2515
Ind. Eng. Chem. Res. 1995,34, 2515-2519
Initiation Mechanism of the Copolymerization of 1,3,5-Trioxaneand Ethylene Oxide Hajime Nagahara, Kenji Kagawa, Toshiyuki Iwaisako, and Junzo Masamoto* Asahi Chemical Industry Co., Ltd., 2767-11, Azaniihama, Kojimashionasu, Kurashiki, 711 Japan
Initially, the initiation mechanism for the copolymerization of 1,3,5-trioxane (TO) and ethylene oxide proposed the formation of 1,3-dioxolane from the reaction of ethylene oxide with formaldehyde, and this mechanism was believed plausible for a long time. Recently, we carefully studied the bulk copolymerization of TO and ethylene oxide using boron trifluoride-dibutyl ether as the initiator. Two novel cyclic formals, 1,3,5,7-tetraoxacyclononane (TOCN) and 1,3,5,7,10-pentaoxacyclododecane(POCD), from the initiation reaction of the copolymerization of TO and ethylene oxide were isolated and identified. From this discovery, we proposed a new initiation mechanism for the copolymerization of TO and ethylene oxide. Ethylene oxide reacts directly with TO to form TOCN and POCD. From TOCN, 1,3,54rioxepane (TOXP) is formed, and from T O W , 1,3-dioxolane is formed.
Introduction Polyacetal resin from the copolymerization of 1,3,5trioxane (TO) and ethylene oxide is a commercially useful engineering plastic. Studies conducted by Weissermal (Weissermal et al., 1964,1967) on the copolymerization of TO and ethylene oxide reveal that an induction period, much longer than observed in the case of the homopolymer, prevails. It has been established that, during this period, ethylene oxide is converted to 1,3-dioxolane, 1,3,54rioxepane (TOXP), and low molecular weight linear copolymers. Only after all the ethylene oxide is consumed does the solid polymer form. Weissermel et al. proposed that trioxane is initially decomposed to formaldehyde (eq l), and then the reaction of formaldehyde with ethylene oxide forms 1,3dioxolane for the initiation mechanism of the copolymerization of TO and ethylene oxide (eq 2).
(') O V 0 CHeO
+
\D/
+
3CH20
-b O
V
oxide and formaldehyde, and this mechanism was thought plausible for a long time. However, we recently isolated two novel cyclic compounds (Nagahara et al., 1989a,b), which are the direct reaction products of TO and ethylene oxide, and proposed a new polymerization mechanism. These two novel products disclosed the precise initiation mechanism of the copolymerization of TO and ethylene oxide. The direct reaction of ethylene oxide and TO formed new cyclic products, and from these cyclic products, TOXP and 1,3-dioxolane were formed. We will now discuss the newly discovered initiation mechanism for the copolymerization of TO and ethylene oxide. The previously held theory concerning the mechanism for the copolymerization of TO and ethylene oxide is based on much experimental data. TO is decomposed to form formaldehyde(eq l),and the formaldehydethen reacts with ethylene oxide to produce 1,3-dioxolane (eq 2). From 1,3-dioxolane and formaldehyde, TOXP is formed (eq 3).
V
+
CHzO
-
00
U
(3)
These two chemicals, 1,3-dioxolane and T O P , then copolymerize with TO (eq 4). Previous work only shows the results of the reaction in the last stable zone.
Collins et al. (1981) also reported a fundamental discussion concerning the copolymerization mechanism of TO and ethylene oxide initiated with boron trifluoride dibutyl ether. They confirmed that ethylene oxide is converted to 1,3-dioxolane and TOXP. They stated that the direct reaction of ethylene oxide with TO was impossible because of the weak basicity of TO. Weissermel et al. and Collins et al. proposed the formation mechanism of 1,3-dioxolane from ethylene
* To whom correspondence should be addressed.
