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The genesis of CO2 and CO in the thermooxidative degradation of polypropylene
Polymer Degradation and Stability 92 (2007) 94e102 www.elsevier.com/locate/polydegstab
The genesis of CO2 and CO in the thermooxidative degradation of polypropylene Steven M. Thornberg a, Robert Bernstein a, Adriane N. Irwin a, Dora K. Derzon a, Sara B. Klamo b,1, Roger L. Clough a,* a
b
Sandia National Laboratories, Albuquerque, NM 87185, USA Arnold and Mabel Beckman Laboratories of Chemical Synthesis, California Institute of Technology, Pasadena, CA 91125, USA Received 27 July 2006; accepted 13 August 2006 Available online 7 November 2006
Abstract Using a set of three isotactic polypropylene samples that had been individually labeled with carbon-13 at each of the three positions in the monomer unit, we conducted experiments to determine the position of origin of carbon monoxide and carbon dioxide that arise during thermal oxidation of this polymer. By GCemass-spectral analysis, we find that 2/3 of the CO2 derives from the C(1) [methylene] carbon and the remaining 1/3 comes from the C(2) [tertiary] carbon, with none coming from the C(3) [methyl group] carbon. The CO also comes mainly from the C(1) [methylene] carbon (80%). This is in contrast to the solid-phase oxidation products, which have been found (by C-13 NMR on these same labeled PP materials) to originate predominantly (80e85%) from oxidation at the C(2) [tertiary] carbon. These results can be understood in terms of the free-radical reactions that underlie the polypropylene oxidation chemistry. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Carbon dioxide; Isotopic labeling; Polypropylene degradation; Oxidation; Degradation mechanism
1. Introduction The partial oxidation of organic substances has long been known to be accompanied by the formation of CO2 and CO, as the most basic and often the major offgassing products. These most fundamental of oxidation products arise almost universally during the unwanted degradation of many organic items, including oxidation of foods or of small-molecule organic liquids, as well as under many other scenarios including industrial processes involving the intentional oxidation of organic compounds in petrochemical syntheses, in the oxygenmediated curing of certain resins and coatings, etc. CO2 and CO have been often discussed as useful indicators of degradation, and have sometimes been evaluated as * Corresponding author. Tel.: þ1 505 821 2729. E-mail address: [email protected] (R.L. Clough). 1 Current address: Dow Chemical Company, 1776 Building, Midland, MI 48674, USA. 0141-3910/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2006.08.020
potential means of quantitating the extent of degradation for materials in laboratory aging experiments or in applications involving sealed containers. Virtually all common commercial polymers indeed generate these simple gaseous molecules when undergoing oxidation under the influence of a wide range of environments, including elevated temperature, ambient-temperature autoxidation, UV light, ionizing radiation, high mechanical stress, etc. The degradation of polypropylene (PP) has been widely studied, owing to its extensive commercial importance and its high susceptibility to oxidative chemistry. Numerous prior studies have made important contributions to the ongoing effort by the polymer community to better understand the complex mechanisms of oxidation chemistry of polypropylene [1e11], including useful reviews [12,13]. A number of papers have focused on volatile oxidation products of PP [14e23], and have reported on the formation of CO and CO2. We report here studies directed toward utilizing these two oxidative outgassing products to gain insight into the degradation of PP, by
S.M. Thornberg et al. / Polymer Degradation and Stability 92 (2007) 94e102
identifying their position of origin from within the macromolecular structure of PP, with the intent of obtaining information pertinent to the chemical reaction pathways leading to their formation. We have prepared [24] three isotactic polypropylene samples having specific C-13 labeling at each of the three positions in the monomer unit. These samples have been subjected to elevated temperature under oxygen in enclosed containers, after which the gaseous species in the atmosphere above the samples were analyzed by mass spectroscopy (MS). This experiment provides the capability to determine the position(s) of origin of CO and CO2, from within the macromolecular framework of polypropylene, thus