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2 Thermal Properties of POE-H-Ps A, B, and C

2 Thermal Properties of POE-H-Ps A, B, and C

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Poly(alkylene H-phosphonate)s



39



Table 1.11 Thermal Properties of POE-H-Ps A1, B1, C1, and PEGs

Component



Mn

VPO



PEG Content

Weight (%)



Glass Transition

Tg ( C)



Melting

Point

Tm ( C)



Heat of

Melting

ΔHm (J/g)



A1

B2

C3

PEG 400

PEG 600

PEG 1000

PEG 5000

PEG 10,000a

PEG 10,500



4900

6500

9300

400

600

1000

5000

10,000

10,500



89

88

92

100

100

100

100

100

100



253.0

253.0

À

À

À

À

À

À

À



24.9

18.2

37.5

25.5

15.4

31.2

53.7

66.9

60.3



11.75

82.62

105.8

116.5

127.9

154.0

183.0

171.6

178.0



a



Data taken from Ref. [120].



TgA



(A)

(B)



TgB



(C)

Exo

ΔT

Endo

TmA



TmB

TmC

–50



0



25



50 T°C



Figure 1.8 DSC thermograms of (A) POE-H-P (A1), (B) POE-H-P (B2), and (C) POE-H-P

(C3).



(Table 1.11). In an analogous fashion, the enthalpy of melting of A, B, and C

increases with their molecular weight, but it is still notably smaller than the ΔHm

of pure PEGs of comparable size, indicating the existence of some unfavorable

geometrical chain alignment [121].



40



2.3



Polyphosphoesters



Reactivity of Poly(alkylene H-phosphonate)s



The reactivity of poly(alkylene H-phosphonate)s is similar to the reactivity of

diesters of H-phosphonic acid. The charge distribution in the molecule of the most

stable conformer of dimethyl H-phosphonate was obtained with Mulliken population analysis on the HF/6À31 1 GÃ //HF/6À31 1 GÃ level in the following.

–0.920

O

H3 C



P

O



2.170

O



0.144

CH3



H



The electron density is lower at the phosphorus atom, i.e., the phosphorus atom

plays the role of an electrophilic center; the α-carbon atom of the alkoxy group is

the second electrophilic center, i.e., it is another potential site of nucleophilic

attack, and this nucleophilic center is considerably weaker than the phosphorus

atom.

The most important reactions of poly(alkylene H-phosphonate)s are: (1) hydrolysis, due to the presence of hydrolytically unstable PaOaC bonds; (2) oxidation;

and (3) additional reactions to double bonds (carbonÀcarbon, Schiff base, and carbonyl group), due to the presence of the highly reactive PaH group.



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 PaOaC 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”).

When the nucleophile attacks the phosphorus atom in the end groups, the corresponding alcohol is eliminated and the PaOH 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

PaOH 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 (ke) is higher

compared to the rate of hydrolysis (km) of the PaOaC 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



Poly(alkylene H-phosphonate)s



41



O

_



HO - P-O-R'- O-P-O-R'



OH



b

O



a



O



ke



O



_



H



H



n



–ROH



R-O - P-O-R'- O-P-O-R' _

H



H



n

_



OH



O



km



O



O



_ _ _

_

H- O-P-O-R'-OH + HO P-O-R' O P OH

H



m



H



km



_

OH



O



O

HO - P - OH



pH



+



HO - P - O- R' - OH



H



+



HO-R'-OH



H



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



δ 5 11.17 ppm (end phosphorus atom bonded to CH3O group) and an increase in

the integral intensity of the signal at δ 5 8.37 ppm (end phosphorus atom bonded to

OH group). No changes in the integral intensity of the signal at δ 5 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 PaOH end group. Results from 31P{H} NMR studies of the acid- and basecatalyzed hydrolysis of dimethyl H-phosphonate revealed that PaO 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 PaOH group, or cleavage of the PaOaC 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 PaH protons, which appear as doublets at 6.86

with 1J(P, H) 5 716.2 Hz, unit, 6.79 with 1J(P,H) 5 708.8 Hz, and at 6.74 with

1

J(P,H) 5 690.3 Hz. These doublets can be assigned to PaH proton in repeating

units, in P(H)OCH3 end groups, and in P(H)OH end groups, respectively. The last



42



Polyphosphoesters



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



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.

