Polyacetal Polymers

Polyacetal polymers containing polyethylene glycol have been prepared which are stable at physiological pH but which readily degrade at lower pH s. Bolton Hunter reagent conjugates prepared from these materials showed a favorable biodistribution profile of the product. 

A reactor containing 2-amino-1,3-propanediol (10.0 mmol) and 25 ml of IM NaOH was cooled to 0.2°C and treated with A-(9-fluorenylmethoxycarbonyl)chloride (13.1 mmol) dissolved in 10 ml of CH2CI2 over a 1 hour period. The solution was stirred for 1 hour at 0°C and 4 hours at ambient temperamre. The organic solvent was evaporated and the aqueous residue poured into 70 ml of EtOAc. The organic phase was isolated, washed with 5% aqueous HCl, dilute NaHC03, brine, and dried. The mixture was concentrated, the residue re-crystalhzed in chloroform, and the product was isolated. 

Polyethylene glycol having a Mn of 3400 daltons (1.47 mmol) and p-toluene sulfonic acid (0.012 g) were added to a 100-ml flask and heated to 80°C to 90°C for 3 hours at 0.5 to 1.0 torr. The mixmre was cooled and treated with the Step 1 product (1.47 mmol) and 10.0 ml of THF it was further treated with divinyl tri(ethylene glycol) (2.94 mmol) in 10 ml of THF. This reaction mixture was stirred for 2 hours at ambient temperature and then treated with 0.3 ml triethylamine. The reaction mixture was precipitated in 100 ml of hexane, and the product was isolated having aM of 25,000 daltons. 

A solution of the Step 2 product (2.050 g) in 20% piperidine containing 10 ml of CH2CI2 was stirred at ambient temperature and monitored by thin layer chromatography. The amino functionalized polyacetal was then isolated by partitioning the piperidine into hexane and next CH2CI2. The residue was dissolved in THF, and the polymer was precipitated by pouring into 100 ml of hexane. The product was isolated having a Mj, of 23,000 daltons. 

The Step 3 product (50 mg) was dissolved in 10 mg/ml of O.IM borate with a pH 8.5 buffer by the addition of a small amount of NaOH and then treated with A-succinimidyl 3-(4-hydroxy 5-[ I]iodophenyl) propionate), 500 pCi, dissolved in benzene containing DMF, and stirred for 15 minutes at ambient temperature. The mixture was diluted 

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Kumar G., Neelakantan N.R., and Subramanian N., Mechanical behaviour of polyacetal and thermoplastic polyurethane elastomer toughened polyacetal, Polym. Plastics TechnoL Eng., 32, 33, 1993. Newmann W. et al.. Preprints, 4th Rubber Technology Conference, London, May 22-25, 1962. Farrissey W.J. and Shah T.M., Handbook of Thermoplastic Elastomers (Walker B.M. and Rader C.P., eds.). Van Nostrand Reinhold, New York, 1988. 

The most stable polyacetal polymer is polyformaldehyde (or polyoxymeth-ylene, POM) this is the only polyacetal that has reached commercial production. This resin has unique properties (e.g., selflubrication) and is very widely used in automotive applications such as engineering plastics. Acetals are widely used engineering thermoplastics with high load-bearing characteristics and low coefficients of friction. Currently, over 200 million lb of acetals are molded and extruded in the United States. 

In 1947, Du Pont began a development program on the polymerization and stabilization of formaldehyde and its polymer. Twelve years later, Du Pont brought the unzipping tendency under control with proprietary stabilizers and commercially announced Delrin polyacetal polymer (Figure 3). The key to the stabilization of poly formaldehyde resins appears to be a blocking of the terminal... 

Celanese joined Du Pont in the market with their proprietary Celcon polyacetal polymer within a year (Figure 3). Celanese managed to obtain basic patent coverage, despite Du Font s prior filing, on the basis of a copolymer variation that led to an enhanced stabilization against thermal depolymerization... 

Polyoxymethylenes or polyacetals (polymers of formaldehyde or trioxane) produce formaldehyde on heating. The chromotropic acid test for formaldehyde is positive (see Section 6.1.4). 

Davis, A. Effeet of climate on the weathering of polyacetal. Polym. Degiad. Stab. 3, 187-198 (1981)... 

Other Rea.ctions, The photolysis of ketenes results in carbenes. The polymeriza tion of ketenes has been reviewed (49). It can lead to polyesters and polyketones (50). The polymerization of higher ketenes results in polyacetals depending on catalysts and conditions. Catalysts such as sodium alkoxides (polyesters), aluminum tribromide (polyketones), and tertiary amines (polyacetals) are used. Polymers from R2C—C—O may be represented as foUows. 