providing insight into the degradation chemistry through which these important products are formed. To our knowledge, this technique provides the first assessment of the genesis of CO2 and CO from an organic material. The isotopic labeling of the three samples used in this study (as illustrated below) was for the C(1) sample, C-13 in the secondary (methylene) carbon position at w97% enrichment; for the C(2) sample, C-13 in the tertiary carbon position at w99% enrichment; and for the C(1,3) sample, C-13 in both the secondary (methylene) carbon position and in the methyl group, at w68% enrichment and w31% enrichment, respectively. The C(1,3) sample had the dual labeling due to a partial scrambling of the C(1) and C(3) positions during polymerization of the C-13-labeled propene monomer. I CH2 CH
CH2 CH
CH3
CH3
n
n unlabeled
C(1)
II CH2 CH
I CH2 CH III CH3
CH3
n
n C(2)
C(1,3)
2. Experimental The polypropylene samples used in this study were prepared by two different polymerization techniques [24], starting with propene monomers which had been prepared with C-13 labeling in specific positions. The polypropylene specimens obtained were largely isotactic, with Mw in the range of 1.6e2.6 105. The isotopic labeling of the polypropylene samples, as measured by C-13 NMR, is summarized in Table 1. Table 1 Relative C-13 abundance of selectively labeled polypropylene samples Sample
CH
CH2
CH3
C(1) C(2) C(1,3)
1.0 98.5 0.9
96.7 0.8 68.3
2.3 0.8 30.8
95
The polypropylene polymers (50 mg), in the form of either powders or pressed film, with no added stabilizers, were subjected to 110 C under pure oxygen (at 1 atm pressure) for 84 h in enclosed stainless steel vials with gold-plated gaskets. Analysis of the headspace gas was performed on a Siemens Quantra Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR/MS) equipped with a 1 Tesla permanent magnet. The sample was introduced into the FT-ICR/MS via a voltage-regulated pulse valve and ions were formed by electron impact ionization. The fundamentals of FT-ICR/MS are described elsewhere [25]. The operating conditions were as follows: filament beam electron energy, 70 eV; filament beam current, 45e650 nA; trap voltage during beam, 1.2e 1.4 V; and trap voltage during excite, 1.0e1.2 V. Each spectrum was collected using 128 co-adds and either 250,000 or 500,000 data points. The typical mass spectral resolution realized was 8300 (m/Dm) and 14,200 (m/Dm) at m/z 44 with 250,000 and 500,000 data points, respectively. The scanned mass range was 12e1000 Da. 3. Mass spectral interpretation The objective of this study depended on a capability to measure quantitatively the relative amounts of 12CO2 versus 13 CO2, and of 12CO versus 13CO, coming from each of the isotopically labeled polypropylene samples following oxidation at elevated temperature. To achieve this, issues of the resolution of the mass spectrometer, the identification of peaks, and the quantification/repeatability of peak heights had to be addressed. The mass spectra obtained after admitting a small portion of the gaseous atmosphere from the vessel containing the oxidized polypropylene also showed peaks corresponding to the constituents of ambient air (N2, O2, Ar, H2O, and others). Carbon dioxide is also present in ambient air, but contamination of our samples is not a problem, because its relative abundance is too low to affect the 13CO2/12CO2 ratio measurements in our samples (for example, argon occurs at 0.93% by volume in air, while carbon dioxide occurs at about 0.033 volume percent). In addition, numerous peaks coming from the fragmentation within the spectrometer of trace organic volatiles produced in the course of the polymer degradation were seen. Several experiments were performed in which the sample container was held at lower temperature (down to 48 C) to ascertain that no significant interference at mass values of 44 or 45 was coming from the decomposition of volatile organic oxidation products, whose vapor pressure would be reduced at the lower temperature (for example, the melting points of acetic and formic acids are 16.5 C and 8.4 C, respectively). Experiments were also performed to confirm that under the mass spectroscopy conditions used in the measurements, carbon monoxide measurements were not significantly affected by fragmentation peaks coming from carbon dioxide. Also seen were numerous species characteristic of the mass spectroscopy technique (such as HCO2, HCO, N2H, etc.) which arise inside the spectrometer by ions picking up an H atom from trace amounts of water adsorbed on the inner walls.
