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Structure and Stability ofPolyvinyl ChlorideMohammad Kazim Naqvi aa Research Centre, Indian PetrochemicalsCorporation Limited, Baroda, IndiaPublished online: 19 Dec 2006.

To cite this article: Mohammad Kazim Naqvi (1985) Structure and Stability ofPolyvinyl Chloride, Journal of Macromolecular Science, Part C, 25:1, 119-155

To link to this article: http://dx.doi.org/10.1080/07366578508079459

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JMS.REV . MACROMOL . CHEM . PHYS., C25( 1 ) . 119-155 (1985)

Structure and Stability of Polyvinyl Chloride*

MOHAMMAD KAZIM NAQVI Research Centre Indian Petrochemicals Corporation Limited Baroda. India

I . INTRODUCTION-SIGNIFICANCE OF POLYVINYL CHLORIDE . . . . . . . . . . . . . . . . . . . . . . . . . 120

I 1 . THERMAL STABILITY OF PVC . . . . . . . . . . . . . . 121

111 . METHODS OF STUDYING THERMAL DEGRADATION . . . 1 2 2

I V . STRUCTURAL DEFECTS IN PVC . . . . . . . . . . . . . 1 2 2 A . Unsaturation . . . . . . . . . . . . . . . . . . . . . 1 2 2 B . Branching . . . . . . . . . . . . . . . . . . . . . . 128 C . Determination of Labile Chlorines in PVC by Chem-

ical Modification . . . . . . . . . . . . . . . . . . . . 133 D . Initiator Endgroups in PVC . . . . . . . . . . . . . . 135 E . Head-to-Head Structures in PVC . . . . . . . . . . . 137 F . Oxygen-Containing Defects . . . . . . . . . . . . . . 138 G . Tacticity . . . . . . . . . . . . . . . . . . . . . . . . 139

V . MECHANISM OF DEHYDROCHLORINATION . . . . . . . . 141 A . Free-Radical Mechanism . . . . . . . . . . . . . . . . 1 4 1 B . Ionic Mechanism . . . . . . . . . . . . . . . . . . . . 142

VI . CONCLUSION . . . . . . . . . . . . . . . . . . . . . . . 146

REFERENCES . . . . . . . . . . . . . . . . . . . . . . . 148

*IPCL communication 66 . 119

Copyright 0 1985 by Marcel Dekker. Inc . 0736-6574/85/250 1 4 1 19$3.50/0

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120 NAQVI

I . INTRODUCTION-SIGNIFICANCE OF POLYVINYL CHLORIDE

Polyvinyl chloride (PVC) was first prepared in the laboratory over a hundred years ago. Due to its inherent instability the commercial applications of the polymer could only be developed after the development of effective means for its stabilization. PVC started to gain commercial significance in the late thirties and since then has continued to gain in importance. By using modifying agents (plasticizers, fillers, stabilizers, and other additives), it can be modified to exhibit an extremely wide range of properties.

placed such materials as metals, wood, leather, rubber, cellulose and other natural and synthetic polymers, textiles, conventional paints and coatings, masonry and ceramics, glass, and paper.

the gramaphone records represent a few examples where produc- tion was revolutionized by the commercial advent of vinyl chloride resins.

plastics in the world. It is the second largest volume thermo- plastic used in the United States and is the lowest priced among the five leading plastics (Table 1).

reasons for its large share of the plastics market. Its growth rate averaged 7% per year in the 1970s, despite the loss of about 20% capacity as a result of compliance with OSHA and EPA vinyl chloride emission standards. Despite the economic recession in 1980, a growth of about 7% per year is expected in the demand for PVC resin [l].

PVC compositions have competed with and successfully dis-

The manufacture of wire and cables, resilient flooring, and

Today PVC is commercially one of the most important thermo-

The low cost of PVC and its great versatility are the major

TABLE 1

United States Plastics Sales in 1980 (Source: Modern Plastics, January 1981)

Volume of production Price Polymer ( x 1000 metric tons) ( @ per pound)

LDPE 3350 41-43

PVC 2469 34-37

HDPE 1953 41-48

PP 1647 38-43

PS 1608 46-48

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STRUCTURE AND STABILITY OF POLYVINYL CHLORIDE 1 2 1

I I . THERMAL STABILITY OF PVC

Ideally, free-radical polymerization of vinyl chloride should result in a regular head-to-tail polymer [2-51. Thermogravi- metric analysis and other studies made on low-molecular weight "model" compounds such as 1,3,5-trichlorohexane [ 6 , 71 corre- sponding to the idealized structure of PVC show these structures to be considerably more stable than the polymer. This "abnor- mal" instability of the polymer is attributed to structural defects in the polymer chain which serve as initiation sites for degrada- tion.

stable polymer in commercial use. To quote Grassie on the sub- ject, "Had the polymer been discovered at the present stage of development of the plastics industry, it would almost certainly have been eliminated as useless because of i ts general instability to all the common degradative agents" IS]. Unstabilized PVC star ts degrading even under mild heating conditions. In the initial stages of degradation, HC1 star ts "zipping-off" from the polymer backbone, resulting in the formation of polyene se- quences which can be observed as a gradual coloration and dark- ening of the polymer. On continued heating the polymer under- goes chain scission-leading to a gradual deterioration of mechan- ical properties and chemical resistance-accompanied by cross- linking which predominates in the later stages. The presence of oxygen leads to rapid oxidation of polyene sequences resulting in chain scission and formation of carbonyl and hydroperoxide groups. The process of degradation is autocatalytic and quickly reduces the polymer to a worthless black char.

tions by using stabilizers, but the exact mechanism of dehydro- chlorination of PVC is still a subject of controversy. There i s also very little certainty about the reactions between PVC and stabilizers. Investigation of the degradation and stabilization of PVC remains one of the most active areas of research in polymer chemistry. Such interest reflects the worldwide economic impor- tance of this unstable polymer.

A polymer is a complex mixture of molecules which is difficult to define and reproduce. The quality of the polymer is markedly affected by the conditions of preparation. Different degrees and types of branching, differences in the number and distribution of various irregular s t ructures , along with the degree and purity of the finished product and conditions of further treatment all influence the thermal stability of the polymer and the course of i t s thermal degradation. This further complicates the study of this polymer and explains the differences between results and conclusions obtained by different workers.

In the absence of additives, PVC is almost certainly the least

I t i s possible to suppress these undesirable degradation reac-

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I l l . METHODS OF STUDYING THERMAL DEGRADATION

When PVC is subjected to thermal degradation, only HC1 is evolved initially, but in the later stages a variety of other deg- radation products (mainly polyenes and aromatics) is also formed. Most workers have used the measurement of HC1 evolved for studying the thermal degradation of PVC. Various forms of apparatus have been described in which the HCl is measured by electrical conductivity [ 91 after removal from the polymer sample. In some cases changes in pressure caused by the gaseous HCl [ 101 evolved in a closed system have been used. The potentiometric measurement of the degradation has also been combined with vis- cometric investigations in a Brabender plastograph [ 111.

with the potentiometric determination of evolved HCl [ 12, 131 and thermal volatilization analysis [ 141 , has also been applied to studying PVC degradation. Another possibility is the photometric determination of discoloration during degradation [ 151 .