Heteroatom Chain Backbone Polymers. This class of polymers includes polyesters, which have been widely studied from the initial period of research on biodegradable polymers, polyamides, polyethers, polyacetals, and other condensation polymers. Their linkages are quite frequendy found in nature and these polymers are more likely to biodegrade than hydrocarbon-based polymers. 

Similar polyacetals were prepared by BASF scientists from CO-aldehydic aUphatic carboxyUc acids (189,190) and by the addition of poly(hydroxycarboxyhc acid)s such as tartaric acid to divinyl ethers (191) as biodegradable detergent polymers. 

There are thus no solvents at room temperature for polyethylene, polypropylene, poly-4 methylpent-l-ene, polyacetals and polytetrafluoroethylene. However, as the temperature is raised and approaches F , the FAS term becomes greater than AH and appropriate solvents become effective. Swelling will, however, occur in the amorphous zones of the polymer in the presence of solvents of similar solubility parameter, even at temperatures well below T. ... 

Weak links, particularly terminal weak links, can be the site of initiation of a chain unzipping reaction. A monomer or other simple molecule may be abstracted from the end of the chain in such a way that the new chain end is also unstable. The reaction repeats itself and the polymer depolymerises or otherwise degrades. This phenomenon occurs to a serious extent with polyacetals, polyfmethyl methacrylate) and, it is believed, with PVC. 

Some polymers such as the polyacetals (polyformaldehyde) and poly(methyl methacrylate) depolymerise to monomer on heating. At processing temperatures such monomers are in the gaseous phase and even where there is only a small amount of depolymerisation a large number of bubbles can be formed in the products. 

The properties of the nylons are considerably affected by the amount of crystallisation. Whereas in some polymers, e.g. the polyacetals and PCTFE, processing conditions have only a minor influence on crystallinity, in the case of the nylons the crystallinity of a given polymer may vary by as much as 40%. Thus a moulding of nylon 6, slowly cooled and subsequently annealed, may be 50-60% crystalline, while rapidly cooled thin-wall mouldings may be only 10% crystalline. 

From the time that formaldehyde was first isolated by Butlerov in 1859 polymeric forms have been encountered by those handling the material. Nevertheless it is only since the late 1950s that polymers have been available with the requisite stability and toughness to make them useful plastics. In this period these materials (referred to by the manufacturers as acetal resins or polyacetals) have achieved rapid acceptance as engineering materials competitive not only with the nylons but also with metals and ceramics. 

Both polymers are linear with a flexible chain backbone and are thus both thermoplastic. Both the structures shown Figure 19.4) are regular and since there is no question of tacticity arising both polymers are capable of crystallisation. In the case of both materials polymerisation conditions may lead to structures which slightly impede crystallisation with the polyethylenes this is due to a branching mechanism, whilst with the polyacetals this may be due to copolymerisation. 

At room temperature there is only a small decrease in free energy on conversion of monomer to polymer. At higher temperatures the magnitude of the free energy change decreases and becomes zero at 127°C above this temperature the thermodynamics indicate that depolymerisation will take place. Thus it is absolutely vital to stabilise the polyacetal resin both internally and externally to form a polymer which is sufficiently stable for processing at the desired elevated temperatures. 

Since acetal resins are degraded by ultra violet light, additives may be included to improve the resistance of the polymer. Carbon black is effective but as in the case of polyethylene it must be well dispersed in the polymer. The finer the particle size the better the ultra violet stability of the polymer but the poorer the heat stability. About 1.5% is generally recommended. For white compounds and those with pastel colours titanium dioxide is as good in polyacetals as most transparent ultraviolet absorbers, such as the benzophenone derivatives and other materials discussed in Chapter 7. Such ultraviolet absorbers may be used for compounds that are neither black, white nor pastel shade in colour. 

Where the polyurethane comprises <30% of the blend, the polyurethane remains in discrete droplets within the polyacetal matrix. In this range the particle size and particle size distribution of the elastomer particles are of importance. Where the elastomer component is in excess of 30%, interpenetrating polymer networks exist in the sense that there are two interpenetrating continuous phases (as opposed to two cross-linked interpenetrating polymer systems). 

Cationic initiators can also polymerize aldehydes. For example, BF3 helps produce commercial polymers of formaldehyde. The resulting polymer, a polyacetal, is an important thermoplastic (Chapter 12) ... 

Ring opening polymerization may also occur by an addition chain reaction. For example, a ring opening reaction polymerizes trioxane to a polyacetal in the presence of an acid catalyst. Formaldehyde also produces the same polymer ... 

The combination of weak links and unzipping can be catastrophic and has been a particular problem in the commercial development of some polymers, in particular polyacetals. 

Polymers that are rigid at high temperatures are known as engineering plastics . This class of polymers includes polyacetal and many nylons. These polymers are used in applications such as small gears in office equipment and under the hood of automobiles. 

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