S.M. Thornberg et al. / Polymer Degradation and Stability 92 (2007) 94e102
96
Table 2 Exact atomic mass values for some species of interest
C(1) Labeled PP
Nominal mass
Exact atomic mass
12 CO N2 13 CO 12 CHO HN2 13 CHO O2 Ar 12 CO2 13 CO2 12 CHO2
28 28 29 29 29 30 32 40 44 45 45
27.9949 28.0061 28.9983 29.0027 29.0139 30.0061 31.9898 39.9624 43.9898 44.9932 44.9977
4. C-13 labeling in CO2 Fig. 1a shows a portion of the mass spectrum obtained from analysis of the gaseous components in the vessel containing C(1) labeled polypropylene, following exposure under oxygen
CO2
CO2
Ar 39
40
41
42
43
44
45
46
47
m/z C(2) Labeled PP
Signal Intensity
CO2
13
CO2
Ar 39
40
41
42
43
44
45
46
47
m/z C(1,3) Labeled PP
CO2
Signal Intensity
A number of molecules relevant to this study share the same nominal mass value. For example, carbon monoxide and nitrogen both have a molecular mass of 28, 13CO2 and HCO2 each have a mass of 45, 13CO and HCO both have a mass of 44. All of the aforementioned species are seen (with varying peak height in different experiments), but peaks corresponding to all of these isomass pairs can be separated with baseline resolution by the instrument used in this study, as each member of the isomass pair in fact differs very slightly in atomic mass. Exact masses of some of the atomic and molecular species of interest are listed in Table 2. The mass value of a given species, as measured by the instrument, varied slightly from day to day, but the differences in mass value between peaks of relatively similar mass were quite reproducible. Thus, 12CO occurs at 0.0112 mass units below N2 and 3.9949 mass units below O2. 13CO occurs 1.0034 mass units above 12CO. 13CO2 falls 1.0034 mass units above 12 CO2, whereas 12CHO2 occurs 1.0079 mass units above 12 CO2. Utilizing the known values for molecular mass differences, confident assignments of the various peaks were thus possible by identifying the patterns which gave correct fits to the expected mass differences. Relative peak heights were assumed to be proportional to relative concentration, for isotopic variants of the same molecule (i.e., 13CO versus 12CO and 13CO2 versus 12CO2). For small peaks (particularly for the 12CO and 13CO peaks), the signal to noise ratio could be improved by selectively removing the ions corresponding to the larger peaks from the spectrometer (such as N2 or O2, depending on the particular sample run) by the technique of ion ejection. This technique was used in the measurements of 13CO and 12CO, for which the higher-concentration 13CO2 and 12CO2 ions were removed. Some variability (scatter) in relative peak height measurements was observed, and so data collected from a number of mass spectral runs, conducted during several different days, were averaged to minimize error in the measurement.
13
Signal Intensity
Formula
13
CO2
39
40
41
42
43
44
45
46
47
m/z Fig. 1. Mass spectra obtained from the gas atmosphere above C-13 labeled polypropylene samples, following exposure to 110 C under oxygen for 84 h. The mass region from 39 to 47 is shown. Top: spectrum obtained from the C(1) labeled polypropylene, center: spectrum from C(2) labeled polypropylene, bottom: spectrum from C(1,3) labeled polypropylene.
