Flameproof linear polyester, a process for its preparation, and articles formed from said polyester

A flameproof linear polyester is described containing, copolymerized in the molecule, a phosphorus compound of formula: ##STR1## where R is a hydrogen atom, an alkyl group with 1 to 4 carbon atoms or a hydroxyalkyl group, and R' is a hydrogen atom, a hydroxyalkyl group or the group --(CH.sub.2 CH.sub.2 O).sub.n H, where n varies from 1 to 4. Said linear polyester is prepared by firstly forming a low molecular weight precondensate from a dicarboxylic aromatic acid (or a relative lower alkyl diester) and an alkylene glycol, and then polycondensing in the presence of said phosphorus compound, in such a quantity as to obtain a content of phosphorus (expressed as the metal) in the linear polyester of between 0.4 and 0.8% by weight. The linear polyester thus obtained can be formed into articles such as fibres, film, sheets and other articles which, besides being flameproof, possess improved dyeing characteristics. A process is also described for preparing the phosphorus compounds corresponding to the aforesaid general formula, where R is hydrogen or an alkyl group containing 1 to 4 carbon atoms, and R' is hydrogen. Said phosphorus compounds are prepared by reacting phenylphosphinic acid or ester with paraformaldehyde, at a temperature of between 60.degree. and 150.degree. C.

This invention relates to a flameproof linear polyester, and the process 
for its preparation. 
The invention also relates to articles formed from said flameproof linear 
polyester. Finally, the invention relates to a process for preparing 
copolymerisable phosphorous monomers able to flameproof linear polyesters. 
Linear polyesters are prepared in the known art by polycondensing 
dicarboxylic aromatic acids with alkylene glycols, particularly 
terephthalic acid (or its methyl diester), with ethylene glycol in the 
presence of suitable catalysts. The linear polyesters thus obtained are 
valuable products in that they constitute the raw materials for 
interesting articles such as fibres, film, sheets and other articles. 
A drawback of these articles is their inflammability, with all its 
consequent risks, especially in the textile and electrotechnical sectors. 
Various flameproofing agents have therefore been proposed for applying to 
the linear polyester either by surface treatment or by addition during its 
processing, for example during spinning. 
Thus tris-dibromopropyl phosphate has been used in the textile sector as a 
finishing agent. However its resistance to washing is not completely 
satisfactory, and it also manifests a certain toxicity. It is also known 
to add brominated flameproofing agents during the spinning stage. However, 
there are difficulties in obtaining stable brominated compounds which do 
not decompose under the temperature conditions during spinning and the 
other fibre treatment operations. 
In the known art it has also been proposed to add special reactive 
phosphorus-containing monomers during polycondensation of the dicarboxylic 
aromatic acid with the alkylene glycol. With these known monomers, linear 
polyesters are obtained which have an acceptable flame resistance only 
when the quantity of phosphorus (expressed as the metal) in the polymer 
exceeds about 1% by weight. However, these phosphorus levels mean that 
undesirable changes occur in the linear polyester characteristics, and 
there is an economical problem because of the relatively high quantity of 
phosphorus monomer introduced into the polymer. A special phosphorous 
monomer copolymerisable with dicarboxylic aromatic acid and alkylene 
glycol has now been found which is able to give the linear polyester good 
flameproof characteristics when present in said polyester at low levels, 
and in any case at levels less than those usual in the known art. 
Thus, one object of the present invention is a new linear polyester with 
flameproof characteristics. 
A further object of the present invention is the process for preparing said 
flameproof linear polyester. 
A further object of the present invention is represented by the articles 
formed from said flameproof linear polyester. 
A further object of the invention is a process for preparing 
copolymerisable phosphorus monomers able to flameproof linear polyesters. 
Further objects of the invention will be apparent from the description 
given hereinafter. 
In particular, the flameproof linear polyester of the present invention is 
the product of the polycondensation of a dicarboxylic aromatic acid (or a 
relative lower alkyl diester) with an alkylene glycol, which contains 
bonded in the macromolecule a phosphorus compound of formula: 
##STR2## 
where R is a hydrogen atom, an alkyl group with 1 to 4 carbon atoms or a 
hydroxyalkyl group, and R' is a hydrogen atom, a hydroxyalkyl group or the 
group --(CH.sub.2 CH.sub.2 O).sub.n H, where n varies from 1 to 4; the 
quantity of said bonded monomer being such as to obtain a quantity of 
phosphorus (expressed as the metal) in the linear polyester of between 0.4 
and 0.8% by weight. 
