Polyphosphoesters (eBook)

2.1.4 Vapor Pressure Osmometry

VPO measurements were performed on a Vapor Pressure Osmometer Model 833 (UIC Inc.), calibrated with a PEG standard (1000 Da) in toluene at 50°C. The VPO yields data that seem more trustworthy. VPO is an absolute method that does not depend on the presence or absence of a particular end group. Molecular weights measured by 31P{H} NMR spectroscopy are higher, except polymer A2, which was compared with those of SEC and VPO. The results for the poly(oxyethylene H-phosphonate) A2 revealed that the values of the molecular weight determined by 31P{H} NMR, SEG, and VPO are very close. The close agreement of all three molecular weight values for these POE-H-Ps indicates that they contain almost exclusively phosphonate end groups.

According to the process of polytransesterification of dialkyl esters of H-phosphonic acid, the side reactions resulting from the breakdown of the chain growth are connected with the nucleophilic attack of the hydroxyl group on the α-carbon atom of the alkoxy groups, connected to phosphorus atom, or at the phosphorus atom—intramolecular transesterification. Obviously, these side reactions are strongly reduced because dialkyl H-phosphonate is in excess, and the content of the terminal hydroxyl groups is very low.

2.3.1 Hydrolysis of Poly(alkylene H-phosphonate)s

Poly(alkylene H-phosphonate)s are hydrolytically unstable due to the presence of the hydrolytically unstable POC bonds, and in the presence of water they undergo hydrolysis. They are very sensitive to moisture. It is known that the process of poly(alkylene H-phosphonate)s hydrolysis occurs by the same reaction as does that of low molecular diesters of H-phosphonic acid, starting with a nucleophilic attack of the oxygen atom of the hydroxyl group on the phosphorus atom—the electrophilic center (Scheme 1.7, pathway “a”).

Scheme 1.7 Hydrolysis of poly(alkylene H-phosphonate).

When the nucleophile attacks the phosphorus atom in the end groups, the corresponding alcohol is eliminated and the POH end is formed. In this case, the molecular weight does not change. When the nucleophile attacks the phosphorus atom in the repeating units, the products of hydrolysis are oligomers with end POH group and end hydroxyalkyl group, and the molecular weight of the polymer decreases. Hydrolysis of poly(alkylene H-phosphonate)s results in replacement of the substituents at the phosphorus atom. That is why hydrolysis can be easily controlled by NMR spectroscopy. The change of the type of substituents at the phosphorus atom can be detected by 31P{H} NMR spectroscopy. The experimental results revealed that the rate of hydrolysis of the end alkoxy groups (k e) is higher compared to the rate of hydrolysis (k m) of the POC bond in the main polymer chain. This conclusion is based on the 31P{H} NMR studies. The addition of water to the sample results in a decrease of the integral intensity of the signal at δ=11.17 ppm (end phosphorus atom bonded to CH3O group) and an increase in the integral intensity of the signal at δ=8.37 ppm (end phosphorus atom bonded to OH group). No changes in the integral intensity of the signal at δ=10.47 ppm for the phosphorus atom in the repeating units were observed. The ratio between integral intensity of the phosphorus atoms in the repeating units and those of the end groups remain the same. It is known that the α-carbon atom in the molecule of the dialkyl esters of H-phosphonic acid is the second electrophilic center. So, it can be assumed that hydroxyl anion attacks this α-carbon, not phosphorus. Such an attack (Scheme 1.7, pathway “b”) results in the elimination of alcohol and the formation of the POH end group. Results from 31P{H} NMR studies of the acid- and base-catalyzed hydrolysis of dimethyl H-phosphonate revealed that PO bond cleavage [79] occurs exclusively. An attack on the α-carbon during hydrolysis of the trimethyl ester of phosphoric acid is proved. If the nucleophile attacks a phosphorus atom, there are two possible products: elimination of alcohol and formation of the end POH group, or cleavage of the POC bond and formation of a monoalkyl ester of H-phosphonic acid and a polymer chain with the end hydroxyl group. The process of hydrolysis of the end alkoxy groups can be controlled by 1H NMR spectroscopy. The 1H NMR spectrum of poly(oxyethylene H-phosphonate)s is shown in Figure 1.9 reveals three types of PH protons, which appear as doublets at 6.86 with 1 J(P, H)=716.2 Hz, unit, 6.79 with 1 J(P,H)=708.8 Hz, and at 6.74 with 1 J(P,H)=690.3 Hz. These doublets can be assigned to PH proton in repeating units, in P()O3 end groups, and in P() end groups, respectively. The last end group is formed as a result of hydrolysis of the end methoxy group of the polymer. In the 31P{H} NMR spectrum (Figure 1.10), there are signals at 11.17, 10.47, and 8.37 ppm.

Figure 1.9 1H NMR spectrum of a partially hydrolyzed poly(oxyethylene H-phosphonate).

Figure 1.10 31P{H} NMR spectrum of a partially hydrolyzed poly(oxyethylene H-phosphonate).

From the 31P NMR spectrum (Figure 1.11), it can be seen that the signal at 11.17 ppm represents a doublet of sextet with 1 J(P,H)=708.8 Hz and 3 J(P,H)=10.5 Hz; at 10.47 ppm, a doublet of quintets with 1 J(P,H)=716.2 Hz and 3 J(P,H)=9.9 Hz; and at 8.37 ppm, a doublet of triplets with 1 J(P,H)=690.3 Hz and 3 J(P,H)=10.97 Hz.

Figure 1.11 31P NMR of a partially hydrolyzed poly(oxyethylene H-phosphonate).

These signals have to be assigned to the phosphorus atom in P()O3 end groups, in repeating units, and in P() end groups, respectively. The acidic POH groups are formed as a result of hydrolysis, not as a result of the attack of the hydroxyl group of PEG on the carbon atom of the end methoxy group. In the 13C{H} NMR spectrum of the poly(oxyethylene H-phosphonate), there is no signal for the CH3OCH2-carbon atom at 58.8 ppm.