at 110 C for 3½ days. The 13CO2 and 12CO2 peaks are indicated. From the average ratio of these two peaks, it is found that 66 5% of the carbon dioxide has the C-13 isotopic label, and therefore 66 5% of the CO2 coming from the polypropylene material must originate from the C(1) carbon atom. Fig. 1b shows a mass spectrum for the gaseous components from the C(2) material, following oxidation as for the sample
S.M. Thornberg et al. / Polymer Degradation and Stability 92 (2007) 94e102
above. Based on the average ratio found for the 13CO2 and 12 CO2 peaks, 33 5% of the CO2 contains the C-13 label, indicating that 33 5% of the CO2 coming from the polypropylene material must originate from the C(2) carbon atom. Determination of the C(3) contribution to the total carbon dioxide generated is an inherently less accurate measurement, compared with C(1) and C(2), because the latter involve samples which are specifically labeled in the positions of interest, whereas for C(3), only the C(1,3) sample, which is labeled to the extent of 31%, in conjunction with 68% labeling in the C(1) position, is available. Determination of C(3) from this sample is inherently less sensitive, as it has labeling that is slightly less than 1/3 of the C(1) amount, and this difficulty is increased in the case of the C(3) CO2 being present in much lesser amount than C(1) CO2. The C(3) determination in any event requires a subtraction, with two possibilities available. The first possibility is to calculate C(3) based on the result of the C(1) and C(2) measurements:
97
5. C-13 labeling in CO Quantitation of the CO product was more difficult compared with CO2, primarily because the amount detected was lower by an order of magnitude or more. Fig. 2a shows data for the 12CO/13CO region of the mass spectrum obtained from the oxidized C(1) sample, which was measured under mass spectral conditions in which several of the larger peaks (12CO2, 13CO2, O2, and Ar) had been ejected to maximize signal to noise. The average value of the 13CO/12CO peak ratios yields a determination of the proportion of CO coming from the C(1) position as slightly over 80%, with larger error bars than for the CO2 measurements. For the C(2) samples, the weak CO peaks were difficult to measure, and in particular the 13CO peak was not visible above the baseline in a number of runs. Fig. 2b shows some of the best data for the 12CO/13CO region of the mass spectrum obtained from the oxidized C(2) sample, for which a 13CO peak is visible. Based on averaging several such runs,
Cð3Þ ¼ 100 ½Cð1Þ þ Cð2Þ
N2
C(1) Labeled PP 13
CO
Signal Intensity
Given the fact that the C(1) and C(2) experiments indicate that essentially all of the carbon dioxide can be accounted for as coming from these two positions in the macromolecule (66 þ 33%), it would be concluded that none of the carbon dioxide originates from the C(3) methyl carbon (or an insignificant amount). The second possibility for obtaining the C(3) value is by analyzing the C(1,3) sample, taking into account the result derived from the C(1) experiment, together with the relative amount of C(1) and C(3) labeling in the sample:
CO
fCð1; 3Þ ½Cð1Þ 0:68g 0:31
N2H
Given the previously discussed measurement indicating 66% of the CO2 coming from C(1), and assuming that C(3) contributes w0%, the equation above predicts that the percentage of C-13 label in the CO2 coming from the C(1,3) sample should be 45%. Fig. 1c shows the mass spectrum obtained from the C(1,3) sample, with the 13CO2 and 12CO2 peaks labeled. The average value for the C-13 labeling in this sample was measured as 46 5%, in excellent agreement with the above. The results on the origin of CO2 from the thermal oxidation of polypropylene are summarized in Table 3. The error bars are estimated as 5% absolute, based on the overall scatter in the measurements coming from repeated experiments for which the values were averaged.
28
28.2
28.4
28.6
28.8
N2
C(2) Labeled PP
CO
13
Table 3 Position of origin of CO2 from the thermal oxidation of polypropylene at 110 C Carbon atom within the polymer
% CO2 from this position
C(1) [methylene] C(2) [tertiary] C(3) [methyl group]
66% [5%] 33% [5%] 0% [<5%]
29
m/z
Signal Intensity
Cð3Þ ¼
28
28.2
28.4
28.6
28.8
CHO CO
N2H
29
m/z Fig. 2. Mass spectra obtained from the gas atmosphere above C-13 labeled polypropylene samples, following exposure to 110 C under oxygen for 84 h. The mass region from 29 to 30 is shown. Top: spectrum obtained from the C(1) labeled polypropylene, bottom: spectrum from C(2) labeled polypropylene.