Experimental Section TO. Chemical grade TO was obtained from Fluka Chemicals. TO was purified as follows. A 30 wt % TObenzene solution was fed t o a 3-m-high laboratory glass sieve tray distillation tower. From the top of the tower, the benzene solution of TO was continuously drawn, and from the bottom of the tower, the high boiling point fraction was removed under the condition of a reflux ratio of 3. The next benzene solution of TO was fed to
0888-5885/95/2634-2515$09.00/0 0 1995 American Chemical Society
2516 Ind. Eng. Chem. Res., Vol. 34, No. 7 , 1995
HA:HB:HC= 1 : 2 : 2
'
HC
t
CH2-CHe
HA
3 5.0
lnteeral Curve
Solv. : cc14 Temp. : Room Temp
4.0
Figure 1. 'H NMR pattern of intermediate A (TOCN).
the 3-m-high laboratory sieve tray. The light boiling fractions like benzene, water, formic acid, and methanol were drawn from the top of the distillation tower and purified TO was drawn from the bottom of the distillation tower according to the method described in our patents (Hamanaka et al., 1982; Masamoto, 1991). The water content in the obtained purified TO was below 1 ppm. TO, which contains various levels of water from 1 to 100 ppm, was used for the polymerization. The water content in TO was adjusted by adding water to TO. Ethylene Oxide. Commercially available ethylene oxide was used without further purification. Boron Trifluoride-Dibutyl Ether. Chemical grade boron trifluoride-dibutyl ether was purified by distillation under reduced pressure and was used in a diluted form with cyclohexane. Polymerization. Purified TO and ethylene oxide (usual case, 4.5 mol % TO) were introduced into a glass ampule. The glass ampule was then immersed in an oil bath at a specific temperature (usually 70 "C). A cyclohexane solution of boron trifluoride-dibutyl ether (usual case, 7 x moVmol of TO) was introduced through the cap of the glass ampule with a microsyringe t o the molten TO-ethylene oxide mixture. The mixture was then intensively shaken in the oil bath (polymerization was usually done at 70 "C). The polymerization was continued for a specific time. At the instance when the catalyst was injected, the reaction mixture was a nonviscous liquid but then became a viscous iiquid. Analysis of PolymerizationProduct. To stop the polymerization reaction, the reaction mixture was poured into a methanol solution containing a small amount of ammonia. At the instance when the catalyst was injected, the methanol diluted products were soluble liquids. When precipitated materials appeared, the precipitated polymerization products were separated. The solution phase was analyzed by gas chromatograPhY* Gas Chromatography. The reaction mixture in the soluble part was analyzed using gas chromatography with a thermal conductivity detector. A 1 m PEG CW20M column was used. The column temperature was increased at the rate of 18 "C/min from 70 to 200 "C. Helium was used as the carrier gas. Its flow rate was 20 mumin. If the column temperature was maintained a t a constant temperature of 150 "C, novel cyclic compounds (which were later identified as TOCN and POCD) could not be detected. Thus, we raised the column temperature from 70 to 200 "C.
Analysis of Novel Cyclic Compounds. Novel compounds were separated and collected by gas chromatography and characterized by lH NMR. lH NMR. Novel compounds were characterized by lH NMR using a Nihon Denshi JNMMH-100. lH NMR was measured using carbon tetrachloride as the solvent and tetramethylsilane as the standard material at room temperature. Results and Discussion The initiation mechanism during the bulk copolymerization of TO and ethylene oxide using boron trifluoride-dibutyl ether as the initiator were carefully studied. The two new intermediates, A and B, were separated and collected by gas chromatography. The structures were then confirmed by lH NMR. Figure 1 shows the lH NMR pattern of the new intermediate A, which is identified as TOCN, a novel cyclic formal. From this figure, the integral value of HA, HB, and HC is equal to 1 to 2 to 2. Figure 2 shows the 'H NMR pattern of the new intermediate B, which is identified as POCD, a novel cyclic formal. The integral value of HA, HB, and HC is equal to 1 to 2 to 4. The new isolated intermediates or novel compounds were T O W and POCD, which disclose the precise initiation mechanism as shown in Figure 3. First, ethylene oxide directly reacts with TO to produce TOCN, which is the reaction product of 1mol of ethylene oxide and 1 mol of TO, and POCD, which is the reaction product of 2 mol of ethylene oxide and 1 mol of TO. T O W and 1,3-dioxolane result from the former compound. The experimental results are shown in Figure 4. In this case, the water content of TO was 1ppm. At first, as the ethylene oxide concentration decreased, TOCN and POCD appeared. After the ethylene oxide was consumed, polymerization started. With the decrease in ethylene oxide, TOCN increased. This means that TOCN is formed from the reaction of ethylene oxide and trioxane (eq 5).
