Phosphorus-containing oligoamides obtained by a novel one-pot degradation of polyamide-6

Polymer Degradation and Stability 91 (2006) 778e788

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Phosphorus-containing oligoamides obtained by a novelone-pot degradation of polyamide-6

K. Troev a,*, N. Todorova b, V. Mitova a, St. Vassileva b, I. Gitsov c,)

a Institute of Polymers, Bulgarian Academy of Sciences, Sofia 1113, Bulgariab University of Chemical Technology and Metallurgy, Sofia, Bulgaria

c The Michael M. Szwarc Polymer Research Institute and Department of Chemistry,

College of Environmental Science and Forestry, State University of New York, Syracuse, NY 13210, USA

Received 11 March 2005; received in revised form 15 June 2005; accepted 16 June 2005

Available online 10 August 2005

Abstract

This paper describes a novel strategy for the recycling of polyamide materials and their transformation into functional reactiveoligomers with new properties. The method is illustrated by the heating of polyamide-6 (20,000 Da) in the presence of diesters of the

phosphonic acid, (RO)2P(O)H, where R could be eCH3, eC2H5 or eC6H5. It is found that the reaction proceeds in several parallelprocesses: (i) phosphorylation of the amide group by the alkyl esters of the phosphonic acids and (ii) degradation of the main chainthrough an exchange reaction between the amide and phosphonic acid ester groups. Alternatively the depolymerization could be

induced via a radical reaction with the participation of the polyamide moieties and the PeH group. The proceeding of theabovementioned reactions and the structure of the phosphorus-containing oligoamides are confirmed by 31P, 1H and 13C NMRspectroscopy. Their molecular weights are determined by size exclusion chromatography.� 2005 Elsevier Ltd. All rights reserved.

Keywords: Depolymerization; H-phosphonic acid diesters; Phosphorus-containing oligoamides; Polyamide-6; Recycling

1. Introduction

The majority of the plastics used for variousapplications in public transportation (both automotiveand aircraft industries), housing and industrial building,electrical and electronic equipment have notableflammability. Consumer safety and increased safetyregulations, however, require drastic decrease of thematerial combustibility [1]. Therefore the developmentof new economically feasible synthetic routes for theproduction of polymers with suppressed ignition andburn tendencies is the focus of considerable scientificand industrial interest. Phosphorus-containing oligomers

* Corresponding authors. Tel.: C315 4706851; fax: C315 4706856.

E-mail address: [email protected] (I. Gitsov).

0141-3910/$ - see front matter � 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.polymdegradstab.2005.06.005

are widely used for polymer modification [2e5] and asflame retardants due to their smoke and corrosionreducing properties. Additional advantages of thesecompounds are the low smoke emission and lowtoxicity. Many of the current flame retardants, however,are not suitable for widespread use because they areeither volatile, or not sufficiently thermally stable. Thenew flame retardants have to demonstrate: (a) morepermanence (reduced vapor pressure at increasingmolecular weight); (b) greater thermal stability and (c)improved compatibility with other polymers.

Recently we reported a highly efficient method forpolyurethane [6] and polycarbonate [7] depolymerizationusing diesters of the H-phosphonic acid. The reactionsyielded phosphorus-containing oligourethanes and oli-gocarbonates, respectively. The driving force behindthese studies was the idea to convert used polymers and

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779K. Troev et al. / Polymer Degradation and Stability 91 (2006) 778e788

polymer waste products into reusable intermediates, inour case e in phosphorus-containing oligomers. Thecurrent paper extends this novel approach to the synthesisof phosphorus-containing oligoamides. We anticipatethat products of this composition and structure could beused as precursors for the production of various polymerssuch as polyurethanes, polypropylene, unsaturated resinsand polycarbonates with improved flame retardancy,adhesion and physico-mechanical properties.

2. Experimental

2.1. Materials

Polyamide-6 (Vidlon) with MnZ 20,000 Da, m.p.213e220 �C, hrelZ 2.4 (1% solution in H2SO4), wassupplied by Vidachim, Bulgaria. Dimethyl H-phospho-nate (98%), diethyl H-phosphonate (98%) and diphenylH-phosphonate (w90%) were purchased from Aldrich.They were used without further purification.