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98
Table 4 Position of origin of CO from the thermal oxidation of polypropylene at 110 C Carbon atom within the polymer
% CO from this position
C(1) [methylene] C(2) [tertiary] C(3) [methyl group]
80% [10%] 5% [10%] Not determined
a measurement of the proportion of CO coming from the C(2) position was somewhat less than 5%. The C(1,3) spectrum suffered from extremely weak signal in the CO region (with CO signals often not clearly distinguishable from the baseline). Given the fact that a determination of C(3) from this sample is inherently prone to large error because of mixed isotopic labeling, with C(3) labeled at only 31%, we did not obtain a satisfactory measurement of the C(3) value. Given the combined C(1) and C(2) values of 85% with large error in the measurement, and given that we did not obtain any direct evidence of a C(3) CO contribution, we can only estimate C(3) as being quite possibly near zero, and very likely less than 15%. The CO results are summarized in Table 4. 6. Discussion of CO2 and CO formation from each chain position in polypropylene 6.1. CO2 and CO from the C(1) methylene carbon Most of the chemistry that takes place in polypropylene undergoing oxidation involves attack by free radicals at the C(2) [tertiary] carbon, and indeed most of the macromolecular oxidation products involve oxidation at the C(2) position [12,24,26]. This is because the tertiary hydrogen atom is much more susceptible to abstraction, compared with the hydrogen atoms at the C(1) and C(3) positions, due to the formation of a more stable tertiary radical. Probably the most important mechanism by which chemistry is initiated at the C(1) [methylene] carbon is in the course of a well known
III CH3 I I II CH2 CH CH2
III CH3 I II CH CH2
III III CH3 CH3 I I II I II CH2 C CH2 CH CH2
OH
I II III CH2 C CH3 + O
.
R
.
III CH3 I II I CH2 C CH2
.
reaction initiated by cleavage of a C(2) peroxide, leading to a mixture of C(2) [tertiary] alcohol and C(2) methyl ketone (Scheme 1). As an indication of the importance of macromolecular oxidation products that can be explained by Scheme 1, it was found in our NMR work [24] on these same isotopically labeled polypropylene samples that following oxidation at 109 C, the amount of these functionalities was as follows: C(2) [tertiary] peroxides w 45%, C(2) [tertiary] alcohols w 20%, and C(2) methyl ketones w 8%, for a total of w73% of all oxidation products found in the polymer. It is from the C(2) methyl ketone route (Scheme 1) that a free radical is simultaneously formed at a C(1) carbon atom, and it is through this ‘‘dangling chain end’’ radical that the formation of large amounts of C(1) CO2 and CO is understandable. This C(1) radical can undergo reaction with oxygen to form a peroxide, as shown in Scheme 2, which will lead to further oxidation products. Because of the structure of this C(1) radical, which is attached to the macromolecule by only one CeC bond, it is understandable that a substantial fraction of the products of further oxidation chemistry of this radical can in fact end up as CO2 and CO. Following thermal decomposition of the C(1) chain-end peroxide, one of the products expected is an aldehyde (Scheme 3). In an environment of ongoing free-radical chemistry, aldehydes are well known to be susceptible to further reaction by hydrogen atom abstraction. Mechanistic studies with small organic molecules indicate that the hydrogen atom on the aldehyde is in fact an order of magnitude more reactive toward abstraction than a tertiary hydrogen atom [27]. Indeed, in our NMR studies of the isotopically labeled polypropylene [26], we found no evidence of any aldehydes (despite finding significant quantities of ketones), which is consistent with high reactivity of the former, leading to other products. Abstraction of the aldehydic hydrogen atom (Scheme 4) gives a carbonyl radical. This radical can readily undergo decarbonylation (i.e., loss of CO), which is a reaction that is well documented from the literature on radical chemistry of small molecules [28]. As one extreme example of decarbonylation, trimethylacetaldehyde is found to decompose to CO and isobutane while sitting at room temperature, which proceeds
III CH3 I II CH CH2
O2
III CH3 I II I CH2 C CH2 OO
.
