the polymer in a useful way. The polymer also may contain small amounts of monomer, entrapped

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Evirometal Health Perspectives Vol. 11, pp. 29-45,1975 Agig of Polymers by David L.

1 Evirometal Health Perspectives Vol. 11, pp ,1975 Agig of Polymers by David L. AIlara* Itroductio This paper provides backgroud o the basic chemical processes which ca occur durig the exposure of polymers to typical ed-use eviromets such as air, sulight, water vapor, ad various atmospheric pollutat gases. The use of polymers as materials for maufacture of may everyday items is costatly icreasig. The large umber of polymers available allow wide variatio i physical properties such as toughess, appearace, ad processig ease. However, all polymers are ultimately susceptible to chemical degradatio uder exposure to various coditios of commoly ecoutered eviromets. If oe excludes more aggressive coditios such as high eergy radiatio, large cocetratios of corrosive gases, ad high temperature such as ecoutered i combustio, a large umber of polymers are still quite susceptible to slow chemical degradatio. The chemical chages which occur durig such degradatio, as a geeral cosideratio, deserve evaluatio as regards alteratios i ay implicatios for health aspects of the polymers' ed uses. A uderstadig of the basic chemical mechaisms ivolved i the degradatio process is therefore sigificat. I this paper the eviromets cosidered are those which ca be arbitrarily classed as "habitable." These are defied as those with temperatures below 100 C, a atmosphere of air (with ad without typical pollutats), varyig amouts of water, preset as vapor or liquid, ad light (sulight ad artificial). Normally, polymers i the state of their ed-use are ot pure materials. I may cases there are added substaces which alter the egieerig ad/or chemical properties of * Bell Laboratories, Murray Hill, New Jersey the polymer i a useful way. The polymer also may cotai small amouts of moomer, etrapped durig the polymerizatio process. Such additives ad impurities may participate i the slow chemical degradatio of the polymer ad, of course, add to the geeral physical complexity of the polymer. However, for purposes of establishig basic chemical mechaisms this paper will discuss oly the degradatio of pure polymers. Sice a polymer is a repeatig chai of simple chemical fuctioal groups liked together, the usual types of chemical reactios that the idividual fuctioal uits udergo i small molecules also apply to a polymer of these groups. However, the rates at which these reactios take place are ofte altered because of morphological effects i the solid polymer which are ot preset i solutios ad vapors of simple orgaic molecules. I geeral, because of the difficulty ecoutered with the petratio of chemical agets ad, radiatio (e.g., sulight) ito a solid polymer, chemical degradatio rates are usually cosiderably slower for the polymer tha for small orgaic molecules made from the same fuctioal groups. Degradatio may be iitiated several ways. Absorptio of ultraviolet radiatio ad/or thermal eergy ca lead to rupture of weak bods. This is ofte followed by formatio of ew bods to give a alteratio of the origial polymer structure. The presece of oxyge, water ad atmospheric pollutats ca alter these processes ad lead to the formatio of ew types of chemical groups. Bod-breakig processes may occur i polymer backboe bods to lower the average legth of Jue

the polymer chais ad/or i pedat group bods leadig to the emissio of small molecules. Bod-makig processes betwee chais give crossliked material of icreased molecular weight ad rigidity.