2.2. General procedure for the phosphorylation anddepolymerization of polyamide

2.2.1. Reaction with dimethyl H-phosphonate (DMP)The polyamide-6 pellets e 4.9 g (0.04 mol on the basis

of polyamide repeating monomer unit) and DMP e15 g (0.14 mol) were placed in a four-necked flask,equipped with a stirrer, thermometer, reflux condenserand an outlet. The reagents were stirred at 160 �C for6 h. After that time the reaction mixture becamehomogeneous. The system was cooled to 60 �C andthe unreacted DMP was removed by vacuum distillation(1! 10�2 mmHg). The reaction products were isolatedas viscous oil and the impurities were removed byprecipitation from methyl alcohol into THF, yield: 9.5 g.The product was soluble in methyl alcohol andHCOOH, and partially soluble in H2O, ethanol, CHCl3and CH2Cl2. It was characterized by 31P, 1H and13C NMR spectroscopy and size exclusion chromatog-raphy, SEC (Table 1).

2.2.2. Reaction with diethyl H-phosphonate (DEP)The polyamide-6 transformation with DEP was

conducted similarly to the case of DMP. Into a four-necked flask equipped with a stirrer, thermometer, refluxcondenser and outlet were charged 4.9 g (0.04 mol)polyamide-6 pellets and 26.1 g (0.19 mol) DEP. Thereaction mixture was allowed to stand at 180 �C for 6 h.After that time the reaction mixture became a trans-parent homogeneous solution. Subsequently the systemwas cooled to 60 �C and vacuum (1! 10�2 mmHg) wasapplied to remove the unreacted DEP. The amount ofthe collected DEP was 20.08 g (0.15 mol). The reaction

mixture was dissolved in methanol and precipitatedfrom THF to yield the desired product as a viscous oil,yield 10.2 g. It was soluble in methanol and HCOOH,and partially soluble in H2O, ethanol, CHCl3 andCH2Cl2. The characterization was performed by 31P,1H and 13C NMR spectroscopy and SEC (Table 2).

2.2.3. Reaction with diphenyl H-phosphonate (DPP)The reaction set-up was similar to the previous two

procedures. The reagents quantities were as follows:polyamide-6 pellets 5.05 g (0.04 mol), DPP 24.75 g(0.1 mol). The treatment temperature was set at150 �C for 3.25 h. After that time the reaction mixturebecame homogeneous, was dissolved in CHCl3 andprecipitated into diethyl ether. The reaction product wasa viscous oil, that was soluble in CHCl3, methyl alcohol,DMSO and HCOOH. Yield 9.5 g. The product wasanalyzed by 31P, 1H and 13C NMR spectroscopy andSEC (Table 3).

2.3. Characterization

2.3.1. Size exclusion chromatography, SECSEC analyses of the oligomers were performed on

a Waters 244 instrument, equipped with a set of fourUltraHydrogel columns with pore sizes 100, 100, 500and 500 A. The eluent was MeOH/H2O (1:1 v/v) witha flow rate of 0.8 mL/min. The molecular weights weredetermined by conventional calibration constructedwith narrow poly(ethylene glycol), PEG, standards.

Table 1

Phosphorus-containing products from the interaction of polyamide-6

with dimethyl H-phosphonate (see Scheme 1, RZ CH3)

N ofproduct

Structure of phosphorus-containing fragment

31P NMR (ppm), J(Hz)

3a - C - NH-(CH2)5-

O

+CH3

OP(O)(H)OCH3

_

8.22, dq1J(P,H) = 672.83J(P,H) = 12.4

80.2

4a - C - NH2- (CH2)5-

O

+

OP(O)(H)OH_

7.2 4.79, d1J(P,H) = 666.2

7a CH3O-P-OCH3

O

O =C-NH (CH2)5-

9.4 2.11, septet3J(P,H) = 11.0

8aCH3O-P-OCH3

O

3.2

3.31, octetJ(P,H) =11.0

NHC(O)-(CH2)5-

Content,*

The content of phosphorus-containing products is calculated from the

* 31P{H} NMR spectra.