III CH3 I II CH CH2
R-H
R H
III CH3 I I II CH2 CH CH2
III III CH3 CH3 I I II I II CH2 C CH2 CH CH2
O
.
Scheme 1.
III CH3 I II I CH2 C CH2 OOH
III CH3 I II CH CH2
S.M. Thornberg et al. / Polymer Degradation and Stability 92 (2007) 94e102
.
III CH3 I I II CH2 CH CH2
III CH3 I I II CH2 CH CH2
O2
OO
.
R-H
99
III CH3 I I II CH2 CH CH2 OOH
Scheme 2.
through a free-radical process initiated by trace impurities of peroxides [28]. The carbonyl radical shown in Scheme 4 can also react with oxygen [28] to give a peroxy radical, which can further react to give a peracid. Subsequent cleavage of the peroxide bond will yield a carboxyl radical (Scheme 4), which can undergo hydrogen abstraction to give an acid, or it can undergo loss of CO2. These reactions of carboxyl radical are widely known from small molecule chemistry, and are very analogous to the widely used thermal decomposition of benzoyl peroxide, which gives both phenyl radicals and benzoic acid [28,29]. As a further example from the organic chemistry literature, it is known that the decomposition of acetylperoxide yields the radical CH3CO2 which undergoes loss of CO2 to produce methyl radicals in high yield [28,29]. As discussed above, the majority of the CO2 and CO emanating from polypropylene oxidative degradation likely comes from reaction of C(1) carbon atom in a mechanism which is initiated by a chain cleavage event. It may thus be reasonable to consider the measurement of these outgassing products as potentially effective indicators of the extent of degradation of macroscopic properties in this polymer. 6.2. CO2 and CO from the C(2) tertiary carbon As mentioned earlier, studies have consistently shown that the oxidation of polypropylene involves the formation of oxidation products primarily at the tertiary carbon atom, as seen by analysis of the partially degraded, solid polymer [12]. Our NMR studies of the same isotopically labeled polypropylene samples utilized in this study afforded a measurement indicating that approximately 80e85% of the macromolecular thermooxidative products are formed at the C(2) [tertiary] carbon atom. Given the knowledge about the preponderance of polymer oxidation products occurring at C(2), it might seem at first surprising that CO and CO2 come mainly from the C(1) carbon atom. However, upon reflection on the fact that the C(2) carbon in polypropylene is bonded to three other carbon atoms, and that all three of these CeC bonds must be broken in order to release a molecule of either CO or CO2, it becomes apparent that a rather extensive series of chemical reactions must be
required to generate either of these two degradation products from the C(2) position. From this point of view, it may seem somewhat remarkable that any significant amount of C(2) carbon proceeds all the way to CO or CO2. As noted in Section 6.1, the most abundant oxidation products at the C(2) position following oxidation at 109 C are tertiary peroxides (representing w45% of all of the solid-phase polypropylene oxidation products), and tertiary alcohols (w20%). In neither of these products has any CeC bond breakage occurred. The third largest C(2) oxidation product, representing w8% of total products found, is the methyl ketone, for which one of the three C(2) CeC bonds has been cleaved, which is at least one step in the right direction for formation of CO and CO2, and some amount of these products may arise through further oxidation of the C(2) methyl ketone. This could be envisioned as proceeding through free-radical abstraction of a hydrogen atom from the methyl group attached to the ketone (which is reasonable, since the resultant radical would be relatively favorable due to stabilization by conjugation with the adjacent carbonyl group). Further oxidation at this methyl group, via chemical routes similar to those shown in Schemes 2e4, could result in eventual loss of the methyl carbon as CO, CO2, or other small molecule product such as formaldehyde, leaving a carbonyl radical at the C(2) carbon, which could then undergo reactions analogous to those in Scheme 4, leading to C(2) CO and CO2. The problem with this mechanism is that one might likely expect to see C(3) CO and CO2 coming from the methyl group that is being oxidized away. However, the results found in this paper are not consistent with any significant amount of these products coming from C(3). An interesting aspect of the results for the C(2) carbon atom is the fact that very different (relative) amounts of CO versus CO2 are generated; C(2) may be viewed as contributing at least six or seven times less to the total amount of CO, compared with the C(2) contribution to CO2. One possibility suggested by this observation is a reaction that leads directly to CO2 but not to CO (unlike the sequential reactions in Schemes 2e4, which seem most suited for chain-end or side-chain reactive groups). One possibility that is intriguing in addressing the C(2) CO2 formation mechanism, would be a reaction analogous
III CH3 I I II CH2 CH CH2
III CH3 I I II CH2 CH CH2
OOH
O
.