Thermogravimetric analysis of the polymer, in combination

I V . STRUCTURAL DEFECTS IN PVC

A. Unsaturation

1 . Mechanism of Double-Bond Formation

Since it is well known that chloroalkenes are often much less stable than the corresponding alkanes, olefinic unsaturation may be an important source of thermal instability in PVC. Chain-end unsaturation could arise by disproportionation during bimolecular reaction of polymer radicals, e .g . ,

2(---CH2CHC1) -W ---CH2CH2C1 + ---CH=CHCl

Chain transfer reactions involving the monomer could also result in unsaturation at the chain ends according to the following two reactions:

- VCM

( 1)

--CH2CHCl + CH2=CHC1 -W -CH CH C1+ CH =CC1 2 2 2

C H q=C C1C H 2CHC1-

(A)

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STRUCTURE AND STABILITY OF POLYVINYL CHLORIDE 123

( A ) can react further to give a branched polymer:

-CHCICH CCI=CH2 + CHClCH2CHClCH2-- -b 2

VCM -CHClCH 2CC1CH 2CHC1CH2CHClCH 2---

-CH CHCl+CH =CHCl+ 2 2

CHQCHCl + -CH=CHCl

( B ) is a 1,2-disubstituted olefin and generally unreactive in free-radical reactions.

a reaction can occur between the polymer and free radicals which leads to the elimination of hydrogen chloride and formation of a double bond. The formation of HCl during the polymerization of vinyl chloride has been observed 1171 . system can also give rise to unsaturation in the polymer back- bone.

mechanism of formation of a double bond next to the chain end:

Braun and Schurek [ 161 assumed that during polymerization

The presence of acetylenic impurities or butadiene in the

Starnes et al. [ 181 appear to have evidence for the following

b -CH2CHClCHClCH2 VCM

head- to- head addition

Polymer with CH 2C1 . VCM -CH2CHCICHCH 2C1

branch

-CH2CHCl

1,2-C1 migration 1 I -c;

2 -CH2CH=CHCH C1+ c1

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2. Unsaturation and Thermal Stability

the vinyl proton signals in a pulsed Fourier transform 'H- NMR spectrum to disappear when only 1-2% of the available HC1 has been lost [ 191, This may be attributed to signal broadening for protons in conjugated polyene structures and thus seems to con- stitute direct evidence for the involvement of olefinic defect sites in the early stages of the degradation process and their incorpo- ration into runs of conjugated double bonds in the later stages.

There are many conflicting reports regarding chain-end un- saturation as initiation sites for thermal degradation. Arlman 1 201 found a linear dependence between the rate of dehydrochlorination and the reciprocal value of the average molecular weight of several PVC samples whose molecular weight varied between 56,000 and 175,000. The same linear dependence was found by Bengough and Sharpe [ 21, 221 and Talamini and Pezzin 1231. Bengough and Sharpe used four samples of PVC initiated by AIBN at different temperatures in bulk. Talamini and Pezzin's results were based on three samples obtained by fractionation of a commercial sus- pension-polymerized product, Sicron 548. The molecular weights in both cases lay within the range of about 12,000 to 60,000.

no systematic dependence between the rate of dehydrochlorination and molecular weight for samples obtained by fractionation of Geon 111.

sition of PVC divided into 30 fractions by measuring the HC1 evolved and monitoring the changes in UV spectra. They con- cluded that fractions having lower and higher molecular weights were less stable than the medium fractions. This is in agreement with the findings of Crosato-Arnaldi and co- workers 1 271 . Stud- ies on model compounds also suggest that unsaturated chain end- groups should not have an important influence on the thermal stability of PVC [ 281 . In conclusion, it may be said that the ef- fect of unsaturated endgroups on the stability of PVC is minor. Definite effects have been demonstrated on low molecular weight samples, and this is to be expected because reactions favoring the formation of unsaturated endgroups favor low molecular weight. The nature of this effect is not clear.

A direct connection between the number of internal double bonds and the rate of dehydrochlorination of PVC has been established, and it seems quite likely that the allylic chlorines associated with internal double bonds along with tertiary chlorines at branch points are mainly responsible for the low thermal sta- bility of the polymer,

Thermal degradation of PVC at 18OOC under nitrogen causes

However, Bengough and Varma 124, 251 affirm that there is

Onozuka and Asahina [ 261 investigated the thermal decompo-

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STRUCTURE AND STABILITY OF POLYVINYL CHLORIDE 1 2 5

Braun [ 291 showed from ozonolysis that for fractions of bulk PVC the number of internal double bonds and the rate of thermal degradation, though dependent on each other, were independent of the molecular weight. This clearly demonstrated the role of internal unsaturation on the stability of the polymer. After care- ful chlorination of the double bonds, an increase in thermal sta- bility was observed and the number of double bonds a s determined by oxidation with potassium permanganate was reduced. It was also shown that one polyene sequence was formed from each iso- lated double bond.

bonds has been demonstrated by other workers [ 27, 30 , 311. In work reported by Lindeschmidt [ 321 , this co-relation was not observed . compounds have shown that the structure

This correlation of stability with the number of internal double

Studies of thermal degradation on low molecular weight model

R'CH=CHCHCICH ~ f f

(R ' = alkyl, R" = H or alkyl)

2

in the gas phase [ 33- 351 and in the liquid phase [ 361 is the most labile one. It is now generally accepted that chlorine atoms allylic to internal double bonds are the most labile and play the most significant role in the thermal degradation of PVC . 3 . Determination of Unsaturation

by bromination. The method described by Morikawa [37 ] based on liquid-phase bromination has been found to be better than gas-

phase bromination [ 381. Boissel [ 381 determined 2 x double bonds per monomer unit under liquid-phase conditions. Recently, Hildenbrand and co-workers [ 391 claimed they had determined double bonds in PVC with an increased accuracy in comparison with earlier methods by the addition of iodine monochloride (Wijs reaction) [ 39al to PVC coupled with x-ray fluorescence analysis to determine the iodine content of the polymer. The number of double bonds per unit weight of polymer was found to increase with an increase in polymerization temperature. In the technically important polymerization temperatures of 30 to 8OoC, 0 .9 double bonds per polymer molecule were found.

NMR techniques using signal accumulation appear to hold the greatest promise for conclusive identification and enumeration of the unsaturated structures in PVC.

The total number of double bonds in PVC has been measured

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Pethiaud and Pham [ 401 studied ether-soluble fractions of extremely low molecular weight (DP = lO-12), in which the end-

group frequency is much higher, by H-NMR, and they conclu- sively demonstrated Structure I to be the main unsaturated struc- ture:

1

-CHCl-CH -CH=CH-CH2Cl 2

I

They also argued that this structure is present in ordinary un- fractionated PVC .