2 the polymer chais ad/or i pedat group bods leadig to the emissio of small molecules. Bod-makig processes betwee chais give crossliked material of icreased molecular weight ad rigidity. All of these effects will be discussed i the ext sectio. More detailed reviews of these processes ca be foud elsewhere (1-4) ad the reader is ecouraged to cosult these refereces for a more comprehesive view of the field. As a fial ote of itroductio, it is importat to realize that although the basic qualitative processes ivolved i degradatio of commo polymers are reasoably well determied, the preset quatitative uderstadig, i geeral, is isufficietly advaced i order that quatitative predictios ca be made of the extet of degradatio ad the levels of products (particularly mior oes) uder typical evirometal exposures. I additio, for such predictios oe eeds to kow exactly such variables as temperature variatios, light flux ad wavelegths, processig coditios, localized cocetratios of reactive gases, ad the ature of ay additives. Ufortuately, these usually caot be easily evaluated ad thus quatitative predictios of chemical degradatio are redered difficult. More studies aimed at allowig predictios of levels of degradatio products of iterest would be most welcome i this field. Accordigly, the discussio preseted here is iteded to be quite broad i scope ad detail is thus ecessarily restricted. Reactios Requirig No Exteral Molecules These are reactios which occur thermally ad/or photolytically. The basic reactios are show i Figure 1. Whe exteral radiatio is abset, these reactios are geerally uimportat below 100 C, with a few exceptios (see below). I the presece of light the degradatios are sometimes greatly accelerated if a absorbig chromophore group is preset, e.g., carboyl groups, hydroperoxides, imio groups, aromatic rigs ad cojugated C=C bods. Degradatio of commo polymers by thermally iduced rupture of radom chai liks ca be regarded as uimportat at ambiet temperatures. However, with strog radiatio such processes may occur i certai polymers. Other reactio paths such as deploymerizatio or elimiatio may be regarded as more likely routes of thermal degradatio. 30 Various eergy-iduced processes of degra- FIGURE 1. datio.,ch% ElijmiK -CH - CH + HX CH CH'- L j ~ Radom Scissio, CH -CH2 *. CH2 A typical example of a depolymerizatio process is the degradatio of poly (methyl methacrylate). This polymer depolymerizes extesively [eq. (1)] upo strog heatig. However, at 100 C it ca be estimated that it will take about 1015 yr for 50%o of the material to depolymerize. Cl OOCH3 CH2 = CH3 COOCH3 C = CH2 CHS Sice poly(methyl methacrylate) depolymerizes at temperatures cosiderably lower tha most other commo polymers, oe ca make the geeral coclusio that degradatio of commo polymers by depolymerizatio is ot sigificat at ambiet temperatures. A excellet example of a very facile elimiatio is give by the thermal degradatio of poly (viyl acetate) [eq. (2)]. CH3 C 0 (1) 0 H = CH =CH + HOCCHs (2) CH -CH Evirometal Health Perspectives

This material degrades very clealy to acetic acid ad a C=-C chai bod, ad it ca be estimated from higher temperature data that it will take about 20 yr for 50% degradatio at 100 C.

