Polymerization of Formaldehyde Using Tetravalent Organo-Tin

DOI: 10.1021/ie970856l. Publication Date (Web): April 7, 1998 ... and Junzo Masamoto. Industrial & Engineering Chemistry Research 2000 39 (7), 2275-22...
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Ind. Eng. Chem. Res. 1998, 37, 1729-1735

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Polymerization of Formaldehyde Using Tetravalent Organo-Tin Compounds as an Initiator Kazuhiko Matsuzaki and Junzo Masamoto* Department of Polyacetal Research and Development, Asahi Chemical Industry Co., Ltd., Kojimashionasu, Kurashiki 711, Japan

Polymerization of gaseous formaldehyde in hexane using various initiators including tetravalent organo-tin compounds was investigated. The end group of the obtained polymer was examined and it was concluded that the chain-transfer agents (such as methanol, etc.) were almost completely consumed during the polymerization of gaseous formaldehyde. Only two series of initiators, quaternary ammonium salt and dialkyltin dimethoxide compounds, produced a polymer with a narrow-molecular-weight distribution (Mw/Mn ) 2). The polymer with the narrowmolecular-weight distribution (Mw/Mn ) 2) produced a product with high Dart impact strength. The polymer with the broad-molecular-weight distribution (Mw/Mn > 2.9) gave the product with low Dart impact strength. Introduction Acetal resin (alternatively, polyoxymethylenes, sometimes called polyacetals or aldehyde resins) is a term used to describe high-molecular-weight polymers and the copolymers of formaldehyde. First commercialized as a homopolymer in 1960 by Du Pont, acetal resins are engineering thermoplastics which have found broad use in traditional metal applications (Schweizer et al., 1959; Koch and Lindvig, 1959; Linton and Goodman, 1959). Shortly thereafter, Celanese (presently Ticona L. L. C) researchers developed an acetal resin based on the copolymerization of trioxane and cyclic ethers, such as ethylene oxide (Walling et al., 1962). In 1962, a commercial plant began producing this acetal copolymer. Since then, the rapid expansion of acetal resin production has occurred worldwide. Up to 1971, Du Pont, Celanese, and Celanese joint ventures have been the sole producers of acetal resins. In 1972, Asahi Chemical started to produce the acetal homopolymer utilizing the world’s third type of polyacetal technology (Kobayashi et al., 1972). Asahi Chemical also industrialized the acetal copolymer in 1985. The acetal homopolymer can be obtained by the homopolymerization of formaldehyde or trioxane. However, only from formaldehyde polymerization is the acetal homopolymer industrially produced. At present, Du Pont and Asahi Chemical are the only producers of the acetal homopolymer. Although the details of the process for making the acetal homopolymer have never been made public, the general processing steps can be outlined as monomer purification, polymerization, endcapping, and finishing (Barker and Price, 1970; Blair, 1976). A narrow-molecular-weight distribution (Mw/Mn ) 2) of the obtained polymer is one of the important factors for the polymer for injection molding use. Only a quaternary ammonium salt has been known for this purpose. Control of the particle size of the product is also important for subsequent isolation, washing, and so forth. The nature of the initiator, the solvent, * To whom correspondence should be addressed. Present address: Department of Polymer Science and Engineering, Kyoto Institute of Technology, Matsugaski, Sakyo-ku, Kyoto 606-8585, Japan.

Figure 1. Polymerization apparatus.

contaminants, reaction conditions, and equipment design all contribute to the physical characteristics of the polymer slurry (Persak and Blair, 1978). Thus, these polymerization systems are very complicated. Furthermore, polymer deposition on the reactor wall is a serious problem during production of the formaldehyde polymer. This deposit mainly depends on the nature of the initiator, reactor design, polymerization conditions, solvent, and so forth and its clarification is very complicated. In this paper, we will mainly discuss a new type of initiator investigated by us at Asahi Chemical for the commercial production of the acetal homopolymer using gaseous formaldehyde. This type of initiator produced a polymer with a narrow-molecular-weight distribution (Mw/Mn ) 2) and a low polymer deposit on the reactor wall. Experimental Section Polymerization. The laboratory-scale polymerization apparatus is shown in Figure 1. Highly purified formaldehyde, in which no water and methanol were detectable by gas chromatography (detection limit: for water, 0.02 wt % relative to formaldehyde; for methanol, 0.01 wt % relative to formaldehyde), was fed into a hexane solution using various initiators. Purified gaseous formaldehyde was obtained using a poly(ethylene glycol) derivative as a purifying agent according to our