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 1J(P,H) 5 708.8 Hz and 3J(P,H) 5

10.5 Hz; at 10.47 ppm, a doublet of quintets with 1J(P,H) 5 716.2 Hz and

3

J(P,H) 5 9.9 Hz; and at 8.37 ppm, a doublet of triplets with 1J(P,H) 5 690.3 Hz

and 3J(P,H) 5 10.97 Hz.

These signals have to be assigned to the phosphorus atom in P(H)OCH3 end

groups, in repeating units, and in P(H)OH end groups, respectively. The acidic

PaOH 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

13

C{H} NMR spectrum of the poly(oxyethylene H-phosphonate), there is no signal

for the CH3OCH2-carbon atom at 58.8 ppm.



pH Dependence of Hydrolysis

A combination of NMR spectroscopy and SEC is used [111] to explore the changes

in the polymer structure and composition in aqueous environments. A 31P{H}

NMR kinetic study of hydrolysis of PEO-H-Ps at acidic (1.66), basic (8.8), and



14.0



13.0



12.0



11.0



1.5984



8.3704



10.4664



43



14.294



1.0000



11.1715



Poly(alkylene H-phosphonate)s



10.0



9.0



8.0

(ppm)



7.0



6.0



5.0



4.0



3.0



2.0



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

H-phosphonate).



neutral (7) pH, at an initial polymer concentration of 1.23 3 10À3 M, is shown in

Figure 1.12.

The degree of hydrolysis of PEO-H-Ps in the three different media is calculated

from the increase in the concentration of phosphonic acid end groups as a function

of time. It is seen that at neutral pH the degree of hydrolysis does not exceed 20%

even after 24 h. In contrast, under strong acidic conditions (pH51.66), a hydrolysis

level of 90% is reached after 11 h. The process carried out under slightly basic conditions (pH58.8) reaches 40% after 12 h. It is known that the primary hydrolysis

of low-molecular-weight H-phosphonate diesters to the H-phosphonate monoesters

is rather straightforward under basic conditions [79], whereas the secondary hydrolysis of H-phosphonate monoesters to the free phosphonic acid proceeds more

quickly under acidic conditions. A more detailed analysis of the 31P{H} NMR

spectra of POE-H-P A1 after 24 h hydrolysis time in the three different media

(Figure 1.13) reveals the following: (1) Hydrolysis under slightly basic conditions

proceeds by a random scissoring of the polymer main chain and in a predominantly

primary mode. In the spectrum (Figure 1.13c), there is an observable peak at

δ 5 16.35 ppm (1), confirming the presence of CH3OP(O)(H)OCH2-phosphonate

end groups. The presence of CH3OP(O)(H)OCH2-phosphonate end groups can be



15



Figure 1.11



31



10



4.1306

3.9954



6.1723

6.0506

5.9281

5.8063



7.0077

6.8787

6.7337

6.6059

6.2949



12.6027

12.4551



Polyphosphoesters

15.7771

15.6303

15.4352

15.3503

15.1350

15.0135

14.8911

14.7590

14.6482



44



5



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



explained with their low concentration, the result of which is that the rate of hydrolysis of methoxy group is very low. Their existence, as well as the lack of a signal

at δ 5 6.75 ppm (4) assigned to the phosphonic acid (HO)2P(O)(H), is in agreement

with the primary mode of hydrolysis. The increased intensity of the signal at

δ 5 9.55 ppm (3), attributed to the HOP(O)(H))CH2-phosphonic acid end groups, is

used as a quantitative measure for the degree of hydrolysis of PEO-H-P A1. (2)

Hydrolysis under strongly acidic conditions leads to a complete degradation of the

polymer through both primary and secondary modes.