780 K. Troev et al. / Polymer Degradation and Stability 91 (2006) 778e788

2.3.2. NMR1H and 13C NMR spectra were recorded on a Brucker

200 MHz spectrometer using tetramethylsilane as aninternal standard in CDCl3 as the solvent. 31P NMRspectra were recorded on a Brucker 200 MHz spectrom-eter with chemical shifts reported in ppm relative toexternal 85% H3PO4 in CDCl3.

3. Results and discussion

The heating of polyamide-6 with the diesters of theH-phosphonic acid at elevated temperatures (160e180 �C) substantially decreases the molecular weightof the starting polymer. Obviously, these reactionconditions facilitate the chemical bonds cleavage in thepolyamide main chain. 31P NMR spectroscopy is theessential tool to study the reaction pathway due tothe sensitivity of 31P chemical shifts to the changes in thesubstituent type (the electron density) at the phosphorusatom. Based on the results from 31P{H} NMR studies itcan be assumed that the interaction of polyamide-6 withphosphonic acid dialkyl esters at elevated temperatures

Table 2


with diethyl H-phosphonate (see Scheme 1, RZ C2H5)

3b -C - NH- (CH2)5 -

O

+C2H5

OP(O)(H)OC2H5

_7.90, dt1J(P,H) = 655.3 3J(P,H) =8.0

65.4

4b -C - NH2- (CH2)5 -

O

+

OP(O)(H)OH_

16.4 5.54, d1J(P,H) = 667.1

5b CH3CH2O-P-NH-(CH2)5 -

O

H

6.6 12.38, dquintets1J(P,H) = 719.53J(P,H) = 9.8

7b C2H5O-P-OC2H5

O

O =C-NH (CH2)5-

6.8 1.24, quintet3J(P,H) = 7.3

8b C2H5O-P-OC2H5

O

1.0 0.96, t3J(P,H) = 7.3

9b

3.8 0.60, sextet3J(P,H) = 7.1

NHC(O)-(CH2)5-

C2H5O-P-O-

O

O = C-NH (CH2)5-+

C2H5

N ofproduct



Content,*

The content of phosphorus-containing products is calculated from the* 31P{H} NMR spectra.

includes: (a) alkylation of the backbone amide groupby the alkoxy group of the H-phosphonic acid; (b)depolymerization via an exchange reaction with theparticipation of the ester moiety of the H-phosphonicacid and the amide group; and (c) depolymerizationthrough an interaction between the ester PeH groupand the polyamide chain. All possible reactions aresummarized in Scheme 1. The identification of com-pounds 3e9 was made by comparing their chemical shiftvalues with those reported in the literature [8].

3.1. Interaction of polyamide-6 with DMP

The 31P{H} NMR spectrum of the reaction mixturecontains a signal at dZ 11.60 ppm that is typical forthe DMP phosphorus atom. Its intensity stronglydecreases (to less than 1.5%) after a vacuum distilla-tion. The spectrum shows also four new signals at 8.22,4.79, 3.31 and 2.11 ppm, Fig. 1. In the 31P NMRspectrum the signal at 8.22 ppm (Fig. 1) appears asa doublet of quartets with 1J(P,H)Z 672.8 Hz and3J(P,H)Z 12.4 Hz (Fig. 2). This spectral signature istypical for the phosphorus atom in product 3a (Table 1).The P-H proton and the PeOCH3 protons of 3a

yield, respectively, the doublet at 6.74 ppm with1J(P,H)Z 671.9 Hz and the signal at 3.76 ppm with3J(P,H)Z 12.0 Hz in the 1H NMR spectrum of thereaction mixture (Fig. 3). The singlet at 3.43 ppm in thesame spectrum is characteristic for the NeCH3 protons.The formation of product 3a is confirmed also by thechange in the values of the 1J(P,H) constants for the

Table 3


with diphenyl H-phosphonate (see Scheme 1, RZ C6H5)