One example of a hydroxyalkyl group of the aforesaid general formula is the 
group --CH.sub.2 CH.sub.2 OH--. 
One class of phosphorus compounds useful for the purpose of the present 
invention is that corresponding to the aforesaid general formula in which 
R' is hydrogen, and R is hydrogen or an alkyl group containing 1 to 4 
carbon atoms. 
A process for preparing phosphorus compounds pertaining to said class is 
described hereinafter. 
In particular, said process consists essentially of reacting phenyl 
phosphinic acid or ester and paraformaldehyde at a temperature of between 
60.degree. and 150.degree. C. in the absence of a solvent. Advantageously, 
the chosen temperature is such as to maintain the reaction mass in the 
molten state, and thus lower (between 80.degree. and 110.degree. C.) if 
reacting the esters of phenylphosphinic acid, which are liquid, but higher 
(between 130.degree. and 140.degree. C.) if reacting the free acid. 
It is known (DOS No. 2,226,406) that alkylphosphinic esters react with 
formaldehyde, preferably paraformaldehyde, to give esters of 
hydroxymethylalkylphosphinic acid: 
##STR3## 
The literature also describes the synthesis of hydroxymethylphenyl 
phosphinic acid by reacting phenylphosphinic acid with formalin 
(formaldehyde in solution) catalysed by ammonium chloride (Zhuernal 
Obshchei Khimii vol. 47, No. 7, pag. 1465-1468, July 1977), but it is 
apparent from the reported data (melting point of 80.degree. greater than 
that effectively found) that the product obtained either did not 
correspond to the exact structure attributed to it, or was polluted by 
considerable quantities of different reaction products. It has now been 
found that phenylphosphinic acid esters and even the free acid also react 
with paraformaldehyde in the absence of any catalyst, to give 
hydroxymethylphenylphosphinic acid with high yields which exceed 80%. 
The reaction takes place very simply in the absence of solvent. In reacting 
the phenylphosphinic acid esters, the operation is preferably carried out 
between 80.degree. and 110.degree. C., as these are liquid substances. 
When operating with phenylphosphinic acid, a higher temperature must be 
used, between 130.degree. and 140.degree. C., in order to always keep the 
reaction mass completely molten. 
Under these conditions a partial condensation of hydroxymethylphosphinic 
acid can take place, shown by the development of water and by TLC 
analysis. 
Oligomers of type: 
##STR4## 
are formed, but these can be easily hydrolysed by adding water to the 
reaction mass and boiling for some hours. 
The acidity of the monomer itself is sufficient to complete the hydrolysis. 
The starting substances, ie the phenylphosphinic acid and its esters, can 
be prepared form dichlorophenylphosphine by known methods (see Houben 
Weyl, XII, I, pag. 294 and 322). 
As also stated by DOS No. 2,226,406 for analogous products of this class, 
these new products are useful as intermediates in the production of 
flameproofing agents, plant protectives etc. Their bifunctional structure 
and their molecular characteristics also enable them to be used as 
monomers for polycondensation resins, which is not so in the case of the 
corresponding hydroxymethylalkylphosphinic acids or esters. In this 
respect, even alone, hydroxymethylphenylphosphinic acid tends to 
polycondense above 130.degree. to evolve water, whereas the ester 
derivatives transesterify. The phenomenon becomes particularly apparent 
above 200.degree. C. 
The new products obtained have been identified by elementary analysis, by 
acidimetric determination of equivalent weight, by infrared spectroscopy, 
by NMR and by mass spectrometry. 
The process for preparing the flameproof linear polyester according to the 
present invention comprises the initial formation of a low molecular 
weight precondensate from the dicarboxylic aromatic acid (or a relative 
lower alkyl diester) and the alkylene glycol. A suitable quantity of the 
aforesaid phosphorus compound is then added to the precondensate, and 
polycondensation is carried out until the proper molecular weight values 
of the linear polyesters are obtained. 