ScJwenk et al. [ 411 studied low molecular weight fraction of PVC (Mn - 1500), obtained by Soxhlet extraction of mass PVC with

methanol, by H- and 13C-NMR. They concluded that 1,5- dichloro-3-pentyl ( I ) , 2,5-dichloro-J-pentyl ( I I ) , and 1,2- dichloroethyl (111) are the main structures of endgroups in PVC , their ratios per molecule being I : I1 : 111 = 2: 1: 5

1

-CH CHCH=CHCH2C1 -CHCl-CH2Cl 2 1 c1

I1 111

Schwenk et al. [ 411 also observed Structure IV which they attributed to methanolysis of Structure I:

-CHCl-CH -CH=CH-CH2-OCH3 2

IV

The presence of Structures I , 11, and 111 has recently been confirmed by Maddams [ 421 and Vincent et al. [ 431, but they found that the intensity of a doublet at 6 = 4.0-4.1 (due to methylene protons in -CH=CH- CH f 1 ) represented only

60% of the intensity of the multiplet at 6 = 5.8-5.9 (due to the two olefinic protons of Structures I and 11). This observation was explained by Heuvel et al. [ 441 who demonstrated that Structure I (and perhaps 11) i s present both in cis and trans forms in the ratio 3 : l .

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STRUCTURE AND STABILITY OF POLYVINYL CHLORIDE 127

H jertberg and Sorvik [ 451 investigated S-PVC samples with

'H- and 13C-NMR and found Structure I to be the main unsatu- rated chain-end structure and 111 to be the main saturated chain- end structure. These two groups accounted for 80% of the end- groups and had only a weak influence on the thermal stability of

the polymer. Their 13C -NMR measurements confirmed that other unsaturated structures besides I were present to a smaller extent.

It is a well-known hypothesis that polymer with reduced sta- bility is formed at the end of conventional polymerizations, i .e. , after the pressure drop. Hjertberg and Sorvik also compared stability and structural details of S-PVC and U-PVC prepared at subsaturation conditions. They concluded that observed struc- tural differences could not account for the lower thermal stability of U-PVC . Apparently other more labile structures a re also formed at subsaturation conditions.

Barboiu et al. [ 461 have determined unsaturated endgroups 1 in low molecular weight PVC by H-NMR spectroscopy of hy-

droxyphenyl-substituted product. They were able to distinguish between the structures -CHCI-CH -CH=CH-CH C1 2 2 and -CH -CHCI-CH=CH-CH C1, and even their trans

and cis forms. Direct measurement of conjugated double bonds have been

attempted by resonance Raman spectroscopy 1471, a highly sensi-

tive technique. Spectra display fine bonds at 1115 and 1500 cm-l. Peitscher claimed to have detected one long sequence =f+C-C'n (n = 10 to 20) in 10,000 molecules. Simak [48al

has developed an IR method to characterize the double bond and crystallizable syndiotactic sequence content of PVC . The results obtained for the double bond content of the polymer by IR were in good agreement with those obtained by chemical methods.

double bond content of PVC by measuring the changes in molec- ular weight resulting from oxidative double-bond scission. Oxida- tion by potassium permanganate in N-dimethyl acetamide [ 311 and ozonolysis [48b] have been used. From the change in intrinsic

viscosity, Braun and Quarg 1311 calculated 0.3 to 2.5 x 10 (-C=C) or +C=C j

higher for PVC-S than for PVC-M. Results obtained by ozonolysis

[ 481 seemed to be lower, 0.05 x 10 , but constant over the 0.51-0.91 conversion range. The last-mentioned authors insisted on controlled ozonolysis conditions a s opposed to the findings of Abbas and Sorvik [ 281. Lideschmidt [ 321 found the permanganate

2 2

Several attempts have been made to determine the internal

-3

per monomer unit with results n

-3

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method to be reproducible but not necessarily exact, and the molecular distribution showed a considerable spread. Both these methods do not take into account the double bonds situated near or on chain ends. Conjugated sequences, if present, and isolated double bonds very close to each other are detected a s single double bonds.

rines associated with unsaturation , by selective nucleophilic sub- stitutions is an indirect measure of double bonds and is discussed in a later section.

Determination of labile chlorines in PVC , mainly allylic chlo-

B . Branching

1 . Mechanism of Branch Formation

polymerization. Short branches in PVC have usually been con- sidered to be formed by backbiting mechanisms similar to those occurring in polyethylene. Such branches should have a tertiary chlorine at the branch point. Thus, in many earlier investiga- tions the methyl content was taken as a measure of the number of tertiary chlorines present in the polymer. Another possible route to tertiary chlorines is copolymerization with unsaturated moieties at the ends of chains [ 491. However, the latter path requires terminal CH =C(CI)-groups and thus is not consistent with

the work of Enomoto [ 171 , which suggests that the 6-hydrogens of vinyl chloride are involved in the process of chain transfer to monomer. Thus chain transfer to dead or growing polymer seems to be the most likely mechanism for the introduction of tertiary chlorine atoms.

Branches in PVC can be formed by transfer to polymer during

2

CI

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STRUCTURE AND STABILITY OF POLYVINYL CHLORIDE 129

CH2 = CHCl - C H ~ - C - C H Z - C H c

I I CI CI CI

I

I I - CH2 - C - C H 2 - CH -

:H2 c1 CHCl

2. B r a n c h i n g and Thermal S tabi l i ty

instability for a long time. 3-chloropentane are much less stable than the corresponding secondary chlorine compounds [ 501 . Comparison of 3-chloro-3- ethylpentane with 3-chloro-4-ethylhexane showed that the latter was by far the most stable [ 511. Macromodels containing tertiary chlorine atoms have also been studied. Caraculacu [ 521 studied the degradation of a polymer obtained by copolymerization vinyl chloride with 2-chloropropene and showed that tertiary chlorine atoms readily initiate degradation at temperatures lower than is usual for pure PVC . Copolymers of vinyl chloride and 2,4- dichloropent-1-ene ( I ) confirmed these findings [ 521 .

Branching in PVC has been suggested as a possible source of 2-Methyl- 2-chloropropane and 3-ethyl-

c1

-CH CHC1CH2C-CH CHCl- I I CH 2 I

I C H 3

2 2

CHCl

Recent work with vinyl chloride/ 2-chloropropene copolymer has suggested that only 0.1-0.2 mo18 of such groups would be needed to account for the instability of PVC [ 4 9 ] . Thus, the type of atom attached to the tertiary carbon at the branching sites in PVC should have a substantial effect on the thermal degradation of the polymer.

On the other hand, investigations with copolymers from vinyl chloride and 2,4-dichloropentene-1 led to the conclusion that be- cause of steric reasons PVC could not contain branch points with tertiary chlorine atoms [ 531. Braun and Wiess [ 54, 551 confirmed thesc findings by further investigations with copolymers of vinyl chloride and 2-chloropropene, The thermal degradation of such

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copolymers, with the same content of methyl groups as of branch points in radically prepared PVC , was much faster. Also, the distribution of the formed polyene sequences of different length was quite different from that of PVC after the same degree of degradation. In copolymers, a remarkable shift to shorter polyene sequences was observed.