3c - C - NH2- (CH2)5 -

O

+

OP(O)(H)OPh_

61.5 2.621J(P,H) = 641.8

5c PhO-P-NH-(CH2)5 -

O

H

9.3 5.41, t,1J(P,H) = 654.83J(P,H) = 9.5

7c PhO-P-OPh

O

O =C-NH (CH2)5-

16.5 – 10.46, s

8c PhO-P-OPh

O

NH-C(O)-(CH2)5-

4.1 – 4.94, s

4c - C - NH2- (CH2)5 -

O

+

OP(O)(H)OH_

8.6 1.351J(P,H) = 641.8

N ofproduct



Content,


- C-NH-(CH2)5- +

O 1

P

O

O

OH

R

R

C-NH-(CH2)5-

O

R

OP(O)(H)OR

3

2

RO-P-NH(CH2)5- +

ROC-(CH2)5-

O

O

5 6H

AlkylationExchangereaction

2b R = C2H5

2c R = C6H5

2a R = CH3

RO-P-O-

O

C(O)NH-(CH2)5 -

9R

RO-P-OR

O

C(O)NH-(CH2)5-7

RO-P-OR

O

NH-C(O)-(CH2)5-

8

Depolymerization

Alkylation

(HO)2P(O)(H)

Hydrolysis

-C-NH2-(CH2)5-

O OP(O)(H)OH

4

1 Saltformation

Scheme 1. Interactions of polyamide-6 with diesters of the H-phosphonic acid.

initial and reaction products. Thus, the presence of thenegative charge on the oxygen atom, bonded to thephosphorus, determines a decrease of the 1J(P,H) valuesfrom 700 Hz for the starting dimethyl H-phosphonate to672.8 Hz for product 3a, Fig. 2 [9]. The 13C{H} NMRspectrum contains a signal at 50.57 ppm (a doublet with3J(P,H)Z 11.97 Hz), which could be assigned to thePeOCH3 carbon atom. It is known [10] that dimethylH-phosphonate is a very strong alkylating agent. In thisreaction the a-carbon atom of the methoxy group playsthe role of the electrophilic center. The nucleophile (thenitrogen atom of the polymer amide group) prefers toattack this position instead the phosphorus atom, whichis the strongest electrophilic center in the dimethylH-phosphonate molecule. That is why the alkylation ofamide group at 150 �C favors the formation of product3a in an 80% yield, Scheme 1.

The hydrolysis of the dimethyl H-phosphonate atthese reaction conditions also yields H-phosphonic acid,which participates in a salt formation with the amidegroup in the polymer backbone (4a, Scheme 1, Table 1).The signal in 31P{H} NMR spectrum at 4.79 ppm(Fig. 1) appears as a doublet with 1J(P,H)Z 666.2 Hzin the 31P NMR spectrum (Fig. 2) and can be assignedto the phosphorus atom in 4a. The PeH proton fromthis compound appears in the 1H NMR spectrum as

Fig. 1. 31P{H} NMR spectrum of the reaction mixture obtained after

heating polyamide-6 with DMP.


Fig. 2. 31P NMR spectrum of the reaction mixture obtained after heating polyamide-6 with DMP.

a doublet at 6.81 ppm with 1J(P,H)Z 667.1 Hz, Fig. 3.The broad signal at 11.02 ppm can be assigned to theproton in a PeOH group.

The remaining two signals at 3.31 ppm and 2.11 ppmin the 31P{H} NMR spectrum (Fig. 1) appear in thecorresponding 31P NMR spectrum as an octet with3J(P,H)Z 11.0 Hz (Fig. 4) and a septet with3J(P,H)Z 11.0 Hz (Fig. 5), respectively. It should benoted that the signal at 2.068 ppm in Fig. 5 belongs tothe 4a doublet (Fig. 2). The 31P NMR data suggest thatin these products the phosphorus atoms are not bondedto a hydrogen atom, i.e., the PeH group has reacted withthe polyamide. A model compound is prepared to reveal

the possible origin of these peaks. It is synthesized bya reaction of dimethyl H-phosphonate and hexamethy-lenediisocyanate according to Scheme 2 under reactionconditions similar to those previously reported [11].