Dicarboxylic acids suitable for preparing the linear polyester are the 
dicarboxylic aromatic acids such as terephthalic acid, isophthalic 
acid,5-sulphoisophthalic acid, diphenyl-para,para-dicarboxylic acid and 
para-phenylenediacetic acid. Of these, terephthalic acid is preferred. The 
alkylene glycols suitable for the purposes of the present invention are 
ethylene glycol, 1,3-propanediol, 1,4-butanediol and higher homologues. Of 
these, ethylene glycol is preferred. In preparing the low molecular weight 
precondensate, the prechosen dicarboxylic aromatic acid and alkylene 
glycol are reacted in a molar ratio of the order of 1:1.5, operating at a 
temperature in the range of 180.degree.-240.degree. C. and under 
autogenous pressure, (from about 1.8 to 2.2 kg/cm.sup.2), possibly in the 
presence of catalysts such as amines, quaternary ammonium bases, metals, 
metal oxides or organic or inorganic metal salts. In industrial practice, 
metal oxides such as Sb.sub.2 O.sub.3 or metal acetates or their relative 
mixtures are used. Conveniently, the quantity of catalyst is of the order 
of 0.15-0.03% with respect to the dicarboxylic aromatic acid. Under these 
conditions, a precondensate is obtained having a molecular weight of 
between about 400 and about 1000, and typically of the order of 500, with 
a reaction time of between 1 and 6 hours. 
Instead of the dicarboxylic aromatic acid, the relative lower alkyl diester 
can be used (generally methyl diester) and the precondensate can be formed 
by transesterification. In all cases, a phosphorus monomer chosen from 
those of the aforesaid general class is added to the precondensate thus 
obtained. 
Specific examples of these phosphorus-containing monomers are: 
phenylhydroxymethylphosphinic acid and methyl and ethyl 
phenylhydroxymethylphosphinite. 
The poycondensation is then effected, operating at a temperature of between 
265.degree. C. and 280.degree. C. under a pressure which decreases from 
760 torr to less than one torr in the presence of catalysts chosen from 
those indicated in relation to the precondensation, and in particular 
metals, metal oxides and organic and inorganic metal salts, preferably 
metal oxides such as Sb.sub.2 O.sub.3, possibly adding stabilizing agents 
for protecting the polymer from degradation in the subsequent fusion for 
further formation, such as phosphoric acid, triphenylphosphite and the 
like. 
Conveniently, said catalysts are used in a quantity of the order of 
0.1-0.03% by weight with respect to the initially fed aromatic 
dicarboxylic acid. 
With a total reaction time of the order of 3-8 hours, the content of three 
carboxylic groups is reduced to values of the order of 10-20 meq/kg, and 
the linear polyester of the present invention is obtained with an 
intrinsic viscosity typically within the range of 0.60-0.65. 
This linear polyester shows good flame proof characteristics, as appear 
from its LOI (Limiting oxygen index, ASTM 2863) values given in the 
experimental examples which follow. 
These LOI values are obtained with low phosphorus levels in the linear 
polyester, confirming the marked flame proofing characteristics of the 
phosphorus-containing monomers according to the present invention. The 
presence of these monomers does not negatively influence any of the 
inherent characteristics of the linear polyesters, and infact improves 
their facility for dyeing. 
The flame proof linear polyesters of the present invention can be formed 
into articles such as fibres, film and sheets by the methods usually used 
for this purpose.

Some experimental examples of the preparation of preferred phosphorus 
compounds are described hereinafter. 
EXAMPLE 1 
100 g (0.70 moles) of phenylphosphinic acid (M.P. 80.degree.-81.degree. C.) 
are heated to 120.degree. C. Keeping the molten mass under agitation, 22 g 
(0.73 moles) of paraformaldehyde are added in small portions. The 
temperature rises due to the exothermic nature of the reaction, and the 
addition of paraformaldehyde is regulated so that the temperature does not 
exceed 140.degree. C. Having completed the addition, the reaction mass is 
left at 140.degree. C. for about one hour, until TLC analysis shows the 
complete disappearance of the starting substance. 100 ml of water are then 
added, and the mixture is kept under agitation for 5 hours. The water is 
evaporated under vacuum, the mixture is taken up in isopropanol (200 ml), 
and the isopropanol is partly evaporated in order to completely remove the 
water by utilising the azeotrope provided by the alcohol. 
Hydroxymethylphenylphosphinic acid crystallises on cooling. 103 g of 
product are obtained with a M.P. of 138.degree.-9.degree. C. and a yield 
of 85.1%. 
A further 14 grams of product with a M.P. of 133.degree.-6.degree. C. are 
obtained by concentrating the mother liquors. 