III CH3 I II I H C CH CH2 O (Aldehyde)
Scheme 3.
S.M. Thornberg et al. / Polymer Degradation and Stability 92 (2007) 94e102
100
III CH3 I II I H C CH CH2
O
.
.
.
R
R H
O
I CO
III CH3 I II CH CH2
.
+
O
O2
III CH3 I II I OO C CH CH2
III CH3 I II I C CH CH2
III CH3 I II I HOO C CH CH2
.
O
III CH3 I II I O C CH CH2
O
I CO2
+
III CH3 I II I HO C CH CH2
R H
O
III CH3 I II CH CH2
.
Scheme 4.
to that discussed by a number of workers [17,30,31] for the formation of acetic acid from a methyl ketone. In this mechanism, a hydroxy radical ( OH) adds to the carbonyl carbon of the methyl ketone, creating an alkoxy radical at the oxygen that was formerly part of the ketone group. Chain cleavage, analogous to Scheme 1, directly gives acetic acid. Experimental evidence for this mechanism has been obtained by Walling and Gibian [30]. It is known that the tertiary peroxy radical of Scheme 1, which is formed in the first oxidation step of a C(2) carbon atom in polypropylene and is therefore the precursor to virtually all oxidation at the C(2) carbon, is a relatively long-lived radical. If, in overall similarity to the above-described mechanism involving hydroxyl radical, this peroxy radical (of Scheme 1) were to add to an existing C(2) methyl ketone (also in Scheme 1), the result would be an alkoxy radical which could then undergo chain cleavage (analogous to Scheme 1). The resultant product would be a C(2) perester, as shown in Scheme 5 (interestingly, our previous NMR study on the C-13 labeled polypropylene samples indeed found
III CH3 I II I CH2 C CH2 OO
.
evidence for what appears to be substantial quantities of C(2) peresters) [24,26]. Cleavage of the peroxide bond in the perester of Scheme 5 (or in a similar perester formed by any other mechanism) would give the radical CH3CO2 , which, as discussed in Section 6.1, is well known [28,29] to decompose into CO2 and methyl radical. The CO2 thus formed would be from the C(2) carbon. It is not known whether the peroxy radical of Scheme 5 would in fact be reactive enough to add to a ketone group; certainly this radical is far less reactive than the hydroxyl radical. At the same time, its long lifetime, and the fact that oxidation of a bulk polymeric material can be expected to consist of many chain reactions that are highly localized on a molecular scale, would give a high probability of the peroxy radical sitting for extended time periods in close proximity to methyl ketone groups formed in the course of the ongoing oxidation. A C(3) methyl radical is formed in this pathway, and while some fraction might undergo reaction with oxygen, much of this highly reactive species would likely terminate by hydrogen abstraction, and its mobility would also facilitate coupling with other radicals.
III CH3 I II I CH2 C CH2
III CH3 II I CH CH2
+
I II III CH2 C CH3
III CH3 I II CH CH2
O O I II III CH2 C CH3
O
.
O
.
