited information on the TOX/DOP copolymer is available from the literature (1,2). Commercial products from T O X and DOP coplymerization, such as Ultraform, have been described in patents. The initiation mechanism of bulk cationic copolymerization of T O X with DOP has not been reported, although homopolymerization of DOP was first described in 1935 (3). The polymerization of DOP can be initiated with various types of initiators. N M R spectra show that the polymer of DOP consists of regular sequences of - 0 - C H - 0 - ( C H ) - units derived from DOP. For T O X copolymerization with ethylene oxide (EO), E O was first converted to low molecular weight copolymer and cyclic oligomers such as dioxolane (DOL) and 1,3,5-trioxepane (TOP) (4). For either E O and D O L as comonomer with T O X , they were found to be preferentially incorporated during the induction period (5). We report here an investigation on the copolymerization of T O X with DOP using in situ C N M R based on data obtained from two-dimensional N M R . The microstructure of T O X / D O P copolymer, including pentad, heptad, and nonad sequences, is developed. The kinetic profiles of comonomer DOP, counit butylène oxide (B; C H C H C H C H 0 ) , and microsequences of the counit distribution in copolymer are also obtained. The initiation mechanism and the process of the incorporation of butylène oxide ( Έ ' ) counits originating from DOP are discussed through analyzing kinetic data obtained from C N M R . Nonad copolymer sequences are also observed and monitored for the first time. 2

2

4

1 3

2

2

2

2

1 3

230

Experimental


Materials Trioxane, 1,3-dioxepane, j boron trifluoride diethyl etherate were obtained from Aldrich Chemical Co. T O X was refluxed continuously in the presence of sodium for at least 48 hours to remove trace amounts of water in a Fuchs style reflux apparatus under dry argon at 120 °C. Dioxepane was purified using a similar procedure. Initiator B F O E t was added with dried 1,4-dioxane as carrier. a n (

3

2

Copolymerization of T O X with DOP Copolymer samples for N M R structural analysis were prepared as follows. T O X and DOP were charged into dry polymerization tubes in the required molar ratio through a septum stopper. To initiate the polymerization, B F O E t solution was added to the monomer solution at 70 °C in an oil bath with constant stirring. The reaction was allowed to proceed for 20 hrs. The resulting copolymers were pulverized, the powder neutralized by stirring in methanol containing 1% triemanolarnine for 1 h to destroy remaining initiator, and were then collected by filtration. Unstable end groups were removed by base hydrolysis using dimethylformamide, benzyl alcohol and triethanolamine mixture (volume ratio: 50:49:1) (6). 3

2

NMR Measurements !

A Varian Instruments IJNYTYplus 600 N M R spectrometer operating at a H resonance frequency of 599.939 M H z and a C resonance frequency of 150.866 M H z was used for copolymer structural analysis. H N M R spectra for base hydrolyzed copolymers in DMSO-d^ were obtained at 140 °C using a 5mm Indirect Detection Probe (ID600-5, Nalorec Cryogenics Corporation). Parameters used were: delay time, 3 s; pulse angle, 24.3°; acquisition time, 3.506 s; and 128 transients for each spectrum For 2D N M R experiments, P F G - H M Q C and P F G - H M B C , a 5mm Quad Resonance Gradient Probe (IDQG600-5, Nalorac Corporation) was used. Pulse field gradient experiments were carried out at 50 °C on 0.7 mL polymer solutions in a mixed solvent of ca. 20% v/v l,l,l,3,3,3-hexafluoro-2-propanol in CDC1 . The 2D spectra were obtained by l 3

]

3

231 collecting 32 transients of 2048 points each for each of 640 increments. Spectral analyses were performed using Varian V N M R software.