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patent (Morishita et al., 1990). Gaseous formaldehyde was continuously fed into an intensively stirred hexane polymer slurry. Almost all of the gaseous formaldehyde fed to the reactor was polymerized. A hexane solution containing the initiator and a small amount of a chaintransfer agent was continuously fed to the polymerization reactor, and the polymer slurry was continuously withdrawn; thus, the polymer content in the slurry was adjusted to about 20 wt %. The polymerization temperature was maintained usually at 50 °C using a controlling thermal jacket. As the chain-transfer agent to control the molecular weight of the polymer, methanol was used. The withdrawn polymer slurry was first filtered, washed with methanol and acetone, and then dried under a nitrogen atmosphere. End-Capping. The hydroxyl end group of the polymer was acetylated with acetic anhyride containing sodium acetate (0.1 wt % based on acetic anhydride). The polymers were acetylated at 140 °C under a N2 atmosphere. Two methods were used for the endcapping of the polymer. Method 1: The polymers were acetylated in acetic anhydride without any dilution. Method 2: The polymers were acetylated in acetic anhydride diluted with kerosene (10 times the volume of acetic anhydride). The acetylated polymers were first washed several times with acetone, next with water several times, and then with acetone several times, and finally dried under a nitrogen atmosphere at 100 °C. The dried polymers were heated to 222 °C under reduced pressure for 1 h to check the degree of acetylation. Every polymer residue was over 97 wt %. Thus, we concluded that both acetylation methods gave sufficiently acetylated polymers. In some polymerization cases, method 1 decreased the value of the reduced viscosity of the acetylated polymer compared to that of the raw polymer. However, method 2 resulted in no substantial difference in reduced viscosity between the acetylated polymer and unacetylated polymer. Thus, we used method 2 to estimate the contents of the end groups of the polymer. Determination of the End Groups. The direct measurement of the hydroxyl end group of the raw polymer was very difficult; therefore, we measured the acetyl group of the acetylated polymer. The acetyl end group of the polymer was determined by the infrared spectroscopy method (Frank et al., 1968). For the infrared spectroscopy measurement, we used a transparent polymer thin film. The polymer powders were placed between two sheets of aluminum foil, and the aluminum foils were heated for several seconds at 190 °C using a thermal press under a pressure of 200 kg/ cm2 and then quenched in water to obtain a transparent thin film. The ratio of the extinction of the carbonyl band (ECdO) at 1755 cm-1 and the reference band of methylene (ECH2) at 1470 cm-1 represent the ratio of the acetyl group of the polymer end group to the polymer main-chain oxymethylene. The methoxyl end group was determined by the Zeisel method. Assuming that the polymer chain is linear and that both polymer end groups consist of a carbonyl group or methoxyl group, the number-average molecular weight was calculated from the ratio of the methylene group of the polymer skeletal bond to the sum of the polymer end groups of the carbonyl group and methoxyl group. In a preliminary experiment, we confirmed that the