Braun and Schurek [ 161 could not find any relationship be- tween the number of branch points and the rate of degradation. A s mentioned earlier, short branches in PVC are mainly chloro- methyl groups with a hydrogen attached to the tertiary carbon. The methyl content is thus a measure of structures which are more stable than tertiary chlorines. For a model substance re- sembling the chloromethyl type of structure, Suzuki and Nakarnura [ 361 reported a decomposition temperature exceeding 18OOC. Abbas and Sorvik [ 301 found no obvious correlation between de- hydrochlorination rate and the methyl content in reduced PVC. There was a slight trend toward higher degradation rates at higher amounts of branching but the data were rather scattered. Suzuki et al. [ 55al found similar results. For very high degrees of branching they observed a much higher dehydrochlorination rate. The degradation rate was considered to be almost constant within the interval common for commercial polymers. During de- hydrochlorination in oxygen, however, a greater dependence was observed.

3. Determination of Branching

Study of the methyl content of reduced PVC (PVCH) using

the infrared absorption at 1378 c m - l was one of the methods of estimating the total branch content of PVC. Since the optical

density at 1378 cm-l is also sensitive to the number of carbon bonds between the methyl group and the next tertiary carbon atom, prior knowledge of the branch is also necessary [56] . In this way Baker et al. [ 571 could explain some of the discrepancies between results of different authors. Corrected values of branch- ing were restricted to the range 1.8 to 16 per 1000 m.u.

Reductive dechlorination by LiAlH 4 , followed by spectroscopic

examination of the hydrocarbon product, has been the main method of studying branching in PVC [ 58- 601 .

Rigo et al. [ 581 found that radiolysis of PVCH gave methane and butane as had been reported by Schroder et al. [61]. This was only compatible with an intermolecular process of branching.

Rigo suggested the following mechanism for the formation of chloromethyl branches following head- to- head addition :

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STRUCTURE AND STABILITY OF POLYVINYL CHLORIDE 131

VCM -- C H 2C HC 1C HC H 2C1 -C H 2C HClC HClCH shift b * 1 , 2 - c 1

-C H C H C 1C H - C H 2C HC1 2 1

CH2Cl

T h i s process w a s considered to be in compe t i t i on wi th chain g r o w t h ,

' VCM 2 -C H - C H C 1 - C H C 1 - C H 2

-CH CHClCH-CH -CH -CH 2 l 2

c 1 2 1

c 1

w h i c h preserved a 1 , 2 - d i c h l o r o structure in the polymer.

strated experimentally by Starnes [ 621 w h o investigated the structure of short branches f o r m e d during the po lymer i za t ion of a - d e u t e r a t e d vinyl chloride:

The mechanism suggested by R i g o has been elegantly demon-

CH =CDCl head- to-head

2 \ . 2 b 2 2

2

-CH -CDCl-CDCl-CH

head- to-tail

-CH -CDCl-CH -CDC1 c . c .

-CH -CDC~-CH-CHDC~ 2 I c H 2 = c D c i -CH -CDC~ICD-CH c i 2 I 2

-CH -CDCl-CH-CH -CDCl -CH -CDCl-CD-CH -CDC1 2 I 2 2 I 2

CHDCl CH2Cl

LiAlH

2 1 LiA1H4

I CH3

2 2 I 1

-CH -CHD-CH-CH -CHD- -CH -CHD-CD-CH -CHD- 2

CH 2D

a b

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132 NAQVI

The exclusive formation of the b -type structure evidenced in

"C-NMR spectra confirms Rigo's mechanism.

to be 5 per 1000 m .u . along with about 0.5 long branches per 1000 m.u. [631.

result of y-radiolysis of polyethylene obtained by LiAlH

tion of PVC, Schroder and Byrdy [64, 651 reported that in emul- sion polymerization, intramolecular 1,5 transfer leading to the formation of butyl branches is more pronounced than in bulk o r suspension polymerization, and this could explain the higher con- centration of butane found in the radiolysis products.

Recently Starnes and co-workers [ 66, 671 have demonstrated that LiAlH reduction of PVC suffers from several serious disad-

vantages, including incomplete removal of halogen and the oc- currence of various side reactions leading to undesirable struc- tural complications. They also demonstrated that these disadvan- tages could be overcome if tri-n-butyltin hydride (Bu SnH) was

used as the reducing agent. The new measurements on branching were somewhat modified under the new conditions. According to recent data [ 68, 691 , the following branchings could be identified: 1-2 .4 long branches/1000 m.u., 0 .4-2 .4 chloroethyl branches/ 1000 m.u., 4-6 chloromethyl branches/1000 m.u., 0.4-1.6 2,4- dichlorobutyl branches / l o 0 0 m .u. (uncertain value).

Bu SnH have appeared. By analyzing the products resulting

from 1 -radiolysis of polyethylene obtained in this way, Bowmer et al. [70] found a strong increase of butane concentration, and that supports the existence of side butylation reactions.

Recently Starnes et al. [ 711 provided conclusive evidence for the presence of 2,4-dichloro- n-butyl branches in reductively dehalogenated samples of PVC . For a series of polymers prepared in conventional ways at temperatures of 43 to 100°C, the branch concentration ranges from about 0.6- l .O/( 1000 C) .

Bowmer et al. [ 721 characterized short chain branches in PVC by y-irradiation of reduced polymers and clearly demonstrated the presence of methyl groups and had evidence for ethyl and butyl branches.

applied to the study of branching in PVC [ 73, 741. It has been demonstrated that detailed microstructural information can be ob- tained with these methods by using carefully controlled experi- mental conditions and appropriate reference systems.

The short chloromethyl branches in PVC have been determined

Analyzing by gas chromatography the products formed as a reduc- 4

4

3

Unfortunately, some doubts on the new reduction method with

3

The method of pyrolysis-gas chromatography has also been

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STRUCTURE A N D STABILITY OF POLYVINYL CHLORIDE 133

A s mentioned earlier, the polymerization of vinyl chloride at subsaturation pressure ( U -PVC) simulates the conditions in or- dinary vinyl chloride polymerization after the pressure drop. A well-known hypothesis is that polymer with reduced stability is formed at the end of conventional polymerization, i . e . , after the pressure drop, and hence, preparation, characterization, and thermal degradation studies on such polymers have recently gained importance. According to Sorvik [ 751, polymerization at subsatu- ration pressure results in products with a high degree of long chain branching due to increased probability of chain transfer to polymer and broad molecular weight distribution.

and have recently published a detailed characterization of S-PVC and U-PVC 1451. They found 0 .1 -1 .8 /1000 m.u. long chain branches in S-PVC fractions and 0 . 9 - 3 . 5 / 1 0 0 0 m.u. in U-PVC but concluded that the observed structural differences could not ac- count for the lower thermal stability of U-PVC .