Kinetic Experiments l 3

A Varian UNYTYplus 300 N M R spectrometer operating at a C frequency of 75.435 M H z was used for obtaining the in situ C N M R spectra of the bulk copolymerization of T O X with DOP at 70 °C. Parameters used were: delay time of 4 s, pulse angle of 37.4 degree, and an acquisition time of 1.19 s, with 128 transients for each spectrum. Since gated decoupling was not used, the results obtained can be considered only semi-quantitative. The comonomers T O X and DOP at a 9:1 molar ratio of methylene oxide unit (M) to butylène oxide unit (B) were injected into a dried 10 mm N M R tube. The acquisition of the first spectrum was followed by the addition of the initiator B F O E t at a molar ratio of 30 ppm to M . The process of acquisition was begun immediately after the initiation of the sample. Chemical shifts were as referenced on the T O X resonance at 93.23 ppm calibrated on the chloroform triplet at 77.00 ppm. 1 3

3

2

Results and Discussion

Assignment of Pentad Sequences of TOX/DOP Copolymer For the TOX/DOP copolymers butylène oxide units, Έ ' , are not expected to be adjacent to each other because a highly unstable carbonium ion on butylène unit would have to be the propagating center. A positive charge on the methylene carbon stabilized by α oxygen is greatly preferred, giving rise to what is perhaps best thought of as an oxycarbonium ion. Therefore, the number of pentad sequences observed is limited to nine: six 'M'-centered pentad sequences ( M M M M M , B M M M M , B M M M B , M B M M M , M B M M B , and M B M B M ) and three Έ'-centered pentad sequences ( M M B M M , B M B M M , and B M B M B ) , where ' M ' is a methylene oxide unit and ' B ' is a butylène oxide unit. The assignment of proton chemical shift of the pentad sequences was first established by comparison with pentad sequences of T O X / D O L copolymers (7). The counit CH CH CH2CH 0 ( Έ ' ) in T O X / D O P copolymer is expected to show higher shielding effect than C H C H 0 ( Έ ' ) in T O X / D O L copolymer. The chemical shifts of the pentad sequences of T O X / D O P copolymers are therefore usually upfield from the corresponding pentad sequences of T O X / D O L copolymers (Table I). Six ' Μ ' - centered pentad sequences from T O X / D O P were 2

2

2

2

2

232

assigned based on the data from T O X / D O L copolymer system. For each Έ ' unit, two different protons in Έ'-centered region were observed: MMCH2CH CH2CH 0 and B M C H 2 C H C H C H 0 . The resonance at 1.59 ppm was assigned to the two central methylene units of - O C H C H C H C H 0 - , B . 2

2

2

2

2


2

l

2

2

2

2

13

Table I. Proton ( B) and Carbon ( C) Chemical Shift* of Pentad Sequences from TOX/DOP and TOX/DOL Copolymers Pentad Sequence

Pentad Sequence

12

C (ppm)

Ή (ppm)

n

C (ppm)

Ή (ppm)

M M M M M

4.84

90.04

M M M M M

4.84

90.04

B M M M M

4.82

89.60

E M M M M

4.83

90.74

B M M M B

4.80

89.18

E M M M E

4.82

89.34

M B M M M

4.72

92.74

M E M M M

4.75

92.92

M B M M B

4.70

92.24

MEMME

4.74

92.62

M B M B M

4.59

95.14

M E M E M

4.66

95.56

MMBÎ**

3.54

68.70

M M E M M

3.67

67.72

BMB]**

3.50

68.04

E M E M M

3.66

67,14

B **

1.59

26.13,

EMEME

3.64

67.22

2

26.07 *The proton chemical shift (Ή) data were obtained at 140 ° C with D M S O - d as solvent and referenced at 2.49 ppm of D M S O - d ; the carbon chemical shift ( C ) data were obtained at 50 ° C with solution of ca. 20% v/v l,l,l,3,3,3-hexafluoro-2-propanol in CDC1 as solvent and referenced on chloroform triplet 77.00 ppm. 6

13

5

3

** 'B]' and Έ ' denote O Ç H C H C H C H ' and O C H Ç H C H C H 2

2

2

2

2

2

2

2

, 2

respectively

)