number-average molecular weight obtained by the end group analysis was in good accordance with the value obtained by the osmotic method (Masamoto, unpublished data), using the polymers obtained with the dimethyl dioctadecylammonium acetate initiator and dibutyltin dilaurate initiator. These facts meant that the obtained polymers are completely linear polymers. Similar results were also reported by other researchers (Schweizer et al., 1959; Frank et al., 1968). It was reported that it had not been possible to detect branch points, carbon-carbon bonds, or other extraneous inchain linkages in any formaldehyde-derived polyoxymethylenes as prepared (Brown, 1967; Schweizer et al., 1959). It was also reported that, similarly, thermal or hydrolytic depolymerization of polyoxymethylene yielded exclusively formaldehyde as the primary product, except for fragments derived from the polymer end group (Schweizer et al., 1959; Brown, 1967). Estimation of Molecular-Weight Distribution (Mw/Mn). The polymer was dissolved in p-chlorophenoltetrachloroethane mixed solvent (1/1) at 90 °C (polymer concentration, 0.5 g/100 mL of solvent), and the reduced viscosity was measured at 60 °C. The viscosity-average molecular weight was calculated (Masamoto, unpublished data) using the following experimentally obtained equation: Mv ) (4.813 ηsp/C - 1.7831) × 104, where ηsp/C > 1.0. The molecular-weight distribution (Mw/Mn) was estimated (Masamoto, unpublished data) using the following experimentally obtained equation: Mw/Mn ) (Mv/Mn)1.182. To derive this equation, as a preliminary experiment, we measured the number-average molecular weight by the osmotic method, and we also measured the weightaverage molecular weight by a light-scattering method using various types of polymers (Masamoto, unpublished data). Flow Properties and Mechanical Properties of the Polymer. The obtained end-capped polymer powders were mixed with an antioxidant (2,2′-methylene bis 4,6-dimethyl phenol, 0.3% based on the polymer) and a thermal stabilizer (nylon 6, 66, 610 terpolymer, 0.5% to the polymer), and we then extruded the mixture into a pellet at 200 °C. The melt flow index (MFI) of the polymer pellet was examined under a load of 2.13 kg at 190 °C. The extruded pellets were injection molded (polymer temperature, 200 °C; injection pressure, 50 kg /cm2 (gauge pressure); mold temperature, 70 °C) onto a plate (130-mm length × 110-mm width × 3-mm thick) and then used for the Dart impact test. Results and Discussion In Table 1, the results of the polymerization are shown using various types of initiators. The reduced viscosity of the raw polymer is in the following order: tributyltin laurate > cobalt acetylacetonate . dibutyltin dilaurate > dibutyltin dimethoxide ) dioctyltin dimethoxide ) dimethyl dioctadecylammonium acetate. At first, it seemed that the initiators which gave the polymers with the highest viscosity were inactive toward the chain-transfer agents such as water, methanol, and so forth, which were usually contained in the polymerization grade formaldehyde. The result of the polymer deposit on the reactor wall is also shown in Table 1. Tetravalent organo-tin compounds gave the least deposit on the reactor wall among the initiators tested.

Ind. Eng. Chem. Res., Vol. 37, No. 5, 1998 1731 Table 1. Results of Polymerization Using Various Initiatorsa initiator