1 Braun et al. [ 771 have used H - and 13C-NMR for studying

Hjertberg and Sorvik [76] studied butyl branches in U-PVC

branched structures in PVC . For a series of U-polymers, the total amount of branching ranged from 7 . 5 to 13 .5 /1000 C .

"C-NMR measurements pointed to ratios of methy1:butyl = 1: 1 and short chains:long chains = 6: 1 .

C. Determination of Labile Chlorines in PVC by Chemical Modification

The presence of allylic chlorines and tertiary chlorines and their influence on the thermal stability of PVC has now been established with some degree of confidence, and together they a re considered to constitute the labile chlorine structures in the polymer. Numerous chemical modification methods involving the selective nucleophjlic substitution of labile chlorines in PVC with other chemical moieties for identifying and quantifying labile structures have been reported in the literature.

of labile chlorines in PVC by acetate groups by treating the polymer with cadmium acetate under vigorous conditions and using IR spectroscopy. In other work the subjection of PVC to Ben- gough and Onozuka's treatment resulted in polymers which were less stable than the untreated polymer as determined by the rate of thermal dehydrochlorination [ 791 . aluminum compounds efficiently alkylate labile chlorines in PVC and alternatively PVC carbonium ions could alkylate aromatic

Bengough and Onozuka [ 781 showed evidence of substitution

Thane et al. 1801 reported that in pentane suspension, alkyl-

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compounds to give rise to polymers of increased stability. The values of 2-38 for labile chlorines estimated by them were con- siderably higher than now generally believed.

a method based on substitution of labile chlorines with phenol. IR [ 811 and U V [ 821 were used for determination of incorporated phenol. The published data indicate a detection limit of 2 and 0.5 ClL/1O0O m.u.

chlorines in PVC by isotopic exchange with SO The selec-

tive exchange of chlorine in the polymer was verified by experi- ments with model compounds. The number of allylic chlorines in PVC was found to be between 0.12 and 0.16 for 100 m .u.

Using thiophenol instead of phenol, Michel et al. [84] found a new selective reaction which takes place exclusively with allylic chlorines and not with tertiary chlorines. A single product of thioether structure is formed:

In a series of papers, Caraculacu and co-workers described

Caraculacu et al. [ 831 also quantitatively determined allylic 36 C1 2 '

SH

In usual PVC having 1.53-2.54 C1 /lo00 m.u. and in a A benzene-soluble low molecular weight PVC fraction, a higher value of 3.07 C1 / 1000 m . u. was obtained.

The methoC has come under certain criticism recently be- cause it was found to give significantly higher values for labile (allylic) chlorines than do other methods, e .g . , phenolysis [ 851 .

Starnes and Spitz [ 861 , in their studies of PVC stabilization with di(n-butyl) tin bis(n-dodecyl mercaptide) or with mixtures of the mercaptide and di(n-butyl) tin dichloride, found the rate of dehydrochlorination was inversely related to the amount of sulfur chemically bound to the polymer up to a sulfur content of approximately 0.9%. The following mechanism of ally1 chloride substitution was proposed:

A

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STRUCTURE AND STABILITY OF POLYVINYL CHLORIDE 135

.

. '. S

Labile chlorines in PVC have also been determined by a crown ether catalyzed acetoxylation of PVC [ 871 and the thermal degradation characteristics of the modified polymer [ 881 . The values were comparable with those obtained by the phenolysis method.

Recently Kennedy anti co-workers [ 891 have developed a method for controlled introduction of allylic chlorines into PVC . Potassium tert-butoxide ( t -BUOK) is used for controlled random dehydrochlorination of PVC in THF at low temperatures. The method presents very interesting possibilities for increasing an understanding of the role played by allylic chlorines in the ther- mal degradation of PVC and further confirmed the validity of the various chemical and other techniques so far used for studying such structures. Ivan et al. [85] recently published a detailed structural characterization of PVC having enhanced labile chlorine content.

D. Initiator Endgroups in PVC

1 . Initiator Endgroup Incorporation

initiator incorporated at the beginning of the chain is a contro- versial one. If the polymerization of vinyl chloride is initiated with organic peroxides which decompose according to

The effect of the decomposition products of the polymerization

(R-COO)Z 4 2R-COO

R-COO + R + C 0 2

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they lead to the structures of the type

RCOO + nCH2=CHCl --D R - C O O \ C H 2 C H C 1 j n

or

R + nCH2=CHC1 + R . + C H 2 C H C 1 j n [ 901

where R is an alkyl or aryl group.

to produce unsaturated endgroups which may result in a less stable polymer. In the case of benzoyl peroxide an additional possibility is initiation by phenyl radicals to give a polymer with terminal phenyl groups:

Peroxide initiators may also undergo primary radical transfer

Instability at the chain end could then arise via the benzylic hy- drogen atoms due to the possible resonance stabilization of the resulting radical :

C H - C H C I - C H - CHCl 1

2. Initiator Endgroups and Thermal Stability

In earlier investigations chain ends were suggested to be important initiation sites for dehydrochlorination . Provided there are no transfer reactions during polymerization, at least half the polymer chain ends will carry initiator fragments. In practice, transfer reactions swamp the normal termination pro- cesses and less than 30% of the chain ends carry initiator residues

Cittadini [ 921 and Corso [ 931 found that azo-initiated polymc?r [911.

liberates HC1 more rapidly than peroxide-initiated material. Stromberg et al. [ 941 confirmed these findings and in addition found that PVC prepared by gamma irradiation was still more

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STRUCTURE AND STABILITY OF POLYVINYL CHLORIDE 137

stable. Talamini and co-workers [ 951 kept the molecular weight relatively constant and compared the rate of dehydrochlorination for samples obtained with different initiators. Their results were consistent with those mentioned above. Park and Smith [ 961 have shown that PVC prepared in solution by free-radical initiation contains 0.1- 1 . 0 initiator fragments per polymer molecule. These polymers are more suitable for investigating the effects of initiator endgroups than polymers prepared in suspension or emulsion systems where chain transfer to monomer is much more predomi- nant.

initiator endgroups for causing dehydrochlorination was in the ratio lauroyl peroxide: isopropyl peroxydicarbonate: benzoyl peroxide:axoisobutyronitrile = 9.7: 5.8: 4 . 4 : 1.6. These values were obtained at 22OOC and the degradation rates were taken as the mean value for 0 to 20% dehydrochlorination.

However, the effect of initiator has been suggested to be less important as model substances for such structures are much more stable than other possible structural irregularities already discussed [ 29, 26, 98, 991 .

Park and Skene [ 971 found that the effectiveness of the

E. Head-to-Head Structures in PVC

Head-to-head units formed in a molecule have not only been considered as initiation sites for the dehydrochlorination but also a s termination points for the growing polyene sequences [29, 98, 1001. Head-to-head units can either be formed through termina- tion by combination or by head-to-head addition during propaga- t ion .