1 3

The chemical shifts of pentad sequences in C N M R spectra (Table I) were then assigned through P F G - H M Q C experiments following the assignment of proton chemical shifts. The assigned pentad sequences both in H and C N M R spectra were further ascertained with P F G - H M B C experiments. In P F G - H M B C experiments, the pentad sequences should have multiple-bond correlation to two flanking pentads. For example, pentad M M M B M sequence should show correlation to two 'M'-centered sequences M M M M B and B M M M B and to two 'B'-centered sequences M M B M M and M M B M B . l

1 3

Kinetic Study on Initiating Mechanism Under selected conditions of comonomer ratio and initiator concentration, the polymerization reaction is sufficiently slow to allow in situ monitoring of

233 1 3

monomer and polymer species using C N M R . Bulk copolymerizations of T O X with DOP initiated by B F O E t were carried out at a 9:1 molar ratio of methylene oxide unit to butylène oxide unit. Figure 1 shows the C N M R spectra accompanying the process of copolymerization of T O X with DOP. Figure 2 displays the kinetic profiles derived from Figure 1. After injection of initiator, the resonance at 95.02 ppm arose quickly in the first spectrum (15.5 min.). According to the 2D N M R assignment, this resonance was from the M B M B M pentad sequence. This assignment of M B M B M sequence was also established by an in situ C N M R experiment of DOP homopolymerization under comparable conditions for TOX/DOP copolymerization. At 15.5 minutes, about 50% DOP had been consumed and almost all had been incorporated as M B M B M sequence. No other pentad sequences were observed in the first spectrum. It follows in the copolymerization of T O X with DOP that initially DOP homopolymerization dominated. Then, accompanying the continuing increase of M B M B M sequence, the pentad sequences with Έ ' separated by more ' Μ ' , i.e.. M B M M B and B M M M B , emerged. At 93 minutes, all of the pentad sequences appeared with the exception of the pentad without Έ ' , M M M M M . At this time, 80% DOP and 20% T O X were consumed and T O X had been contributing to ' Μ ' incorporated with Έ ' into the pentad sequences. Associated with the M B M B M pentad, BMÇH2CH2CH2CH2O should also appear in the Έ'-centered region. A peak at 67.24 ppm was assigned to B M Ç H 2 C H C H C H 0 overlapping with the resonance of the carbon adjacent to oxygen in Έ ' unit of DOP (67.28 ppm). This resonance was not resolved until 80% DOP consumption reached. Although the integral value of this peak could not be accurately determined due to its overlap with DOP, its rapid increase was clearly parallel to M B M B M . On the other hand, the central carbon in Έ ' units (OCH2ÇH2ÇH2CH2O) of M B M sequences showed a well-resolved resonance at 26.43 ppm, over 3 ppm upfield from the central C H in Έ ' units of the DOP ring (29.11 ppm). This peak emerged in the first acquired spectrum after initiation. Similar to M B M B M sequence, the resonance of the central C H in Έ ' units originating from DOP increased rapidly within the first 62 minutes (Figure 2), followed by a much slower growth to a plateau at about 140 minutes. At the same time, M B M B M declined at a significant rate due to its conversion into pentads with Έ ' separated by more than one ' Μ ' unit through processes including transacetalization. In addition to the contribution from transacetalization involving already formed sequences, a significant portion of the growth of the pentads M B M M B , M B M M M , B M M M B and Β M M M M , was due to copolymerization of T O X with DOP. At this time, M M M M M pentad sequence was not observed. For the first acquired spectrum after the injection of initiator, the resonances of C-2 (acetal carbon) and C-4 of comonomer DOP shifted downfield by ca. 0.2 3

2


1 3

l 3

2

2

2

2

2


234

'

1

' ' ' ' ' ' ' ' ' ' ' ' ' · 29 28 27

- ' 26

. ppm

Figure 1. C NMR spectra as a function of time for the bulk copolymerization of TOX with DOP at 70 °C. Molar ratio: M/B= 9; BFyOEtJM =30 ppm; upper Figure, 'M'-centered region; lower Figure, 'B'-centered region.