(5)
As the concentration of TOCN decreased, the concentration of TOXP increased. This means that TOXP was
Ind. Eng. Chem. Res., Vol. 34,No. 7, 1995 2517 HA:HB:HC= 1 : 2 : 4
-C
HC
Integral Curve SOIV. CCI.4 Temp : Room Temp
I
I
5.0 Figure 2. 'H N M R pattern of intermediate B (POCD).
4.0
HA
with a decrease in the concentration of TOXP, the concentration of 1,3-dioxolane increased. This suggests that 1,3-dioxolane was formed from TOXP (eq 7). The
0
()+A-
O
V
?U ?
U
Trioxane Ethylene Oxide TrioxeDane 1,3.5.7-Tetraoxac~clo nonane CTOCNI
1
\
U
U
1 , 3 . 5 . 7 , 1O-Pentaoxacyclododecane tPOC01
Figure 3. Initiation mechanism of copolymerization of trioxane and ethylene oxide. ,100
100,o
H e 0 : 1 DPm
Time ( s e d Figure 4. Concentration profile of reactants and intermediates in trioxandethylene oxide copolymerization (HzO: 1 ppm). EO, ethylene oxide; TOCN, 1,3,5,7-tetraoxacyclononane; POCD, 1,3,5,7,10-pentaoxacyclododecane;TOXP,1,3,5,-trioxepane; DOXL, 1,3dioxolane.
formed from TOCN (eq 6).
< :>-< A
U
/O\
>
0 0
+
("1 0 0
Q"Q
\
PDms value
CH2O
U
TOCN decreased with a n increase in TOXP and then finally disappeared. From eq 6, TOCN was thought to be a far more thermodynamically unstable material compared to TOXP, and the reverse reaction, the reaction from TOXP and formaldehyde to TOCN, seems to be impossible. Soon after the appearance of TOXP, 1,3-dioxolane appeared. After the maximum concentration of TOXP,
2
0
V
+
CH2O
concentration of TOXP and 1,3-dioxolane decreased steadily with polymerization time. There seemd to be a thermodynamic chemical equilibrium between TOXP and 1,3-dioxolane. During the early stage of polymerization, the reacted ethylene oxide can be almost quantitatively analyzed. For example, at the polymerization time of 20 s, 55%of the ethylene oxide was consumed. As for this consumed ethylene oxide, 64% was changed to TOCN, 20% was changed to TOW, 12% was changed to POCD, and 5% was changed to 1,3-&oxolane. Another example is given for the polymerization time of 40 s. Almost 100% of the ethylene oxide was consumed. As for this consumed ethylene oxide, 10% became TOCN, 58%became TOXP, 14% became 1,3-dioxolane. Thus, 82% of the ethylene oxide is consumed as a monomeric form of ethylene oxide. As for the other 18%of residue, most of it is thought t o be consumed as the dimeric form of ethylene oxide via POCD. Collins et al. (1981) reported that the first reaction of ethylene oxide was the formation of 1,3-dioxolane, which was assumed t o occur along with the reaction products of formaldehyde and ethylene oxide according to eqs 1 and 2. They assumed that, from the reaction of l,&dioxlane and formaldehyde,TOXP was formed (eq 3). However, by careful checking of their experimental data, their data showed that the formation of TOXP occurred earlier than the formation of 1,3-dioxolane,and after the maximum concentration point of TOXP the concentration of 1,3-dioxolaneincreased with polymerization time. Their data are quite similar t o ours though they did not find TOCN and POCD. Their experimental data actually suggest that 1,3-dioxolane was formed from TOXP, and this interpretation is in accordance with our experimental data. The initial copolymerization mechanism is much more clearly shown when TO contains a small amount of water. Figure 5 shows the concentration profile of reactants and intermediates when the water content in TO is 20 ppm. Figure 6 shows the concentration profile of reactants and intermediates when the water content in TO is 100 ppm.