DMDOAA

TBTL

DBTDL

DBTDM

DOTDM

Co(AA)3

η (sp/C) Mw/Mn (1) η (sp/C) (1) Mn (2) η (sp/C) (2) Mn deposit

1.74 2.2 1.73 34 100 1.70 34 000 good

18.7 47 4.62 34 600 18.3 33 800

2.38 3.5 2.20 32 400 2.37 33 600 excellent

1.67 2.1 1.71 32 200 1.68 32 900 excellent

1.70 2.2 1.73 33 800 1.72 33 300

11.6 26 3.93 34 800 11.9 34 300 fair

a Polymerization temperature, 50 °C; chain-transfer agent, methanol, 4 × 10-3 mol/L of hexane (0.7 × 10-3 mol/mol of feed CH O). 2 DMDOAA: dimethyl dioctadecylammonium acetate, 3.4 × 10-5 mol/L of hexane (6 × 10-5 mol/mol of feed CH2O). TBTL: tributyltin laurate, 1.7 × 10-4 mol/L of hexane (3 × 10-5 mol/mol of feed CH2O). DBTDL: dibutyltin dilaurate, 1.7 × 10-4 mol/L of hexane (3 × 10-5 mol/mol of feed CH2O). DBTDM: dibutyltin dimethoxide, 1.7 × 10-4 mol/L of hexane (3 × 10-5 mol/mol of feed CH2O). DOTDM: dioctyltin dilaurate, 1.7 × 10-4 mol/L of hexane (3 × 10-5 mol/mol of feed CH2O). Co(AA)3: Cobalt acetylacetonate, 1.7 × 10-4 mol/L of hexane (3 × 10-5 mol/mol of feed CH2O). (1): acetylation method 1, 0.1 wt % sodium acetate in acetic anhydride, 140 °C. (2): acetylation method 2, 0.1 wt % sodium acetate in acetic anhydride, 10 times the volume of kerosene relative to acetic anhydride, 140 °C.

The reason tetravalent organo-tin compounds gave excellent results for the reactor deposit is thought to be attributed to the high polymerization activity. The reason cobalt acetylacetonate gave fair results for the reactor deposit is thought to be attributed to the lower polymerization activity. Ishii and Suzuki reported that cobalt acetylacetonate gave a polymer with the highest value of reduced viscosity, although it gave the least polymerization activity (Ishii and Suzuki, 1969) among the metal chelate initiators. Dimethyl dioctadecylammonium acetate has excellent polymerization activity, and it gave good results regarding the reactor deposit. The tetravalent organo-tin compounds gave slightly better results, though the difference was very small. Anyway, the reactor deposit involves very complicated phenomena, and it is very difficult to clarify. The results of the acetylation are shown by comparing the two different methods of acetylation. In some polymers, method 1, which used bulk acetic anhydride, decreases the value of the reduced viscosity compared to that of the raw polymer. However, acetylation method 2, which used acetic anhydride diluted with kerosene, resulted in no substantial difference in the reduced viscosity between the acetylated polymers and the raw polymer. The number-average molecular weights determined by the end group analysis were compared between acetylation methods 1 and 2. We found that no substantial difference was observed between the polymer obtained by the two different methods, though acetylation method 1 decreased the value of the reduced viscosity in some polymers. The polymer with Mw/Mn ) 2, whose molecular-weight distribution is thought to be the most probable distribution and which was obtained using dimethyltin dimethoxide, dioctyltin dimethoxide, and dimethyl dioctadecylammonium acetate as the initiators, showed no substantial change after the acetylation using the two different methods. It seemed that, for the broader molecular-weight distribution, the reduction in the reduced viscosity was larger and the molecular-weight distribution of the acetylated polymer by method 1 seemed to approach the most probable distribution. These facts suggested that during the acetylation, for acetylation method 1, some kind of acetal exchange reaction occurred though its nature was not clear. The number-average molecular weight of all polymers determined by the end group analysis is almost equal, though different types of initiators were used for this experiment. This fact suggested to us that all of the chain-transfer agents such as methanol would react during the polymerization, though different types of initiators were used. In fact, in the past, some patents

(Wagner and Koch, 1967; Brown, 1967) claimed the specific initiator to be inactive toward impurities such as water and methanol, because these initiators produced a polymer with a high viscosity value even though the gaseous formaldehyde included some water and methanol. Also, one paper reported that some type of initiator (such as cobalt acetylacetonate) produced a polymer with an apparent molecular weight of millions based on the reduced viscosity of the raw polymer, and thus the polymerization system using these initiators was proposed to give a living polymerization system (Ishii and Suzuki, 1969). However, all the polymers obtained in our experiments, which used different types of initiators and which gave different values of the reduced viscosity, gave almost the same number-average molecular weight. Judging from these results, though we did not evaluate every type of initiator, the initiator which gave a higher value of the reduced viscosity, gave a polymer with a broader molecular distribution. It seemed plausible that the chain-transfer agent, such as water and methanol, would be consumed during the polymerization of gaseous formaldehyde, though in the past, some specific initiators were said to be inactive toward the chain-transfer agents such as water and methanol. Various types of initiators gave different molecularweight distributions. In the case of quaternary ammonium acetate, the polymerization scheme may be shown as follows using methanol as a chain-transfer agent (Brown, 1967):

initiation

CH3COO- +NR4 + CH2O f CH3COOCH2O- + +NR4

propagation

+ CH3COO(CH2O)n NR4 + CH2O f + NR4 CH3COO(CH2O)n+1

chain transfer + CH3COO(CH2O)n+1 NR4 + CH3OH f CH3COO(CH2O)n+1H + CH3O- +NR4 reinitiation