The 31P{H} NMR spectrum of the reaction productcontains one signal at dZ 2.42 ppm. The 31P NMRspectrum shows that this signal is a septet with3J(P,H)Z 11.0 Hz. The 1H and 13C NMR character-istics of the model compound contain signals identical tothose observed in the reaction mixture. For example,a doublet in the 1H NMR spectrum of the reactionproduct at dZ 3.74 ppm with 3J(P,H)Z 12.01 Hz couldbe assigned to the PeOCH3 protons. A doublet at

Fig. 3. 1H NMR spectrum of the reaction mixture obtained after heating polyamide-6 with DMP.


dZ 50.67 ppm with 2J(P,H)Z 6.4 Hz in 13C{H} NMRspectrum is due to the carbon atom in the samePeOCH3 group. The 1H NMR spectrum containsalso signals at dZ 2.98, 1.51 and 1.40 ppm, which canbe attributed to the NeCH2, NeCH2eCH2e andNeCH2CH2eCH2 protons, respectively. The carbonatoms in these groups yield signals that appear in the13C{H} NMR spectrum at 42.80, 32.86 and 26.56 ppm,respectively. These results provide sufficient proofto believe that the signal at 2.11 ppm (Figs. 1 and 5)is due to the phosphorus atom in a product with thestructure 7a listed in Table 1. This product is formed

Fig. 4. 31P NMR spectrum of the reaction product 7a obtained after

heating polyamide-6 with DMP.

as a result of the interaction between the PeH groupand the polyamide. The possible mechanism of thisinteraction is shown in Scheme 3.

It can be assumed that at the first stage a thermallyinduced homolytic cleavage of polyamide could occur attwo positions in the main chain [12] under the reactionconditions used (Scheme 3.1). Then the abstraction ofa proton from the dimethyl H-phosphonate would leadto the formation of a dimethyl phosphonate radical(Scheme 3.2). The recombination of this radical with theradicals, which form during the homolytic cleavageof polyamide (Scheme 3, reaction pathways A and B),results in the formation of products 7a and 8a. Thedoublets with 3J(P,H)Z 12.0 and 11.9 Hz observed at3.69 and 3.72 ppm in the 1H NMR spectrum couldbe assigned to products 7a and 8a, respectively.The doublets at 50.76 ppm and 50.49 ppm with2J(P,C)Z 6.1 and 5.8 Hz in 13C{H} NMR spectrumoriginate most probably from the PeOCH3 carbonatoms of the same products 7a and 8a. The SEC analysis(Fig. 6) shows that the Mn of the reaction product is2850 Da. The phosphorus content is 13.9%.

In summary, the heating of polyamide-6 with di-methyl H-phosphonate leads to the phosphorylationof the polyamide (80%) due to the alkylation abilityof dimethyl H-phosphonate. Depolymerization of thepolyamide also proceeds to a lower extent (12.6%) viaa radical reaction with the participation of amide andPeH groups.

Fig. 5. 31P NMR spectrum of the reaction product 8a obtained after heating polyamide-6 with DMP.

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3.2. Interaction of polyamide-6 with DEP

It is expected that DEP would reduce the degree ofthe alkylation reaction and would favor the exchangereaction. The polyamide-6 is heated with DEP at 180 �Cand the reaction mixture is subjected to vacuumdistillation in order to remove the unreacted diethylH-phosphonate. The 31P{H} NMR spectrum (Fig. 7)of the isolated reaction products contains signals at12.38 ppm, 7.90 ppm and 5.54 ppm, and a set of threepeaks at 1.24, 0.96 and 0.60 ppm. Their possible originswill be discussed below.

2 CH3O-P-OCH3 + O = C = N-(CH2)6-N = C = O

O

H

CH3O-P-OCH3

O

O = C- NH- (CH2)6 - NH- C = O

CH3O-P-OCH3

O

= 2.42 ppm, septet 3J(P,H) = 11.0 Hz

cat.CH3ONa,80 ºC

Scheme 2. Synthesis of bis-dimethylcarbamoylphosphonate.