The analytical data of the product are as follows: 
Elementary analysis: 
______________________________________ 
C 49.0% (theoretical 48.9%) 
H 5.34% (theoretical 5.3%) 
P 18.12% (theoretical 18.0%) 
______________________________________ 
Acidimetric equivalent 171.4 (theoretical 172.1) 
Infrared spectrum: 
______________________________________ 
.nu.O--H (alcoholic) 
3480 cm.sup.-1 s 
.nu.O--H (acid) 1600 cm.sup.-1 m-broad 
.nu.P = O 1150 cm.sup.-1 s 
______________________________________ 
Further bands: 1430 cm.sup.-1 s; 1210 cm.sup.-1 ms; 1060 cm.sup.-1 s; 960 
cm.sup.-1 s; 850 cm.sup.-1 m; 760 cm.sup.-1 m; 730m; 680m. (s=strong; 
ms=medium-strong; m=medium). 
Ultraviolet spectrum (solvent water): 
______________________________________ 
.lambda. (nm) 
.epsilon. 
______________________________________ 
262 330 
210 5600 
195 13000 
______________________________________ 
H-NMR(CD.sub.3 OD) (ppm) 
.delta.3.90(2H,d,J=5H.sub.z) 
.delta.7.4-8(5H,m,arom) 
Mass Spectrometry (executed on the silanised compound): m/c (relative 
intensity %) 73 (100); 45 (60); 121 (40); 75 (36); 135 (34); 301 (28); 43 
(25); 47 (24); 316 (0.8) M.sup.+ (carried out with a mass spectrometer 
mod LKB 9000 at 70 eV). 
EXAMPLE 2 
100 g (0.5 moles) of phenylphosphinic acid ethyl ester are heated to 
80.degree. C., and 15 g (0.52) of paraformaldehyde are added in small 
portions. The paraformaldehyde addition is regulated so that the 
temperature does not exceed 110.degree. C. by virtue of the exothermic 
nature of the reaction. When the addition is complete, the mixture is kept 
for about one hour under agitation at 110.degree. C., and the termination 
of the reaction can be seen by gas chromatography or thin layer 
chromatography. 100 cc of a 10% HCl solution are then added, and the 
mixture kept at 80.degree. C. for 30 min. The water and hydrochloric acid 
are then evaporated under vacuum, and the mixture taken up in isopropanol. 
On cooling, 69.3 g of hydroxymethylphenylphosphinic acid are obtained with 
a yield of 80.5% and a melting point of 138.degree.-9.degree. C. 
On concentrating the mother liquors, a further 9 g are obtained with a M.P. 
of 134.degree.-6.degree. C. 
The analytical data of the product are analogous to those of Example 1. 
EXAMPLE 3 
The procedure of Example 2 is followed, with the exception that the 
hydrolysis is not effected. 
The reaction mass is kept under vacuum (0.1 torr) for 1 hour at 80.degree. 
C. The ethylhydroxymethylphenylphosphinite which forms is shown to be 
present to the extent of 95% by gas chromatography. Any attempt to distil 
the ester leads to the development of ethyl alcohol, while a polymer of 
vitreous appearance is obtained which on hydrolysis with concentrated HCl 
gives hydroxymethylphenylphosphinic acid. 
The analytical data of the obtained hydroxymethylphenylphosphinite are as 
follows: 
Ultraviolet spectrum (solvent water): 
______________________________________ 
.lambda. (nm) 
.epsilon. 
______________________________________ 
262 740 
216 8500 
195 12000 
______________________________________ 
Elementary analysis: 
______________________________________ 
P 15.20% (theoretical 15.5%) 
______________________________________ 
Infrared spectrum: 
______________________________________ 
.nu.O--H 3240 cm.sup.-1 s 
.nu.A.sub.r --H 3035 cm.sup.-1 w 
.nu.P = O 1190 cm.sup.-1 s 
______________________________________ 
Other bands: 2990 cm.sup.-1 w; 2910 cm.sup.-1 w; 1590 cm.sup.-1 w; 1440 
cm.sup.-1 s; 1120 cm.sup.-1 s; 1030 cm.sup.-1 s; 960 cm.sup.-1 s; 860 
cm.sup.-1 m; 740 cm.sup.-1 m; 690 cm.sup.-1 m. (s=strong; m=medium; 
w=weak). 