Evidence of this is the disappearance of the phosphonate end groups signal 1 at

δ 5 16.35 ppm and the relatively low intensity of the signal 2 at δ 5 15.00 ppm,

corresponding to the phosphorus atom in the main chain repeating units

(Figure 1.13B). In addition, the appearance of a signal for the phosphonic acid (4)

is further proof of the proposed breakdown. The peak of highest intensity, 3, shows

that the hydrolyzed system is composed mainly of monohydrolyzed species. The

downfield shift of 3 by 0.5 ppm (Figure 1.13B) is most likely due to the deshielding of the phosphorus atom resulting from the hydrogen-bond formation between

the PQO moiety in the phosphonic acid end and the H3O1 ions existing in the



Poly(alkylene H-phosphonate)s



45



Degree of hydrolysis (%)



100

90

80

70

pH 1.66



60



pH 7



50



pH 8.8



40

30

20

10

0

0



200



400



600



800



1000



1200



1400



1600



Time (min)

Figure 1.12 Degree of hydrolysis of PEO-H-P A1 (Table 1.10) versus time at

1.23 3 10À3 M, 37 C, and various pH.



medium; (3) hydrolysis under neutral conditions (pH 5 7.4) proceeds slowly at

1.23 3 10À3 M and through a predominantly primary method (Figure 1.13A). A

quantitative estimate of the two rate constants of hydrolysis, ke and km, cannot be

provided by 31P NMR due to the low concentration of the phosphonate end groups

in the polymers (5 times less compared to the inner phosphonate groups) and the

complex character of the hydrolysis mixtures.



Concentration Dependence of Hydrolysis

A further 31P{H} NMR study of hydrolysis of PEO-H-Ps under neutral conditions

shows that the degree of hydrolysis is affected by the initial concentration of the

polymer. A 70% higher degree of hydrolysis is observed when the initial concentration of the polymer is increased from 1.25 3 10À3 to 7.29 3 10À3 M (Figure 1.14).

The change in the molecular weight distribution during hydrolysis of POE-H-Ps

is analyzed using SEC. The SEC profiles of the samples, taken after 1, 3, 5, and

24 h of hydrolysis at concentrations 1.23 3 10À3 and 7.29 3 10À3 M, respectively,

are shown in Figures 1.15 and 1.16.

A comparison of Figures 1.15 and 1.16 shows significant differences in the

trends of hydrolysis as a function of the initial concentration of the polymers. As

evidenced by 31P{H} NMR, hydrolysis of POE-H-Ps at 7.29 3 10À3 M proceeds

faster, and degradation is complete after 24 h (Figure 1.16). To avoid any misinterpretation of the SEC data, a comparative 31P{H} NMR analysis of the same samples was performed, and the results are plotted in Figure 1.16.

It is obvious that the degree of hydrolysis of POE-H-Ps obtained by in situ

31

P{H} NMR (Figure 1.14) and SEC/31P{H} NMR (Figure 1.17) is rather close,

providing confirmation for the validity of the results obtained.



46



Polyphosphoesters



2



3

(C)



1



16



15



14



13



12



11



10



9



8



7



6



ppm



3



4



2



16



15



14



13



12



11



10



9



8



7



(B)



6



ppm



2



3

1



(A)

16



15



14



13



12



11



10



9



8



7



6



ppm



Figure 1.13 31P{H} NMR spectra of hydrolysis of POE-H-P (A1) at concentration

1.23 3 10À3 M and (A) 24 h, pH 5 7.4; (B) 24 h, pH 5 1.66; and (C) 24 h, pH 5 8.8.



The observed concentration differences might be explained by the fact that

hydrolysis at 7.29 3 10À3 M results in a higher concentration of phosphonic acid

end groups, which in turn self-catalyze the process.



Dependence of Hydrolysis on the PEG Segment Length

Hydrolysis of polymers A, B, and C was performed under neutral conditions at the

same concentrations (1.23 3 10À3 and 7.29 3 10À3 M). The results from 31P{H}

NMR analysis are graphically presented in Figures 1.18 and 1.19 and reveal a similar pattern to that shown in the previous section.