The 31P NMR spectrum (Fig. 8) of the same mixtureshows that the signal at 7.90 ppm is a doublet of tripletswith 3J(P,H)Z 9.6 Hz, which is the characteristicsignature of a phosphorus atom surrounded by OCH2

protons. This arrangement is further confirmed by thedoublet in the 13C{H}NMR spectrum at 61.70 ppm with2J(P,C)Z 4.6 Hz, characteristic for a POCH2 carbonatom. The calculated 1J(P,H)Z 655.3 Hz characterizesa phosphorus atom, which is connected with a negativelycharged oxygen atom. Such environment could begenerated through the alkylation of the amide group

Fig. 6. SEC of the main fraction obtained after heating polyamide-6

with DMP.

(1) -(CH2)4-CH2- C -NH - CH2 -(CH2)4 -

O

A BA

B

- (CH2)4-CH2 + C-NH-CH2(CH2)4-

O

..

..- (CH2)4-CH2 + NH-C-CH2(CH2)4 -

O

(2) - (CH2)4-CH2 + H-P(OCH3)2 - (CH2)4-CH3 + .P(OCH3)2

O

Reaction pathway A

C-NH-CH2(CH2)4- + .P(OCH3)2

O

O

(CH3O)2P - C-NH-CH2(CH2)4-

O

O

O

7a

Reaction pathway B

NH-C-CH2(CH2)4 - + .P(OCH3)2

O

O

(CH3O)2P - NH-C-CH2(CH2)4 -

O

O 8a

.

.

.

Scheme 3. Radical reaction with the participation of polyamide and dimethyl H-phosphonate.


by DEP (see Scheme 1). Thus, the signal at 7.90 ppm canbe attributed to the phosphorus atom in the fragment3b (Table 2). The PeH proton in 3b appears atdZ 6.73 ppm in the 1H NMR spectrum (Fig. 9) with1J(P,H)Z 661.9 Hz.

The signal at 12.38 ppm, which is a doublet ofquintets with 3J(P,H)Z 7.2 Hz and 1J(P,H)Z 719 Hz,can be assigned to the phosphorus atom in 5b (Table 2).The doublet in the 13C{H} NMR spectrum at 63.17 ppmwith 2J(P,C)Z 6.9 Hz could originate from the POCH2

carbon atom of 5b and the one at 39.18 ppm with2J(P,C)Z 6.4 Hz can be assigned to the PNHC carbonatom. Phosphorus atoms with these substituents can beobtained as a result of the exchange reaction betweenthe ethoxy group in diethyl H-phosphonate and anamide group (Scheme 4).

The DEP hydrolysis at the reaction conditions usedleads to the formation of H-phosphonic acid (see


heating polyamide-6 with DEP.
Scheme 1), which participates in the reaction of thesalt formation with the amide group. The phosphorusatom in 4b presents a doublet at 5.54 ppm with1J(P,H)Z 667.1 Hz. As it is mentioned above the31P{H} NMR spectrum of this reaction mixture showsa set of three peaks at 1.24, 0.96 and 0.60 ppm. The31P NMR spectrum reveals that the signal at 1.24 ppmpresents a quintet with 3J(P,H)Z 7.2 Hz; the signal at0.96 ppm is a triplet with 3J(P,H)Z 7.3 Hz and that at0.60 ppm presents a septet with 2J(P,H)Z 8.6 and3J(P,H)Z 7.1 Hz. These peaks could be attributed tothe phosphorus atoms in the products 7b, 8b and 9b,respectively (Table 2). These compounds are formed byradical processes proceeding similarly to those withdimethyl H-phosphonate. SEC analysis (Fig. 10) in-dicates that the fraction with Mn equal to 1480 Daamounts to 93% of the reaction mixture. The phospho-rus content there is 11.3%. The results obtained showthat the heating of polyamide-6 with diethylH-phosphonate includes: (a) phosphorylation of thepolyamide due to the alkylation reaction (65.4%);
Fig. 8. 31P NMR spectrum of the reaction mixture obtained after

heating polyamide-6 with DEP.


Fig. 9. 1H NMR spectrum of the reaction mixture obtained after heating polyamide-6 with DEP.

(b) depolymerization of the polyamide due to theexchange reaction with the participation of ethoxy andamide groups (6.6%) and (c) depolymerization due toradical processes (11.6%).