13

I • • ' ' I • ' ' ' I // 68 67 66

>85miii

124Ûmin

« 1379.5min


400

2

800

Time (min.)

600

2

1000

TlU

—,

1200

1400

BMMMB

l"T>lTTr

MBMMB

ΒΜΜΜΜ

ΜΒΜΜΜ

MMMMM

y

2

2

Figure 2. TOX/DOP copolymerization: kinetic profiles of pentad sequences and central carbon ofCH2CH CH CH20 counit, B . Molar ratio: M/B= 9; BF OEt /M =30 ppm

200

y:

MBMBM

OCH2CH2CH2CH2O

18


237 ppm (Figure 1, 15.5 min. curve). This is likely caused by the protonation of the DOP, leading to an equilibrium between protonated and unprotonated DOP with exchange at a rate faster than the N M R time scale. The process of initiation of polymerization by protonated DOP was supported by the immediate observation of the end group HOCH CH CH CH OCH2- (HOÇH CH CH CH OCH -, 61.59 ppm and ΗΟΟΗ2ςΗ ΟΗ ΟΗ ΟΟΗ -, 29.55 ppm). These two resonances, with chemical shifts in agreement with literature values (#), were also observed in our control experiment of DOP homopolymerization. The resonance at 61.59 ppm was well resolved whereas the resonance at 29.55 ppm partially overlapped with the corresponding carbon of comonomer DOP. The resonance at 61.59 ppm was clearly observed to increase first and then decline at the time the M B M B M sequence started to decline. After the initial growth of the pentad sequences with at least one ' Β ' , the M M M M M pentad sequence corresponding to Έ ' separated by at least five ' Μ ' emerged and was accompanied by the continued decline of M B M B M sequence. Then, the sequence M M M M M showed a sustained increase together with the rate leveling off for M B M M M and B M M M M at a later stage. At the same time, T O X resonance decreased rapidly, followed by the growth of M M M M M pentad at the highest rate. Unlike the M M M M M pentad, the rate of growth for pentads M B M M M and B M M M M eventually slowed down. Eventually, all three curves became less steep. The two pentads with two Έ ' ( B M M M B and M B M M B ) first increased and then decreased in a manner similar to M B M B M but with their maxima occurring much later. The decline of M B M B M and M B M M B reflected the randomization of the counit Έ ' , resulting in a T O X / D O P copolymer having thermal stability.


2

2

2

2

2

2

2

2

2

2

2

2

2

Nonad Sequences , 3

It is noteworthy that in the in situ C N M R experiment, not only pentads but also heptads and even nonads were observed for some sequences. Heptad sequences have been reported previously for T O X / D O L copolymer (9). In the present study, a number of nonads were identified for 'M'-centered sequences. For the pentad M M M M M , i.e. (M) , six nonads are possible through adding ' Μ ' or Έ ' to both sides of the pentad: M M ( M ) M M , B M ( M ) M B , M M ( M ) M B , M B ( M ) M M , M B ( M ) M B and M B ( M ) B M . Only five distinct resonances were observed (Figure 3). It is reasonable to assume that the pentad with nine ' Μ ' , M M ( M ) M M , was not detected due to its very low concentration. Even at the end of the period of our observation, the consumption ratio of ' Μ ' t o - Έ ' was only 5 to 1. Similarly for the pentad B M M M M , i.e. B ( M ) M , six nonads are possible: M M B ( M ) M M M , B M B ( M ) M M M , B M B ( M ) M M B , M M B ( M ) M M B , M M B ( M ) M B M , and B M B ( M ) M B M . However, only three 5

5

5

5

5

5

5

5

3

3

3

3

3

3

3

T

89.6

r

r

... .._._._. . τ

τ

_.^._ ^

89.4

5

89.2

5

89.0

ppm

5.5min

155min

- 310min

— 465min

620min

775min

930min

1085min

1240min

1379.5min

(Β/Μ)ΜΒΜΜΜΜΜΜ Χ ( Β/Μ )Μ Β M MM Μ Μ Β j (Β/Μ)ΜΒΜΜΜΜΒΜ

Figure 3. Nonad sequences as a function of time for the bulk copolymerization of TOX/DOP, From Figure 1.