Shimizu and Ohtsu (101) have proposed a chemical method to determine head-to-head structures in PVC . Mitani et al. [ 1021 found 2.5 to 7.0 head-to-head structures per 1000 m.u. , increas- ing with the polymerization temperature. It has not been possible

to detect internal head-to-head structure by 13C-NMR spectros- copy with the detection limit of 2/1000 m.u. [103] . Starnes et al. [ 1031 found evidence for the absence of neighboring methylene

groups by I3C-NMR spectroscopy. On the other hand, the pro- posed real rangement of head-to-head units at the radical chain ends resulting in chloromethyl branches would partially explain their consumption during polymerization and their absence in the final product.

various PVC samples using Shimizu and Ohtsu's method [ 1011. In a recent investigation, Hjertberg et al. [lo41 studied

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138 NAQVI

They concluded that this method mainly gave a measure of the content of saturated 1,2-dichloroethyl chain endgroups, the presence of which has been conclusively demonstrated recently [45] . With caution they contend there a re 0 to 0.2 head-to-head structures per 1000 m.u.

Head-to-head PVC prepared by adding chlorine to cis- poly- butadiene has been found to be less stable than ordinary head-to- tail polymer [ 1021 .

From the results obtained by thermal decomposition of both low molecular weight vicinal dichlorides in the gas phase [ 105, 1061 and of the copolymers of vinyl chloride and trans- 1,2-dichlo- roethylene [ 1041 , it is not possible to attribute the cause of the thermal stability of PVC to the individual head-to-head structures. Recently, Crawley and McNeill [ 1081 chlorinated cis- 1,4-poly- butadiene in methylene chloride, leading to a head-to-head and tail- to-tail PVC , They found, for powder samples under pro- grammed heating conditions, that head-to-head polymers had a lower threshold temperature of degradation than normal PVC , but reached its maximum rate of degradation at higher temperatures.

The extent of head-to-head units in PVC and their effect on stability of the polymer is yet to be conclusively demonstrated although it would seem that as compared to other structural de- fects their contribution to polymer instability is a minor one.

F. Oxygen-Containing Defects

Various oxygen-containing structures [ 100, 1091 arising either by a reaction with traces of oxygen already present during poly- merization or by oxidation during storage or treatment of the finished polymer in air have also been considered to be a source of labile sites in the PVC molecule. Virgin PVC may contain up to 500 ppm peroxide [ 1101 , which together with hydroperoxides may act a s initiation sites for dehydrochlorination. Popova et al. [ 1111 reported a higher dehydrochlorination rate for the PVC poly- merized in the presence of oxygen ( 0.18). Decomposition temper- atures were also 10-15OC lower. Garton and George [ 1121 also observed decreased stabilities for polymers prepared in the presence of oxygen. This method of preparation introduces poly- peroxide moieties that can decompose into HCHO, HC1, and CO [ 1131. However, the importance of oxygenated structures re- mains unclear for polymers prepared under normal conditions.

Sonnerskog [ 1141 found that PVC could oxidize a number of phenols at room temperature in the absence of oxygen, In addi- tion, a graft copolymer was formed when acrylonitrile was added to PVC. These results were taken as indications for the peroxy groups in PVC.

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STRUCTURE A N D STABILITY OF POLYVINYL CHLORIDE 139

A s far as oxidation of the polymer with oxygen of the air is concerned, the B-hydrogen atom in the neighborhood of the C=C double bond is the most likely one to be attacked by oxy- gen with the formation of hydroperoxide which undergoes further decomposition [ 261. OH and CO groups have been detected spectroscopically in the polymer [ 99, 1151 . radation as a consequence of conjugated ketoallyl groups,

Minsker et al. [ 116, 1171 discussed the initiation of PVC deg-

-CO-CH=CH-CHCl-CH - 2

which should be expected to be more important in causing insta- bility in PVC than chloroallyl groups.

Svetly et al. [ 1181 , in a very interesting study of the effect of cis- and t r ans -a , 6-unsaturated ketones on the thermal stability of PVC , concluded that the propagation of polyene sequences oc- curs via migration of activating groups between polymer chains and that the activating groups are represented by oxygen-con- taining structures. The possible mechanisms proposed by them are discussed in a later section.

G . Tacticity

1. Tacticity and Thermal Stability

Guyot et al. [ 1191 studied the influence of regular structures on thermal degradation of PVC in an inert atmosphere and con- cluded that a higher degree of polymer crystallinity favors inter- molecular condensation and therefore the acceleration of dehydro- chlorination. They [ 1201 also studied the thermal degradation of semicrystalline, low molecular weight PVC fractions, prepared by polymerizations catalyzed by tert-butyl magnesium chloride and observed that crystallinity of some fractions gave considerable thermal stability so long as the pyrolysis temperature remained below the melting point of the sample. structural configuration of monomer units could also influence the course of thermal degradation. Millan and co-workers [ 121, 1221 have shown that rate of nonoxidative thermal dehydrochlorination of PVC is increased by the presence of syndiotactic sequences, although there seems to be no significant effect of tacticity in the overall energy of activation. These observations are consistent with a slow initiation step whose activation energy is tacticity independent, followed by rapid polyene growth which is facilitated by the syndiotactic arrangement.

I t was suggested that

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In a series of studies on the effect of tacticity on the non- oxidative thermal degradation of PVC , Millan and co- workers [ 123- 1261 demonstrated that the higher the syndiotactic content of the polymer, the higher the rate of degradation and the length of polyene sequences formed. The number of oxidative scissions, by ozonolysis, of the degraded samples was found to be low for syndiotactic sequences and high for atactic sequence, which was accounted for by the increased clustering of double bonds to form long polyene sequences in the more syndiotactic polymers.

Millan [ 1271 studied the effect of tacticity on the ionic dehy- drochlorination and chlorination of PVC . For the dehydrochlori- nation reaction both the reaction rate and polyene sequence dis- tribution depend markedly on the syndiotactic content. Chlorina- tion appeared to be easier through heterotactic parts than through

syndiotactic sequences as shown by "C-NMR.

tacticity on the nucleophilic substitution reactions of PVC. thiophenate and phenol were used for these reactions. The cen- tral chlorine in isotactic triads and, to a lesser extent, in hetero- tactic triads were found to be most reactive. It was concluded that initiation of degradation may occur by normal structures, and polyene buildup may be favored by syndiotactic sequences. This is in favor of simultaneous participation of normal chlorines along with defect sites in the thermal degradation of PVC .

In recent years this view has received increased attention [ 131, 1321. Razuvaev et al. [ 1331 suggested that even in the early stages of degradation, dehydrochlorination of normal and of chloroallylic fragments proceeds simultaneously. Chain propaga- tion stages consist of consecutive degradation of chloroallylic fragments of the macromolecules.

during degradation in nitrogen. They concluded that although allylic chlorine atoms seem to be the main points of initiation, other sites cannot be excluded as the number of initiation points increased appreciably during the early stages of degradation.

Millan and co-workers [128-1301 also studied the effect of Sodium

Abbas and Sorvik 1281 studied the structural changes in PVC

2 . Determination of Tacticity

merized PVC by "C-NMR (in terms of triads and tetrads) and IR (in terms of diads). Sorvik [135] has presented an extensive comparative study of the quantitative determination of PVC tac- ticity, concluding that NMR spectroscopy is a more precise tool for these measurements than IR or Raman spectroscopy. I t was shown that a decrease in polymerization temperature leads to a slight increase in syndiotacticity.