.sup.1 H-NMR (C.sub.6 D.sub.6 at 55.degree. C.): 
.delta.4.19 (2H, d, J=4 Hz) 
.delta.7.2-8.2 (5H, m, arom) 
.delta.3.95 (2H, q, J=7) 
.delta.1.08 (3H, t, J=7) 
.delta.1.08 (3H, t, J=7) 
.delta.3.95 (2H, q, J=7) 
.delta.4.19 (2H, d, J=4 HzP-CH.sub.2) 
.delta.7.2-8.2 (5H, m, arom) 
Mass Spectrometry m/e (relative intensity %): 141 (100); 77 (40); 170 (34); 
169 (21); 142 (16); 51 (12); 78 (11); 47 (9); 200 (3) M.sup.+. (Using a 
mass spectrometer mod LKB 9000s at 70 eV). 
Experimental examples are described hereinafter of the preparation of the 
flame proof linear polyester. In these examples, the inflammability was 
determined in accordance with the ASTM 2863 standard, which measures the 
percentage of oxygen (contained in a nitrogen-oxygen mixture) necessary 
and sufficient to maintain the combustion of the linear polyester sample 
under examination. 
The greater the quantity of oxygen necessary for this purpose, the lower 
the inflammability of the sample under examination. 
EXAMPLE 4 
1 part by weight of ethylene glycol and 1.8 parts by weight of terephthalic 
acid are fed into an autoclave. The mixture is prepolymerised at a 
temperature of 235.degree. C. under a pressure of 2.0 kg/cm.sup.2 and in 
the presence of 0.03% by weight (with respect to the terephthalic acid) of 
a quaternary ammonium base, until a precondensate having an average 
molecular weight of about 500 is obtained. 
This precondensate is used in the following experimental polycondensation 
examples. 
EXAMPLE 5 
150 parts by weight of the precondensate of Example 4, 4.7 parts by weight 
of phenylhydroxymethylphosphinic acid and 0.04% by weight (referred to the 
precondensate) of H.sub.3 PO.sub.4 and Sb.sub.2 O.sub.3 are fed into an 
autoclave. The agitated mass is heated to 270.degree.-280.degree. C., and 
the pressure is reduced from 760 torr to less than 1.0 torr during the 
course of 1.5 hours. 
The reaction is made to proceed under vacuum for a further 2 hours, and at 
the end of this period a linear polyester is obtained having the following 
characteristics: 
______________________________________ 
intrinsic viscosity 0.65 
(determined in o-chlorophenol at 25.degree. C.) 
COOH 17 meq/kg 
(free acid groups) 
Tm 248.degree. C. 
(endothermic melting point measured by DSC) 
DEG 1.6% by weight 
(percentage by weight of ether 
units determined by gas chromatography 
measurement on a saponified and acetylated 
sample) 
LOI 29 vol % 
(Limiting oxygen index - by ASTM 2863) 
phosphorus 0.62% by weight 
______________________________________ 
In determining the LOI, the polymer granules are moulded in order to obtain 
test pieces suitable for the purpose. 
EXAMPLE 6 
150 parts by weight of the precondensate of Example 4, 4.7 parts by weight 
of phenylhydroxymethylphosphinic acid and 0.04% by weight (referred to the 
precondensate) of titanium tetraisopropylate are fed into an autoclave. 
The mixture is polymerised in the manner of Example 5, and a linear 
polyester is obtained having the following characteristics: 
______________________________________ 
intrinsic viscosity 0.63 
COOH 18 meq/kg 
Tm 247.degree. C. 
LOI 29 vol % 
phosphorus 0.60% by weight 
______________________________________ 
EXAMPLE 7 
150 parts by weight of the precondensate of Example 4, 5.6 parts by weight 
of ethyl phenylhydroxymethylphosphinite and 0.04% by weight (referred to 
the precondensate) of H.sub.3 PO.sub.4 and Sb.sub.2 O.sub.3 are fed into 
an autoclave. The mixture is polymerised by the method of Example 5, and a 
linear polyester is obtained having the following characteristics: 
______________________________________ 
intrinsic viscosity 0.64 
COOH 16.0 meq/kg 
Tm 248.degree. C. 