CH3O- +NR4 + CH2O f CH3OCH2O- + +NR4

The active site of polymerization is thought to be the anionic species of

-(CH2O)nCH2O-

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Only one active species is possible, and propagation involves the addition of formaldehyde to this anionic species. As the molecular weight is determined by the chain-transfer reaction to water, methanol, and so forth, the molecular-weight distribution should be the most probable distribution (Mw/Mn ) 2). This is in accordance with the experimental results. In the case of the tetravalent organo-tin compounds, the polymerization scheme may be shown as follows considering the polymerization scheme given by Brown (Brown, 1967) for the insertion mechanism of organometallic initiators:

Sn-XR + CH2O f Sn-OCH2XR

initiation propagation

Sn-(OCH2)nXR + CH2O f Sn-(OCH2)n+1XR

chain-transfer reaction Sn-(OCH2)n+1XR + CH3OH f H(OCH2)n+1XR + Sn-OCH3 reinitiation

Sn-OCH3 + CH2O f Sn-OCH2OCH3

where X denotes an ether linkage or ester linkage and R denotes an alkyl group. As for dibutyltin dilaurate of the tetravalent organotin compounds, the polymerization scheme is thought to be the insertion of formaldehyde into the Sn-O linkage. Assuming a slow insertion of formaldehyde into the Sn-ester linkage, there should be two kinds of active sites, though these active sites were not experimentally confirmed: CH3(CH2)10COO-Sn-OCH2- and -CH2O-Sn-OCH2-. In this case, different species are thought to have different activities, and the molecularweight distribution of the polymer should be broad. This was in good agreement with the experimental results. As for dibutyltin dimethoxide and dioctyltin dimethoxide, insertion of formaldehyde into the Sn-ether linkage is thought to be fast, and only one active species is possible, though this active site was not experimentally confirmed: -CH2O-Sn-OCH2-. In this case, the molecular-weight distribution should be the most probable distribution, and these results are in good accordance with the experimental results. In the case of tributyltin laurate, only one active site should be considered, though this active site was not experimentally confirmed: (Bu)3Sn-OCH2-. However, the molecular-weight distribution of the polymer was very broad. The reason tributyltin laurate gave a polymer with a broad molecular weight is not presently clear. In the case of the cobalt acetylacetonate, considering the scheme proposed by Ishii and Suzuki (1969), the polymerization scheme may be shown as follows:

initiation

Co(acac)3 + CH2O f (acac)2Co-OCH2-O-C(CH3)dCHCOCH3

propagation (acac)2Co-(OCH2)n-O-C(CH3)dCHCOCH3 + CH2O f (acac)2Co-(OCH2)n+1-O-C(CH3)dCHCOCH3

chain transfer (acac)2Co-(OCH2)n+1-OC(CH3)dCHCOCH3 + CH3OH f H-(OCH2)n+1-O-C(CH3)dCHCOCH3 + (acac)2Co-OCH3 reinitiation