89.8

•«ΗΠ»···ι--«·-"ΤΎ-τ-?-

5

,'ΒΜ(Μ )ΜΜ; ΒΜ(Μ,)ΜΒ; ΜΒ(Μ )ΜΜ ΜΒ(Μ.,)ΜΒ; ΜΒ(Μ )ΒΜ}


239 separate resonances were observed. We conclude that the two nonads in each of the three pairs have the same chemical shift: M M B ( M ) M M M and B M B ( M ) M M M ; B M B ( M ) M M B and M M B ( M ) M M B ; M M B ( M ) M B M and B M B ( M ) M B M . A reasonable generalization is that without the interruption of Έ ' , which contributes five bonds to the backbone, chemical shift of the center' M ' can be influenced by a counit separated by three ' Μ ' units. Within the B M M M M pentad manifold, three separate peaks were assigned as the nonads (Figure 3): ( B / M ) M B M M M M M M at 89.22 ppm, ( B / M ) M B M M M M M B , 89.18 ppm and ( B / M ) M B M M M M B M , 89.12 ppm. The above nonad assignments are also consistent with the growth pattern of sequences with separation of Έ ' by more ' Μ ' as polymerization proceeds. The nonads ( B / M ) M B M M M M B M contained and therefore contributed to the pentads M M M B M and M M M M B ; all three showed similar growth pattern of emerging at early stage with significant increase followed by a slow leveling off. The nonads ( B / M ) M B M M M M M M contain the pentads M M M M M and M B M M M . The nonads ( B / M ) M B M M M M M M and the pentad M M M M M showed continued significant growth. The nonads ( B / M ) M B M M M M M B , containing the pentads M M M M B and M M M M M , slowed down in growth at later periods and contributed to a portion of the M M M M M growth. Detailed separate assignments are not available at present for the five nonads associated with the pentad M M M M M . The counit Έ ' assumes a range of conformations different from those of Έ ' and ' Μ ' , thus leading to a different manifestation in chemical shift of microsequences. Replacing the two B carbons of the pentad M B M M M (92.74 ppm) with a ' M ' counit gives M M M M M (90.04 ppm), leading to an upfield shift of 2.70 ppm. Subututing the edge-'M' of the M M M M M pentad by a Έ ' or a Έ ' courût gives B M M M M (89.60 ppm) and E M M M M (90.74 ppm), resulting in an upfield shift of 0.44 ppm and a downfield shift of 0.70 ppm respectively. Chemical shift assignments are also complicated by the possible involvement of δ-effecton carbon chemical shift for T O X / D O P copolymers. For Έ'-centered sequences, the following sequences were observed:(M)4BiM(M/B) , B(M) B,M(M/B) , BMMB^M/B^, M B M B i M ( M / B ) , ( M ) B M ( M / B ) , BMMg2M(M/B) , and M B M B M ( M / B ) . Unlike T O X / D O L copolymerization, where the insertion of formaldehyde into D O L forming trioxepane plays an important role, the insertion of formaldehyde into DOP to form the 9-membered ring, 1,3,5-trioxacyclononane (TOCN), is not observed during the copolymerization of T O X with DOP in our experiments. Two plausible contributing factors can be considered: one is the difficulty of forming 9-membered ring T O C N based on thermadynamics. The second factor is the higher basicity of DOP than D O L . The comonomer D O P was found (10) to be more basic and more reactive than D O L . For example, the polymerization of DOP initiated by anhydrous perchloric acid in CH C1 was reported (//) to be a thermodynamically more favored process than that of D O L 3