2518 Ind. Eng. Chem. Res., Vol. 34,No. 7, 1995
.""
The cationic active ethylene oxide complex reacts via a direct insertion mechanism with trioxane to form the cationic active complex of TOCN, which is denoted as TOCN". TOCN is then formed (eqs 9 and 10).
H i 0 : 20ppm
.-o!
+ TO - TOCN* TOCN* + EO - TOCN + EO* EO*
(9) (10)
For the formation of POCD, the insertion reaction of ethylene oxide with TOCN (eqs 11- 13)via the cationic Time [Sec] Figure 5. Concentration profile of reactants and intermediates in trioxane/ethylene oxide copolymerization (H2O:20 ppm). EO, ethylene oxide; TOCN, 1,3,5,7-tetraoxacyclononane; POCD, 1,3,5,7,10-pentaoxacyclododecane;TOXP, 1,3,5-trioxepane; DOXL, 1,3dioxolane. 1 OOQ
1
+ TOCN - POCD* TOCN* + EO - POCD* POCD* + EO - POCD + EO* EO*
+ EO - (EO),* (EO),* + TO - POCD*
Time (sed
The consumption of ethylene oxide was prolonged with increased water content. The consumption of ethylene oxide was extremely prolonged compared with the reaction of a 1 ppm water content in TO. Thus, increases in TOCN and POCD were also prolonged, and they showed a steady state concentration. After a short time when TOCN appeared, TOXP also appeared. TOCN decreased rapidly when ethylene oxide disappeared. With the rapid decrease in TOCN, a rapid increase of TOXP was observed. This truly means that TOXP is formed from TOCN. The appearance of 1,3dioxolane was observed a little later along with the appearance of TOW. During the latter stage of the reaction, an increase in 1,3-&oxolanewas observed with a decrease in TOXP. This truly means that 1,3dioxolane is formed from TOXP. From these figures, it is quite clear that ethylene oxide directly reacts with TO t o form TOCN and POCD. The major product is TOCN. From TOCN, TOXP is formed. From TOXP, 1,3-dioxolane is formed. Considering these experimental results, the following mechanisms for the formation of TOCN, POCD, TOXP and 1,3-dioxolane during the copolymerization of TO and ethylene oxide can be presented. AB water prolonged the reaction and ethylene oxide is a highly basic material, the direct reaction between boron trifluoride-dibutyl ether and ethylene oxide to form the cationic active complex of ethylene oxide seems to be plausible (eq 8). Here, EO denotes ethylene oxide BF,*Bu,O
+ EO
-
EO*
(8)
and EO* denotes cationic active complex of ethylene oxide, though its nature is unclear.
(12)
(13)
complex of POCD, which is denoted as POCD*, is thought plausible. However, there is no proof to deny the direct reaction of 2 mol of ethylene oqde with trioxane forming the cationic complex of bimolecular ethylene oxide, which is denoted as (EO),*(eq 14 and 15).
EO*
Figure 6. Concentration profile of reactants and intermediates in trioxandethylene oxide copolymerization (HzO:100 ppm). EO, ethylene oxide; TOCN, 1,3,5,7-tetraoxacyclononane; POCD, 1,3,5,7,10-pentaoxacyclododecane;TOXP, 1,3,5-trioxepane; DOXL, 1,3dioxolane.