It can be seen that hydrolysis of all POE-H-Ps at neutral conditions is affected

in a similar fashion by their initial concentration. Independently of the POE segment length, a difference of about 70% in the degree of hydrolysis is observed

upon a sixfold increase in the starting concentration of the polymer (Figures 1.18



Poly(alkylene H-phosphonate)s



47



100



Degree of hydrolysis (%)



90

80

70

c 1.25e–3M



60

c 7.29e–3M



50

40

30

20

10

0

0



200



400



600



800



1000



1200



1400



1600



Time (min)

Figure 1.14 Comparison of the degrees of hydrolysis, % (31P{H} NMR) of POE-H-Ps A1

versus time at neutral pH at two different concentrations, 1.25e-3M and 7.29e-3M.



70



0 min



60



60 min

180 min



RI (mV)



50



420 min



40



1440 min



30

20

10

0

17



18



19



20



21



22



23



24



25



Vol. (mL)



Figure 1.15 SEC profile of the molecular weight distribution of PEO-H-P based on PEG

400 in the course of neutral hydrolysis (after 1, 3, 7, and 24 h) at a concentration of

1.23 3 10À3 M.



and 1.19). In both concentrations, PEO-H-P A shows a 10À12% higher degree of

hydrolysis with respect to POE-H-Ps B and C. The slightly better hydrolytic stability of polymers B and C can be attributed to the relatively lower weight content of

hydrolyzable groups.



48



Polyphosphoesters



0 min



250



60 min

180 min



200



420 min



RI (mV)



1440 min

150

100

50

0

17



18



19



20



21



22



23



24



25



Vol. (mL)



Degree of hydrolysis (%)



Figure 1.16 SEC profile of the molecular weight distribution of PEO-H-P based on PEG

400 in the course of neutral hydrolysis (after 1, 3, 7, and 24 h) at a concentration of

7.29 3 10À3 M.



90

80

70

60

50

40

30

20

10

0



c 1.25e–3M

c 7.29e–3M



0



500



1000



1500



2000



Time (min)

Figure 1.17 The degree of hydrolysis (%) of PEO-H-P A1 versus time at pH 5 7.4,

obtained by consecutive SEC and 31P{H} NMR analysis.



The initial stage of hydrolysis involved cleavage of the alkoxy end groups. The

rate of hydrolysis of the end alkoxy groups was higher than that of PaOaC bonds

in the repeating unit. A 31P{H} NMR study of the hydrolytic stability of poly(oxyethylene H-phosphonate) showed that the degree of hydrolysis after 6 h at acid pH

is about 20% and at basic pH is 36%. This result can be explained by the aggregation of poly(oxyethylene H-phosphonate), which prevents further hydrolysis of the

polymer. An aqueous SEC study of poly(oxyethylene H-phosphonate)s showed predominant self-assembly of these polymers in water [111]. Obviously, the acidic



Degree of Hydrolysis (%)



Poly(alkylene H-phosphonate)s



49



30

25

Polymer A1

20



Polymer B2

Polymer C3



15

10

5

0

500



0



1000



1500



2000



Time (min)



Degree of hydrolysis (%)



Figure 1.18 Degree of hydrolysis of PEO-H-P A, B, and C versus time at concentration of

1.23 3 10À3 M and pH 5 7.4, obtained by 31P{H} NMR at 37 C.



100

90

80

70

60

50

40

30

20

10

0



Polymer A1

Polymer B2

Polymer C3



0



500



1000



1500



2000



Time (min)



Figure 1.19 Degree of hydrolysis of PEO-H-P A, B, and C versus time at concentration of

7.29 3 10À3 M and pH 5 7.4, obtained by 31P{H} NMR at 37 C.



PaOH groups, formed as a result of hydrolysis of the PaOaC bond, participate in

hydrogen bonding with PQO groups, resulting in the formation of aggregates. At

elevated temperatures, the final products of hydrolysis were H-phosphonic acid and

the starting hydroxyl-containing compound.



2.3.2 Oxidation (AthertonÀTodd Reaction)

The AthertonÀTodd reaction was used to transform poly(alkylene H-phosphonate)s

into the corresponding poly(alkylene phosphate)s. The reaction of AthertonÀTodd

is an interaction of dialkyl H-phosphonates with chlorocarbons in the presence of a

base [122,123]. This is a method for oxidation of dialkyl H-phosphonates to the



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