3 This material degrades very clealy to acetic acid ad a C=-C chai bod, ad it ca be estimated from higher temperature data that it will take about 20 yr for 50% degradatio at 100 C. Sice most other polymers are much more stable to such elimiatio reactios, oe ca coclude that at ambiet temperatures purely thermal elimiatio reactios are uimportat. However, oe otable exceptio of a commo polymer which gives measureable elimiatio at low temperatures is poly (viyl chloride). Careful studies (5) have show that measurable HCl evolutio ca occur i days at 80 C. The reactio is show i eq. (3). F 'I 01- FCH =CH-1 HCI Cl H 1CH O ~ ~ I R.. R02- (cross likig) R-H+A. - R. +AH R02 R + R02H oradical products (light, metals) + 02 RH 4 H20 + R- HO + RO (/VyA) ROH + R H i(chai \Ov. + HCA 0 scissio) (3) The mechaism is quite complex ad cotroad free- CO, RCO2H, C02, etc. Ioxidatio versial. It may be a bled of ioic radical processes, but the oly sigifiicat prod- HCI. The uct reported at lower temperatures ihs HCI evolutio ca be greatly acce' lerated by FIGURE 2. Free-radical chai-oxidatio sequece for light ad oxyge, depedig upo suich factors polymers with susceptible C-H bods. I L1 as the amout of oxyge preset ad te wave- is overwhelmigly the most commo). The legth ad itesity of the light. C=C group is also quite susceptible to attack by free radicals ad leads to a oxidatio sequece which is similar to that for C-H oxida- Reactios with 02 tio. The evetual result of autoxidatio is Thermal ad photo-oxidative d egradatio formatio of oxygeated fuctioal groups i are the most commo types of delterioratio the polymer backboe (carboyl groups, alcomechaisms. A partial scheme of t;he autoxi- hols, ethers ad hydroperoxides), chai scissio datio process is show i eq. (4) forr oxidatio leadig to lower chai sizes, occasioal crossliks (ether or peroxide) betwee chais, ad of C-H groups. emissio of small molecules such as CO, C02, H20, HO02, ad small orgaic molecules such as carboxylic acids ad ketoes. The presece of F/I CH/IH [PH ] metal particles or salts ofte accelerates the + product oxidatio. This is usually regarded as due to catalysis of the homolysis of hydroperoxide molecules ito reactive radicals. More detailed RH discussios of these processes ca be foud elsewhere (1 ). A typical time for 1% reactio of autoxidatio chai sequece (4) a susceptible polymer (with o additives) such as polyethylee could be several years at am- The reactio sequece is iitiateci by some biet summer temperatures. The types of polytrace amout of free radicals or atc)ms which mers susceptible to thermal-oxidative attack ca abstract a hydroge atom from a C-H group are show i Figure 3 with the geeral types to produce a radical site which ca ireact with of products formed. It should be oted that 02. A complex, free-radical chai re,actio e- poly(viyl chloride) is quite readily degraded sues as show i Figure 2. (Other X--H groups, i the presece of oxyge; the major product is where X is ot carbo are possible but C-H HCl, ad the mechaism is quite complex. Jue

4 R poly olelis, CH2 poly oxymethylee. \[ COC 13 polylviyl acetate), (CH2l) Thermal Oxidatio NH NH (CH214 polyihexamethylee adipamidel I polydiees Cl H C Hr poly(viyl chloride) Chai Scissio Co. CO0. H20. H202. Small orgaic molecules (acids, carboylst ad iclusio of oxygeated groups o polymer backboe. - HCI FIGURE 3. Selected polymers susceptible to thermal oxidatio ad their products. The processes of oxidative degradatio ca be greatly accelerated by ultraviolet radiatio. The absorptio of light eergy by sesitive groups (e.g., hydroperoxides or carboyl groups) ca result i the formatio of reactive species by bod rupture or i some cases just by the formatio of suitable electroically excited states (e.g., siglet oxyge). These species ofte ca accelerate the ormal free-radical chai oxidatio as well as provide ew degradatio paths. The et effects of photo-oxidatio are roughly the same as thermal oxidatio, e.g., formatio of backboe carboyl groups, chai scissio, ad productio of CO, H,O, CO2, ad other small molecules. Some geeral types of polymers which photoxidize are show i Figure 4. Polymers susceptible to thermal oxidatio are also sesitive to photo-oxidatio. I additio, polymers with strog ultraviolet absorbig groups are ofte quite susceptible eve though stable to thermal oxidatios. Photo-oxidatio ormally occurs mostly i the surface 32s C 11~~C Photo-oxidatio "NY ~. CH CHe,CH2 "] polydiees i -CH{CHk poly olefis poly (viyl chloride) polyacryloitri le poly(viyl acetate) ich 2 polyoxymethylee, C6H4-0 polytpheylee oxide), -C6H4 r NH -C6H4 x R, C6H5 Cl CN OAt ~~~~~~~~~~~~~~~~~~~~~~~~~1 -NHC -C6H4 -Cjpoly im- pheylee isophthalamide), {-.C6H4-C(CH3)2-C6H4- -C6H4- SO2- C6H4Jpolysulfoe, products similar to thermal oxidatio FIGURE 4. Selected polymers susceptible to photooxidatio. regios of the polymer because of combiatios of such effects as oxyge diffusio ad the opacity of the polymer to the radiatio. I geeral, fudametal studies have oly bee doe o a few types of polymers (1). Hydrolysis Fuctioal groups such as esters, amides, ad acetals ca be attacked by water to form the paret acid or aldehyde group ad the correspodig alcohols or amies i the first two cases. Whe the hydrolysis-sesitive bods are part of the polymer backboe chai liks, hydrolysis will result i depolymerizatio ad release of the hydrolyzed moomers from the polymer. Whe the fuctioal groups are attached to the side of the backboe chai, the polymer remais itact but small molecules are released. Examples of hydrolyses are give i Evirometal Health Perspectives