3

3

3

3


3

2

3

2

3

2

3

2

3

2

2

2

2

2


240 under comparable conditions. The higher basicity of DOP leads to its much stronger preference to protonation over T O X . The protonation of DOP starts the initial polymerization, hence the immediate appearance of the end group HOCH2CH2CH2CH2OCH2-. The 5-membered ring D O L was also shown by N M R to behave differently from the 7-membered ring DOP under the influence of cationic species. For methoxycarbenium hexafluoroantimonate as the cationic initiator, the initiation steps for cationic homopolymerization D O L and D O P were shown to be different (12). For DOL, based on N M R data the following species are proposed to coexist at -70 °C, with the equilibrium strongly shifted to the right: CH3-O-CH2—O'^o

CHg— ο

o' In contrast, for the DOP the reaction of the methoxymethylium cation at low temperature was considered to give mostly cationated D O P instead of ring expansion. These observations support our finding of initial D O P homopolymerization without formaldehyde insertion in TOX/DOP copolymerization.

Conclusion The initial process of copolymerization of T O X with DOP is markedly different from the T O X / D O L system For the T O X / D O P system, insertion of formaldehyde into DOP to form a larger acetal ring before substantial copolymerization does not play a significant role. The initiation of the homopolymerization of DOP to give a burst of M B M B M sequence dominates the early events. Then a steady decline of M B M B M after its initial burst is accompanied by the emergence of M B M M B , a pentad with Έ ' separated by one more M ' and the growth of pentad sequences with ' Β ' separated by additional 'Μ'. Finally, the growth of M M M M M pentad shows the highest rate. The randomization of Έ ' units is manifested in the decline of pentad sequences with two Έ ' units. The difference in the initiation mechanism between T O X / D O P system and T O X / D O L system is explained in terms of the higher ring strain of T O C N than TOP and the basicity difference between DOP and D O L . The kinetic profiles obtained are based on chemical shift assignment for T O X / D O P copolymers through P F G - H M Q C and P F G - H M B C N M R experiments. Sequence nonads have been observed for the first time in copolymer microstructure N M R analysis. l

241

References


1.

Fejgin, J.; Tomaszewicz, M.; Cieslak, J. Polimery (Warsaw) 1976, 21(7), 298; Chem. Abstr. 1976, 85, 160567. 2. X u , Y.-M.; He, J.-P.; Huang, X.-Y.; Yang, S.-Y.; Hu, Q.-Z. Fudan Xuebao, Ziran Kexueban 1999, 38(6), 696; Chem. Abstr. 2000, 133, 120767. 3. Hill, J. W.; Carothers, W. H . J. Am. Chem. Soc. 1935, 57, 925. 4. Weissermel, K.; Fisher, E.; Gutweiler, K . Kunstoffe 1964, 54, 410. 5. Price, M . B . ; McAndrew, F.B. J. Macromol. Sci. A-I, 1967, 2, 231. 6. Pesce-Rodriguez, R. Α.; Wang, S.; Yang, N . - L . Makromol. Chem. 1990, 191, 99. 7. Werner, M. D. Copolymerization Studies of Trioxane and Dioxolane. Ph.D. Thesis, The City University of New York, Staten Island, N Y , 1996 8. Liu, Y . ; Wang, H . ; Pan, C. Makromol. Chem. Phys. 1997, 198, 2613. 9. Fleischer, D.; Schulz, R. C. Makormol. Chem., 1975, 176, 677. 10. Okada, M.; Yamashita, Y. Makromol. Chem. 1969, 126, 266. 11. Plesch, P. H.; Westermann, P. H . Polymer 1969, 10, 105. 12. Penczek, S., Kubisa, P., Matyjaszewski, K . Adv. Poly. Sci., 1980, 37, 1.

An NMR Study on the Bulk Cationic Copolymerization of Trioxane with

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