Pham et al. [ 1341 examined the tacticities of radically poly-

In a more recent paper Robin-

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STRUCTURE A N D STABILITY O F POLYVINYL CHLORIDE 141

son et al. [ 1361 tried to improve the known NMR methods for quantitative determination of tacticity . In another publication [ 1371 these workers studied the C-Cl stretching vibration region from the Raman spectra of PVC for determination of polymer tac- ticity. The results were close to those determined by N M R . The advantage of the Raman method in comparison with other spectral methods is that no special sample preparation which might alter the conformation content or crystallinity is required. Simak [ 48a] recently developed an IR method for characterizing the crystalliz- able syndiotactic sequence content of PVC . The IR crystallizable content was found to be slightly greater than the x-ray crystal- linity, presumably because not all crystallizable sequences actu- ally crystallize.

Chartoff and co-workers [ 1381 studied infrared spectral changes with crystallization in PVC and showed that IR band ratio measurements can be used to measure relative amounts of crystal- linity within a particular PVC material. The method was found inapplicable when comparing two different PVC materials with grossly different values of crystallinity. The ratio A 635/A 610 was

found to increase consistently with annealing time in accord with changes in both density and x-ray diffraction intensity.

thermal degradation of PVC and used UV-visible spectroscopy to follow the relative tacticity of different samples.

Martinez and Millan [ 1391 studied the influence of tacticity on

V . MECHANISM OF DEHYDROCHLORINATION

Despite numerous investigations on the thermal dehydrochlo- rination of PVC, the mechanism of this reaction is still the subject of much controversy. Radical, ionic, and molecular (concerted) paths and their combinations have been proposed. All three mechanisms have been summarized by Geddes [ 1001 and have re- ceived recent support.

A . Free-Radical Mechanism

A vast amount of information has accumulated over the years in favor of the free-radical mechanism. Stapfer and Granick [ 1401 observed an increased rate of dehydrochlorination on addition of free radical sources. A number of workers have observed the same effect by v-irradiation [141-1431 and by U V irradiation. The use of isotopic tracer techniques has given direct evidence for the formation of polymeric radicals. PVC was dehydrochlo-

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rinated in solution in the presence of either tritium labeled or carbon- 1 4 labeled toluene. Both tritium and carbon- 14 became bound to the polymer, and there was a linear relation between the extent of tritium incorporation and dehydrochlorination [ 1471 . ESR measurements have provided indirect evidence of polymer radical formation [ 148- 1501 .

Recently Yousufzai et al. [ 1511 claimed to have found direct evidence for the presence of chlorine atoms in the dehydrochlo- rination reaction while studying the chloromercury acetaldehyde inhibited degradation of PVC .

Strong indirect evidence for chlorine atoms a s chain carriers was obtained from work with polymer mixtures and graft copoly- mers [ 152-1541. Atmospheric oxidation reactions are mostly free- radical, and enhancement of the rate of dehydrochlorination in air must be regarded as further evidence in favor of radical process- es. A free-radical process has also been reconciled with hydrogen chloride catalysis [ 1421.

Recent evidence for a radical mechanism has been provided by studies of decomposition energetics [ 1551, the degradation behavior of PVC-polystyrene [ 1561 and PVC-polypropylene mix- tures, and the effects of radical traps [ 1571.

B . Ionic Mechanism

The dehydrochlorination of PVC by bases such as lithium chloride follows an ionic mechanism [ 1581 . Wolkober [ 1591 found that sulfuric acid and some heavy metal salts, especially iron salts, accelerated the dehydrochlorination of PVC . Baum [ 1601 and later Rieche et al. [ 941 also discussed an ionic mechanism for pure thermal dehydrochlorination and so explained the catalytic effect of organic bases. However, up to now there is no direct proof of such a mechanism.

Goto and Fujii [ 1611 were the first to mention the effect of various solvents; they found that the degradation of PVC in tetralin and o-dichlorobenzene proceeded at virtually the same rate but was exceedingly fast in nitrobenzene. The same effect was observed by Bengough et al. [ 21, 22, 24, 251. Although the solvents they studied were from different types of compounds (esters, ketones, chlorinated hydrocarbons, etc) , it was again in nitrobenzene that the thermal decomposition of PVC proceeded much faster than in other solvents. This was explained in terms of the much larger polar character of nitrobenzene as compared to other solvents. This was confirmed by Marks et al. [ 1621 who succeeded to some extent in correlating the rate of dehydrochlo- rination of PVC in solution with the dielectric constant of the medium.

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STRUCTURE AND STABILITY OF POLYVINYL CHLORIDE 143

The ionic mechanism is also in accord with the mechanism of thermal decomposition of low-molecular weight model compounds which in the gas phase undergo decomposition via a homogeneous monomolecular first-order reaction [ 34, 35, 501 , most likely through a polar transition state [ 1061 .

A polar transition state during the elimination of hydrogen chloride from PVC has also been assumed by Imoto and Nakaya [ 1631 on the basis of calculations of the energy of the C-Cl bond depending on the distance between both atoms and the dis- tribution of bonding electrons (polarization).

Another observation in favor of an ionic mechanism is the autocatalytic effect of HC1 on thermal degradation. Thermal degradation in inert solvents is not influenced by inhibitors for radical reactions [ 211 , and hydroquinones give no inhibition in the absence of oxygen. Zafar and Mahmood [ 1641 studied the thermal degradation of PVC in various solvents at 18OOC and ob- served the following order: benzonitrile > nitrobenzene > cyclo- hexanone > dioctyl phthalate > a-bromonaphthalene, for the rate of reaction. The effect was explained on the basis of 6-elimination of the E -type favored by polar solvents.

chlorination of 4-chloro- 2-hexene (model of allylic unstable struc- ture in PVC) in various solvents. The reaction was reversible and had an ionic mechanism. It was catalyzed by a charge trans- fe r complex between hexadiene and hydrochloric acid. inhibited by complexing agents of HCl such as THF.

Theoretical considerations have also suggested that an ionic path ( E ) is not unreasonable [ 166, 1671.