LOI 29 vol % 
phosphorus 0.60% by weight 
______________________________________ 
EXAMPLE 8 
A precondensate is prepared by the method of Example 4 from 1 part by 
weight of ethylene glycol, 1.6 parts by weight of terephthalic acid and 
0.2 parts by weight of isophthalic acid. 
This precondensate is then polymerised by the method described in Example 
5. 
A linear polyester is obtained having the following characteristics: 
______________________________________ 
intrinsic viscosity 0.60 
COOH 15 meq/kg 
Tm 240.degree. C. 
LOI 29 vol % 
phosphorus 0.61% by weight 
______________________________________ 
EXAMPLE 9 (reference) 
150 parts by weight of the precondensate of Example 4 and 0.04% by weight 
(referred to the precondensate) of H.sub.3 PO.sub.4 and Sb.sub.2 O.sub.3 
are fed into an autoclave. 
On polymerising by the method of Example 5, a linear polyester is obtained 
having the following characteristics: 
______________________________________ 
intrinsic viscosity 0.66 
COOH 12 meq/kg 
Tm 257.degree. C. 
LOI 21 vol % 
______________________________________ 
EXAMPLE 10 
150 parts by weight of the precondensate of Example 4, 3 parts by weight of 
phenylhydroxymethylphenylphosphinic acid and 0.04% by weight (referred to 
the precondensate) of GeO.sub.2 are fed into an autoclave. 
The mixture is polymerised by the method of Example 5, and a linear 
polyester is obtained having the following characteristics: 
______________________________________ 
intrinsic viscosity 0.64 
COOH 14 meq/kg 
Tm 251.degree. C. 
LOI 27 vol % 
phosphorus 0.41% 
______________________________________ 
The flameproof linear polyester obtained in Examples 5, 6, 9 and 10 is spun 
in the molten state using a 12 bore spinerette at 1200 m/min, and is then 
drawn at 600 m/min, hot-plate treatment at 140.degree. C. and ironing at 
80.degree. C. 
A stocking formed from this yarn is dyed using a bath at 100.degree. C. 
containing Bleu Palanil 3 RE (0.7% by weight) and a carrier (10% by 
weight). 
Table 1 shows the characteristics. In said table, the luminosity was 
determined by a Gardner colorimeter. It should be noted that the greater 
the quantity of dye fixed by the fibre, the smaller the quantity of light 
reflected (luminosity) by it. 
TABLE 1 
______________________________________ 
Linear Count 
polyester 
dtex/fil. 
Toughness Elongation 
Dye luminosity 
______________________________________ 
Example 5 
90/12 20.3 22.3 20.2 
Example 6 
90/12 19.2 20.1 21.3 
Example 9 
90/12 22.7 29.1 32 
Example 10 
90/12 17.4 24.7 21.5 
______________________________________ 
EXAMPLE 11 
An autoclave is charged with 150 parts by weight of the precondensate of 
EXAMPLE 4, 3.5 parts by weight of phenylhydroxymethylphosphinic acid, 
0.02% by weight of GeO.sub.2 (referred to the weight of the precondensate) 
and 3% by weight (referred to the weight of the precondensate) of 
polyethyleneglycol having an hydroxyl termination, having an average 
molecular weight of 1540. Polymerization is carried out according to the 
procedure of EX. 5 and a linear copolymer is obtained, having the 
following specifications: 
______________________________________ 
Intrinsic viscosity 0.55 
COOH 18 milliequiv/kg 
Tm. 248.degree. C. 
LOI 27 
Phosphorus 0.42% 
Diethyleneglycol contents 
1.4% 
______________________________________ 
EXAMPLE 12 
An autoclave is charged with 150 parts by weight of the precondensate of 
EXAMPLE 4, 1.4 parts by weight of phenylhydroxymethylphosphinic acid, 
0.04% by weight (referred to the weight of the precondensate) of 
phosphoric acid and Sb.sub.2 O.sub.3, 0.1% by weight (referred to the 
weight of the precondensate) of sodium methylate and 5.3 parts by weight 
of the hydroxyethyl ester of the isophthalic-5-sodium sulphonate acid. 
Polymerization is carried out according to the procedure of EXAMPLE 5 and 
a linear copolymer is obtained, having the following specifications: 
______________________________________ 
Intrinsic viscosity 0.54 
COOH 16 meq./kg 
Tm 245.degree. C. 
LOI 29 
Phosphorus 0.52% 
Diethyleneglycol contents 
1.8% 
______________________________________