3.3. Interaction of polyamide-6 with DPP

In contrast to the dialkyl esters of H-phosphonicacid diphenyl H-phosphonate cannot participate in analkylation reaction with amino-containing compounds.The conjugation of the p-electron pair at the oxygenswith the aromatic rings in the phenoxy moietiesincreases the electron density at the a-carbon atom.The reaction mixture obtained after heating ofpolyamide-6 with DPP is dissolved in CHCl3 andprecipitated with diethyl ether. 31P{H} NMR spectrum

-(CH2)5 - C - NH -(CH2)5 +

O

C2H5O - P - OC2H5

O

H

- (CH2)5 - C-OC2H5 + CH3CH2O -P- NHCH2-(CH2)4

O

-C-

O

OH5b

Scheme 4. Exchange reaction between the amide moiety and the ethoxy

group of the H-phosphonic acid.

(Fig. 11) of the precipitate contains five new signals atdZ 5.41, 2.62, 1.35, �4.94 and �10.97 ppm. Theanalysis shows that diphenyl H-phosphonate is removedfrom the reaction mixture by the precipitation becausethere is no signal for the DPP phosphorus atom at1.67 ppm with 1J(P,H)Z 731 Hz. 31P NMR spectrum(Fig. 12) shows that the signal at 5.41 ppm in Fig. 11presents a doublet of triplets with 1J(P,H)Z 654.8 Hzand 3J(P,H)Z 9.5 Hz. The value of the 3J(P,H) constantis characteristic for a phosphorus atom connected toOCH2 protons. This signal can be assigned to thephosphorus atom in 5c (Table 3). Obviously, suchphosphorus-containing compound is during anexchange reaction between the amide functions inthe polymer backbone and the DPP phenoxy groupsproceeding in a similar fashion to the case of diethyl


with DEP.


H-phosphonate (see Scheme 4). The signal atdZ 2.62 ppm, which presents a doublet with1J(P,H)Z 641.8 Hz could be assigned to the phospho-rus atom in 4c. The signal at 1.35 ppm appears asa doublet with 1J(P,H)Z 656.9 Hz and could be due tothe phosphorus atom of product 4a. The signals at


heating polyamide-6 with DPP.

Fig. 12. 31P NMR spectrum of the reaction mixture obtained after

heating polyamide-6 with DPP.


�4.94 ppm and �10.97 ppm, which are singlets, belongto phosphorus atoms, which are not bonded toa hydrogen atom e structures 7c and 8c, respectively.As in the case of dimethyl and diethyl H-phosphonates,these products are formed by radical processes with theparticipation of the PeH group and the polyamide.The SEC analysis shows degradation products with anapparent molecular weight of 1270 Da, Fig. 13. Thisfraction has phosphorus content of 9.6%.

It could be summarized that the heating of poly-amide-6 with diphenyl H-phosphonate includes a phos-phorylation of the polyamide through a salt formation,depolymerization of the polyamide due to the exchangereaction with the participation of the phenoxy andamide groups (9.3%) and depolymerization due to theradical process (20.6%).

4. Conclusions

The results obtained from this study could besummarized as follows: (i) it was shown for the firsttime that an amide group can be alkylated by dialkylesters of H-phosphonic acids; (ii) it was established thata polyamide can be depolymerized by an exchangereaction between polymer amide groups and the estermoiety (alkoxy or phenoxy) of the correspondingH-phosphonic acid; (iii) at the reaction conditions useda new radical reaction takes place with the participationof the polyamide and the PeH group of H-phosphonic


with DPP.

acid diesters, leading ultimately to the depolymerizationof the starting polyamide. We were not able to detectphosphorus-free decomposition products with theanalytical techniques at hand. Their formation couldnot be fully excluded, but the large excess of phosphonicacid diesters shifts the reaction pathway towards thephosphorus-containing products. The practical signifi-cance of the one-pot depolymerization of usedpolyamides by diesters of the H-phosphonic acid is thatit produces phosphorus-containing oligoamides withpotential application as fire-retarding polymer additives.

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