(acac)2Co-OCH3 + CH2O f (acac)2Co-OCH2OCH3

where acac denotes acetylacetonate. The polymerization scheme is thought to be the insertion of formaldehyde into the Co-O linkage. Assuming the slow insertion of formaldehyde into the Coacac linkage, there should be three kinds of active sites, though these active sites were not experimentally confirmed: (acac)2-Co-OCH2-, (acac)-Co(OCH2-)2, and -H2CO-Co-(OCH2-)2. In this case, different species are thought to have different activities, and the molecular weight distribution of the polymer should be broad. Cobalt acetylacetonate gave a polymer with a broadmolecular-weight distribution. Considering the fact that almost all metal chelate compounds produced a polymer with a high reduced viscosity value (Ishii and Suzuki, 1969), almost all chelate compounds would give polymers with broad-molecular-weight distribution. This might be due to the multiple active sites. We examined how the chain-transfer agent (in this case, methanol) affected the molecular weight. Figure 2 shows the effects of the chain-transfer agent on the reduced viscosity of the raw polymer using dibutyltin dimethoxide. The reduced viscosity of the obtained polymer decreased with an increase in the methanol feed. Figure 3 shows the relationship between the methoxyl group of the polymer and the methanol fed into the reactor. In this case, hexane solution containing the initiator and methanol was continuously fed into the reactor. There was a good relationship between the methoxyl group of the polymers obtained and the methanol fed into the polymerization system. This fact showed that the methoxyl group of the polymer came from methanol. Figure 4 shows the relationship between the numberaverage molecular weight of the polymer estimated by the end group analysis and the methanol fed into the reactor. In this case, hexane solution containing the initiator and methanol was continuously fed into the reactor. There was also a good relationship between number-average molecular weight of the polymers and the methanol fed into the polymerization system. These figures suggest that the molecular weight of the polymer was determined by the quantity of the chain-transfer agent. Thus, it was concluded that almost all of the chain-transfer agents fed into the reaction system were attached to the polymer end group. We also confirmed that, in the withdrawn polymerization slurry, methanol was an undetectable trace by a gas chromatographycal method (methanol concentration in hexane was below several ppms). This meant that most of the methanol fed into the polymerization reactor was consumed by the chain-transfer reaction. Figures 5 and 6 show the effects of the polymerization temperature on the reduced viscosity- and molecularweight distribution using dibutyl-tin dilaurate. An increase in the polymerization temperature gave a polymer with a reduced value of the reduced viscosity

Ind. Eng. Chem. Res., Vol. 37, No. 5, 1998 1733

Figure 2. Effects of methanol feed on the reduced viscosity of the polymer. Polymerization temperature, 50 °C; initiator, dibutyltin dimethoxide, 1.7 × 10-4 mol/L of hexane (3 × 10-5 mol/mol of feed CH2O).

Figure 3. Effects of methanol feed on the methoxyl end group of the polymer. Polymerization temperature, 50 °C; initiator, dibutyltin dimethoxide, 1.7 × 10-4 mol/L of hexane (3 × 10-5 mol/mol of feed CH2O).

Figure 4. Effects of methanol feed on the number-average molecular weight of the polymer. Polymerization temperature, 50 °C; initiator, dibutyltin dimethoxide, 1.7 × 10-4 mol/L of hexane (3 × 10-5 mol/mol of feed CH2O).

and Mw/Mn, though the number-average molecular weight was almost constant (each Mn was as follows: at 40 °C 33 000, at 50 °C 32 500, at 60 °C 32 000). The reason the increase in the polymerization temperature produced a polymer with a reduced value of Mw/Mn is postulated as follows: The increase in the polymerization temperature will activate the hydroxyl end group of the dead polymer. If the hydroxyl polymer end group acts as a chain-transfer agent, the molecular-weight distribution of the obtained polymer will approach the most probable distribution. One other postulate is

Figure 5. Effects of polymerization temperature on the reduced viscosity of the polymer. Initiator, dibutyltin dilaurate, 1.7 × 10-4 mol/L of hexane (3 × 10-5 mol/mol of feed CH2O); chain-transfer agent, methanol, 4 × 10-3 mol/L of hexane (0.7 × 10-3 mol/mol of feed CH2O).