From their recent studies Svetly and co-workers [ 118, 1681 concluded that initiation cannot be attributed to labile chlorine atoms in the vicinity of structural defects. They argue that if initiation only occurs from labile chlorines and each labile chlorine atom initiates only a single polyene sequence, then, since prop- agating polyenes are subjected to gradual termination, this must lead to a gradual decrease in the rate of dehydrochlorination and after a sufficiently long time, if no other mechanism of initiation is admitted, should drop to zero. This is at sharp variance with experimental findings. This group compared the dehydrochlorina- tion process of a pure polymer and of a polymer with a small quantity of a , B-unsaturated ketones added to it with the fixed cisoid or transoid structure. It was found that the transoid enone had no perceptible effect on the rate of dehydrochlorination even up to concentrations of 0.91 mol%, whereas a s little as 0.058 mol% of cisoid enone raised the reaction rate considerably. Since a cisoid arrangement of C=C and C=O bonds can react in a cyclic manner and not enones with a fixed S-trans conformation, the following mechanism was postulated:

1 Hoang et al. [ 1651 extensively studied the thermal dehydro-

It was

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(1)

( 2 )

Labile chlorines in the vicinity of defect structures are not the cause of low thermal stability of PVC. Constant rate of dehydrochlorination of PVC in an inert atmosphere is a result of dynamic equilibrium between simultaneously proceeding initiation and termination of the chain elimination of HC1. Chain dehydrochlorination is initiated by a reaction of the active group with a regular structural unit of the adjacent polymer chain. It is highly probable that the active groups a re repre- sented by a , %-unsaturated carbonyl groups, and the mechanism involves a cyclic intermediate or transition state.

( 3 )

( 4 )

B

/ A

\

.CoCH R ' = CH-

Catalysis of degradation of PVC by the HC1 evolved and the formation of longer polyene sequences with HC1 catalysis than without are established facts. Kelen et al. [169] studied the deg- radation of PVC in an atmosphere of tritium-labeled HC1 and found fast isotope exchange between polymer and labeled HC1. The following mechanism involving protonation of polymer was proposed [ 1701 :

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STRUCTURE AND STABILITY OF POLYVINYL CHLORIDE 1 4 5

c1 I

-CH -CH=CH-CH=CH-CH=CH-CH -CH-CH - 2 2 2 I+.+ c1

i I +

--CH -CHT-CH-CH=CH-CH=CH-CH -CH-CH - 2 2 2

c1 + I

2 2 -CH -CHT-CHZCH-CH=CH-CH-CH -CH-CH - I 2

IH+ c1 I

-CH -CHT-CH=CH-CH=CH-CH=CH-CH-CH ~ 2 I 2

I-HCl

-CH -CHT-CH=CH-CH=CH-CH=CH-CH=CH-

1 2

etc.

Recently Shipiro [ 171 , 1721 studied the thermal decomposition of solid PVC at low conversion in the presence of HCl and H B r and demonstrated their catalytic activity. A unified mechanism involving three steps was proposed:

(1) Random generation of a single carbon-carbon double bond in a cis configuration via a radical or unimolecular pro- cess. 1,4-Elimination of HC1 via a six-centered transition state yielding a diene. Hydrogen halide catalyzed isomerization of the diene produced in Step (2) to regenerate a 1,4-diene structure which then can undergo Step ( 2 ) repeatedly to form a polyene sequence of length n. Both Steps ( 2 ) and ( 3 ) are molecular reactions.

(2)

( 3)

- C n -

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V . .

CI

- CH - CHQ e C H - C H - C H - 3 \ I - I

\ tHCI CH - CH

,SH CI \ H - CI’

CI

CH = CH

\ H C I I ‘ C H -

- CH - CH + CH - CH’ /

CI

V I . CONCLUSION

The structure and thermal stability of PVC has been an area of intensive research for over three decades. A wealth of useful information regarding the polymer structure and stability has been generated over the years, but due to the complex nature of the polymer and the reactions involved in i ts thermal degradation, a clear understanding of these has still not emerged. The most important reason behind this has been the difficulty in identifying and quantifying the structural irregularities that have been re- garded as the main cause of the low stability of the polymer. The levels of these irregularities in the polymer are extremely low, and the kind of sensitivity of the various spectroscopic techniques used in elucidating the fine structure of PVC that was required has been achieved only in very recent years. No amount of re- search could have possibly answered the fundamental questions of the structure-stability relationship in PVC without the com- plementary development of a level in analytical spectroscopy which was a prerequisite for studying the fine structure of PVC . In fact , the recent achievements in structural analysis of PVC by spectroscopic techniques mark just the beginning of a thorough understanding of the fine structure of PVC and its relation to the stability of the polymer.

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STRUCTURE AND STABILITY OF POLYVINYL CHLORIDE 147

The contributions which have been made by various techniques in studying the structure of PVC have been fairly exhaustively discussed in the preceeding sections. The FT-NMR technique can quite easily be singled out a s being the one which has made a major contribution, and without belittling the contributions made by various other techniques, it may be said with a fair degree of confidence that this technique appears to hold the greatest prom- ise for the future for studying the fine structure of PVC.

identified and quantified have been discussed. The effect of labile allylic chlorines and tertiary chlorines on the thermal sta- bility of the polymer has been fairly conclusively demonstrated. A large amount of methyl branches in PVC contain a tertiary hydrogen at the branch point and hence cannot be considered to contribute very much toward destabilizing the polymer. Also, no definite conclusions can be drawn regarding the role played by other irregularities such as oxygen-containing defects, chain-end unsaturation, initiator endgroups, and head-to-head structures in influencing the stability of the polymer. The question that whether it will ever be possible to isolate and understand the exact mechanism by which a particular structural defect influ- ences the stability of the polymer is a question better left un- answered. It may well be that a particular structure's influence exists only in relation to the influence of other structures and in the absence of which it may have no effect at all. Of course, this is pure speculation.

past has been that different workers were studying polymer samples which were structurally quite different from each other and whose thorough characterization was not possible at that time. This was the main reason behind the lack of correlation between the results of various workers and the apparent disagreement between their results and conclusions.

Chemistry ( IUPAC) working Party on PVC has been a very im- portant step toward eliminating this problem. It is now possible to have studies of the structure and stability of PVC to be per- formed on the same samples in various laboratories of the world, and to have comparison of the results. This collective effort has resulted in a more well defined and clearer understanding of the structure of PVC and its stability.

Since the late fifties the theory put forward by Frye and Horst, that labile chlorines in PVC due to structural defects in the polymer were the main cause of i ts instability and their sub- stitution by other more stable groups was the main step in the stabilization of the polymer, has been the most widely accepted. But the concept that normal chlorines in PVC, either on their own

The various structural irregularities that have been positively

Another major problem in the study of PVC stability in the

The formation of the International Union of Pure and Applied

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or under the influence of the irregular structures, undergo dehy- drochlorination simultaneously with the labile chlorines has re- ceived increased attention in recent years. In a way, this has added a new dimension to the problem, keeping alive interest in this phenomenon.

Ionic, free-radical , and molecular mechanism have been put forward for the thermal dehydrochlorination of PVC . All of these have received support in recent years. What appears more likely is that all the three mechanisms may be operative simultaneously in the thermal dehydrochlorination of PVC , the relative contribu- tion made by each depending on the conditions under which the degradation was studied.

studying this highly complex polymer. Despite the strides that have been made in recent years in understanding the structure and stability of PVC, the picture is still far from clear. Much remains to be done to unravel the mysteries of this intriguing and fascinating polymer, especially now that i ts thorough characteri- zation may be possible.

All this illustrates only too well the difficulties involved in

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] at

09:

45 2

3 Se

ptem

ber

2013