5 Hydrolysis Polyamides 1 ~~~~~~~~0 (CH2) r 2 CNH? H2 NCH2) m C"OH NH2 - (CH2) - Polyacetals E 0 H ~~Hp0 -,OH HO~C 0NXCH.2 H2 + 2 CH2 Polyesters /epolymerizatio CH20 H H20 H 'CH'CH1 H+, OH CHr--'k + CH30H +(CH2)2 -OC- C6H4 - CO H+, OH- - (CH2,- OH + HOOC - C6H4 - COOH HO - FIGURE 5. Hydrolysis reactios of represetative polymers. Figure 5. Some hydrolyses require acid or base catalysis ad ofte these catalysts ca be supplied by exteral agets, such as atmospheric S03, or oxidatio products of the polymer. Peetratio of water ito may polymers is surprisigly great, but i may cases hydrolysis (like oxidatio) may be restricted to surface regios. Other Active Molecules There are several atmospheric gases which are ofte preset i sufficiet amouts to affect the properties of polymers. The most commo of these is ozoe which is resposible for the severe crackig of may rubber products, particularly i urba atmospheres. This aget is quite reactive toward C=C bods ad rapidly breaks them with the formatio of carboyl compouds o the ew chai eds. Such reactios are ormally restricted to the surface of the polymer, sice the 03 is too reactive to peetrate far ito the polymer bulk. The oxides of sulfur ad itroge-so2, SO3, NO, ad NO2-ca react with polymers i the absece ad presece of light to give a complex variety of results. The oxides of sulfur ca cause sulfoatio reactios, catalyze photooxidatio, ad i the presece of water produce strog acids which ca catalyze hydrolysis. Similarly, itroge oxides ca cause itratio reactios ad act as photo, free-radical, ad acid catalysts. Reactios of SO2 ad NO2 with polymers has bee reviewed ad studied by Jelliek (6). Health hazards associated with the degradatio of the polymer to give the formatio of harmful products would appear secodary to the presece of these very reactive reagets themselves. REFERENCES 1. Hawkis, W. L., Ed. Polymer Stabilizatio, Wiley- Itersciece, New York, Niema, M. B. Agig ad Stabilizatio of Polymers. Cosultats Bureau, New York, Reich, L., ad Stivala, S., Autoxidatio of Hydrocarbos ad Polyolefis, Dekker, New York, Grassie, N. Degradatio. I: Ecyclopedia of Polymer Sciece ad Techology, Vol. 4, Itersciece, New York, 1966, p Palma, G., ad Careza, M. Degradatio of poly- (viyl chloride). I. Kietics of thermal ad radiatio-iduced dehydrochloriatio reductios at low temperatures. J. Appl. Polym. Sci. 14: 1737 (1970). 6. Jelliek, H. H. G. Polymer-gas reactios (air pollutats: NO2 ad S02) as fuctio of pressure, UV light, temperature ad morphology-a survey. I: Proceedigs of the 4th Materials Research Symposium, 1970, Garthersburg, Md., Natioal Bureau of Stadards Special Publicatio 357, Washigto, D.C., Jue

Free Radical Polymerization

Free Radical Polymerization Free Radical Polymerizatio Referece: Aspe Polymers: Uit Operatios ad Reactio Models, Aspe Techology, Ic., 2013. 1. Itroductio The free-radical bulk/solutio polymerizatio model is applicable to bulk ad