Figure 6. Effects of polymerization temperature on the molecularweight distribution of the polymer. Initiator, dibutyltin dilaurate, 1.7 × 10-4 mol/L of hexane (3 × 10-5 mol/mol of feed CH2O); chaintransfer agent, methanol, 4 × 10-3 mol/L of hexane (0.7 × 10-3 mol/mol of feed CH2O).

possible; with the increase in polymerization temperature, the reverse reaction will increase. Thus, each active site will approach equal reactivity, that is, equal propagation rate constant and equal chain-transfer rate constant. Thus, the molecular-weight distribution will approach the most probable distribution. The polymers with different molecular weight distributions were pelletized with the antioxidant and thermal stabilizer. The polymer pellets were tested for the melt flow index value. Figure 7 shows the relationship between the polymer reduced viscosity and the melt flow index value of the polymer pellet obtained using two different kinds of initiators. The different initiators gave different curves. The polymer obtained using dibutyltin dimethoxide, which produced a polymer with a narrower molecular-weight distribution (Mw/Mn ) 2) gave the upperside curve compared to the polymer obtained using dibutyltin dilaurate, which produced a polymer with a broader molecular-weight distribution. With the same reduced viscosity, the melt flow index of the polymer with the narrower molecular-weight distribution showed a higher value than those of the polymer with broader molecular-weight distributions. These figures suggest to us that the melt flow index represents the weight-average molecular weight and the reduced viscosity represents the viscosity-average molecular weight. Comparing the same viscosity-average molecular weight polymer, the polymer with the broader

1734 Ind. Eng. Chem. Res., Vol. 37, No. 5, 1998

tin methoxide was used as an excellent initiator for the commercial production of a formaldehyde polymer. Conclusions

Figure 7. Relationships between the reduced viscosity and melt flow index (MFI) using two kinds of organo-tin initiators. Polymerization temperature, 50 °C; O, initiator, dibutyltin dimethoxide (DBTDM), 1.7 × 10-4 mol/L of hexane (3 × 10-5 mol/mol of feed CH2O); chain-transfer agent, methanol ((0.2 × 10-3) - (0.9 × 10-3) mol/mol of feed CH2O); b, initiator, dibutyltin dilaurate (DBTDL), 1.7 × 10-4 mol/L of hexane (3 × 10-5 mol/mol of feed CH2O); chaintransfer agent, methanol ((0.5 × 10-3) - (1.1 × 10-3) mol/mol of feed CH2O).

Figure 8. Effects of molecular-weight distribution on the Dart impact strength. Each polymer melt flow index was adjusted to about 14 g/10 min; samples were obtained using the following initiator: Mw/Mn 2.1, dibutyltin dimethoxide; Mw/Mn ) 2.2, dimethyl dioctadecylammonium acetate; Mw/Mn ) 2.9-4.0 dibutyltin dilaurate.

molecular-weight distribution gave the polymer with higher weight-average molecular weight. The polymers with different molecular weight distributions were pelletized and injection molded into a plate for the Dart impact strength test. Figure 8 shows the relationship between the Dart impact strength and the molecular-weight distribution (Mw/Mn) by maintaining the melt flow index value of the polymer nearly constant (in this case MFI ) 14). The polymer with the narrow molecular-weight distribution (Mw/Mn: 2.0-2.2) showed a Dart impact strength value of around 300 kg‚cm, while the polymer with the broader molecular-weight distribution (Mw/Mn > 2.9) showed a Dart impact strength value below 150 kg‚cm. In the case of the polyacetal resin, the Dart impact strength is one of the very important measures for practical use as an engineering material for injection mold use, and the Dart impact strength was found to be very sensitive to the molecularweight distribution of the polymer. For injection molding use, a polyacetal with a narrowmolecular-weight distribution (Mw/Mn ) 2) is essential. The alkyl tin dimethoxide is an excellent initiator having both characteristic features of high activity for a low reactor polymer deposit and producing a polymer with a narrow-molecular-weight distribution. This alkyl