(11)
(14)
(15)
The cationic active complex of TOCN is thought to change to the cationic active complex of TOXP, which is denoted as TOXP*, by eliminating formaldehyde. From this TOW*, TOXP is formed (eqs 16 and 17). In
- TOW* + CH,O TOW" + TOCN - T O W + TOCN" TOCN*
(16) (17)
the absence of ethylene oxide, TOCN decreased rapidly to form TOXP. TOCN* is thought to be unstable in the absence of ethylene oxide, then to rapidly change to TOXP*. The cationic active complex of TOXP is thought t o change to the cationic active complex of 1,3-dioxolane, which is denoted as DO=*, by eliminating formaldehyde, and from DO=* 1,3-dioxolane,which is denoted as DOXL, is thought to be formed (eqs 18 and 19).
DO=*
-
+ CH,O + TOXP - DOXL + T O P *
TOW*
DOXL"
(18) (19)
Cationic complexes of TOXP and 1,3-dioxolane are thought to react with trioxane to form the polymeric cationic complex, which is denoted as polymer*, and this polymeric complex reacts with TOCN and 1,3-dioxolane to form a new polymeric complex (eqs 20-23).
+ TO - polymer* DO=* + TO - polymer* polymer* + TOXP polymer" polymer* + DOXL - polymer" TOW*
-
(20) (21) (22) (23)
As water prolongs the reaction, water is thought to compete with the reaction of ethylene oxide during the initiation reaction of TO and ethylene oxide. Thus,
Ind. Eng. Chem. Res., Vol. 34,No. 7, 1995 2619 water is thought to form a less reactive complex, though its nature is unclear.
Conclusions The initiation mechanism of the bulk copolymerization of TO and ethylene oxide was clarified. First, ethylene oxide directly reacts to form a cyclic compound such as TOCN and POCD. From TOCN, TOXP results, and from TOW, 1,3-dioxolane results. Two compounds, T O P and 1,3-dioxolane, are then copolymerized with TO. Water prolongs the reaction of ethylene oxide with TO. Thus the formation of TOCN and POCD were also prolonged. The formation of TOXP and 1,3-dioxolane were also prolonged. Overall, water prolonged the induction period of the copolymerizationof trioxane and ethylene oxide.
Hamanaka, K.; Iwaisako, T.; Masamoto, J.; Yoshida, K. Process 1982,assigned for Separating Trioxane. US Patent 4,332,644, to Asahi Chemical. Masamoto, J. Purification of Trioxane. Japanese Unexam. Pat. 3-123777,1991,assigned to Asahi Chemical. Nagahara, H.; Iwaisako, T.; Masamoto, J.; Kagawa, K. A Novel Cyclic Compound. Japanese Exam. Pat. 89-49713,1989a, assigned to Asahi Chemical. Nagahara, H.; Iwaisako, T.; Masamoto, J.; Kagawa, K. A Novel Cyclic Compound. Japanese Exam. Pat. 89-49714,1989b, assigned to Asahi Chemical. Weissermel, K.; Fisher, E.; Gutweiler, K. Zur Copolymerization des Trioxane. Kumtstofe 1964,54, 410-415. Weissermel, K.; Fisher, E.; Gutweiler, K.; Hermann, H. D.; Cherdon, H. Polymerization of Trioxane. Angew. Chem., Znt. Ed. Engl. 1967, 6, 526-533.
Received for review October 27, 1994 Revised manuscript received January 19,1995 Accepted March 27, 1995@
Literature Cited Collins, G. L.; Green, R. K.; Beradinelle, F. M.; Ray, W. H. Fundamental Consideration on the Mechanism of Copolymerization of Trioxane and Ethylene Oxide Initiated with Boron Trifluoride Dibutyl Etherate. J.Polym. Sci., Polym. Chem. Ed. 1981,19,1957-1607.
IE9406198
Abstract published in Advance ACS Abstracts, May 15, 1995. @