Various initiators were investigated for the polymerization of gaseous formaldehyde in hexane solution. 1. Comparison of Various Initiators Are as Follows. Dialkyl tin dimethoxide, a tetravalent organotin compound, was found to be an excellent initiator. This initiator was sufficiently active to give the lowest polymer deposit on the reactor wall, gave a polymer with a narrow-molecular-weight distribution (Mw/Mn ) 2), and produced a tough material. A quaternary ammonium acetate, such as dimethyl dioctadecylammonium acetate, is a good initiator. This initiator gave a polymer with a narrow-molecularweight distribution (Mw/Mn ) 2) and produced a tough material. However, this initiator gave slightly more polymer deposit on the reactor wall compared to dialkyltin dimethoxide, though the difference was very small. Dibutyltin dilaurate, a tetravalent organo-tin compound, was found to be the one of this family sufficiently active to yield the lowest polymer deposit on the reactor wall. However, this initiator gave a polymer with a broader molecular-weight distribution compared with dialkyltin dimethoxide and dimethyl dioctadecylammonium acetate and produced a brittle material. Tributyltin laurate and cobalt acetylacetonate gave a polymer with a broad-molecular-weight distribution. This type of initiator is not suitable for producing a polymer for injection molding use. 2. Molecular Weight of the Polymer Is Given as Follows. The chain transfer agents such as methanol were completely consumed during the polymerization of gaseous formaldehyde, though different types of initiators were examined. Thus, number-average molecular weight is determined by the amount of chaintransfer agent fed to the reaction. The initiators formerly said to be inactive toward impurities such as water and methanol were supposed to give a polymer with a broad-molecular-weight distribution. 3. Effect of the Molecular Weight Distribution Is Given as Follows. The molecular-weight distribution of the polymer is an important factor for producing a tough material. The polymer with a narrow-molecular-weight distribution (Mw/Mn ) 2) produced a polymer with a high Dart impact strength and the polymer with a broad molecular distribution (Mw/Mn > 2.9) had a low Dart impact strength. Literature Cited Barker, S. J.; Price, M. B. In Ritchie, P. D., Ed.; Polyacetals; Iliffe Books, London Books: London, 1970. Blair, L. M. A survey of acetal resins. Technical Pap. Reg. Technol. Conf.-Soc. Plast. Eng. Southeast Ohio Sec. 1976, 164. Brown, N. Polymerization of formaldehyde. J. Macromol. Sci. (Chem.). 1967, A1, 209-230. Frank, H.; Jaaks, V.; Kern, W. Determination of number average molecular weight of polyoxymethylene. Makromol. Chem. 1968, 114, 92. Ishii, T.; Suzuki, T. Polymerization of formaldehyde by metal chelate. J. Chem. Ind. Jpn. (Kogyo Kagaku Zasshi) 1969, 72, 2644-2649. Kobayashi, Y.; Suzuki, I.; Ishida, S. How Asahi makes polyacetals. Hydrocarbon Process. 1972, 51 (11), 111-112. Koch, T. A.; Lindvig, P. E. Molecular structure of high molecular weight acetal resins. J. Appl. Polym. Sci. 1959, 1, 164-168.

Ind. Eng. Chem. Res., Vol. 37, No. 5, 1998 1735 Linton, W. H.; Goodman, H. H. Physical properties of high molecular weight acetal resins. J. Appl. Polym. Sci. 1959, 1, 179-184. Masamoto, J. unpublished data, Asahi Chemical. Morishita, H.; Masamoto, J.; Hata, T. Process for producing high purity formaldehyde. U.S. Patent 4,962,235, 1990, assigned to Asahi Chemical. Persak, K. J.; Blair, L. M. Acetal resins. Kirk-Othmer Encycl. Chem. Technol. (3rd ed) 1978, 2, 112-123. Schweitzer, C. E.; MacDonald, R. N.; Punderson, J. O. Thermally stable high molecular weight polyoxymethylenes. J. Appl. Polym. Sci. 1959, 1, 158-163.

Wagner, K.; Kocher, E.-U. Process for the production of high molecular weight polyoxymethylenes. U.S. Patent 3,316,219, 1967, assigned to Bayer. Walling, C.; Brown, F.; Bartz, K. U.S. Patent 3,027,352, 1962, assigned to Celanese.

Received for review November 13, 1997 Revised manuscript received February 20, 1998 Accepted February 21, 1998 IE970856L