II III O C CH3
+
O
III CH3 I II I CH2 C CH2 O
.
III CH3 II I CH CH2
III CH3 I II I CH2 C CH2 O O II III C CH3
II CO2 +
O
.IIICH3 Scheme 5.
III CH3 II I CH CH2
+
I CH2
.
S.M. Thornberg et al. / Polymer Degradation and Stability 92 (2007) 94e102
6.3. CO2, CO, and the C(3) methyl side chain The results obtained in this study find that all of the CO2 evolved from polypropylene undergoing thermal oxidation can be accounted for by contributions from the C(1) and C(2) carbon atoms, and we find no evidence of any appreciable amount of C(3) CO2. It is also found that the preponderance of CO comes from C(1), with a small amount from C(2). The potential contribution of C(3) CO could not be obtained independently, but if it is present, it is at best small compared with the C(1) contribution. The lack of C(3) contribution to the CO2, and little if any to the CO, may seem, on first thought, somewhat surprising, given that the C(3) exists as pendant side groups all along the polypropylene. As such it is bonded to the macromolecule by only one CeC bond, so that oxidation at this site, once initiated, might be expected to proceed with relatively high yield to CO2 and CO, through reactions analogous to those in Schemes 2e4. The findings for C(3) can be understood as reflecting the low reactivity of the methyl site. As is well known, the ordering of reactivity of hydrogen atoms to free-radical abstraction is tertiary > secondary > primary. The primary hydrogens of the methyl group are thus the least reactive within the polypropylene structure. Moreover, to the extent that some small quantity of radicals may be formed by abstraction at C(3), they have the opportunity within the bulk matrix to abstract a tertiary hydrogen atom from a C(2) site on an adjacent chain, thus converting the primary radical to a tertiary radical, before O2 diffuses to the site. Intrachain H-atom abstraction (i.e., ‘‘hydrogen hopping’’) has been demonstrated to occur rather efficiently within hydrocarbons in the solid phase [32]. One other potential mechanism for activation of C(3) that should be mentioned here is the ejection of a C(3) methyl rad ical, CH3 , as an alternative path to the chain cleavage reaction coming from the C(2) alkoxy radical of Scheme 1. Actually, the formation of a CH3 radical could be expected to be less favored compared with the chain cleavage reaction, because the latter generates a primary radical, which is known to be significantly more stable compared with CH3 . Nevertheless, this reaction forming CH3 has been mentioned many times in earlier publications about polypropylene oxidation. Another possible mechanism for formation of C(3) CH3 was described in a preceding section of this article. If significant quantities of C(3) CH3 are indeed being formed, the products from this radical apparently must come largely through hydrogen abstraction and/or radicaleradical coupling. Previous studies have reported on the lack of participation of the C(3) methyl in the formation of macromolecular oxidation products [8]. Indeed, the NMR studies conducted previously [24,26] on these same C-13 labeled polypropylene materials found no measurable amount of solid-phase oxidation products at the C(3) methyl carbon. If C(3) oxidation products were to be found, one could have imagined that their most likely occurrence might be among single-carbon-atom volatile compounds, given the position of C(3) as a methyl side chain in the polymer. The lack of a significant contribution of C(3) to the CO2 and CO is a further indication of the
101
relative inertness of the C(3) methyl group in an environment of thermal oxidation. 7. Summary and conclusions Samples of polypropylene having carbon-13 isotopic labeling in each of the three positions along the chain were subjected to elevated temperature (110 C) in the presence of oxygen in enclosed vials, after which the gaseous atmosphere over the samples was analyzed by mass spectroscopy. This technique has provided the first ever determination of the positions of origin of the oxidation products CO and CO2, from within a macromolecular framework. The majority of both these gasses is found to originate from the C(1) [methylene] carbon atom in the polypropylene backbone (w66% of the CO2, and >80% of the CO). This result can be understood in terms of chemical reactions initiated by the thermal decomposition of a C(2) [tertiary] peroxide to give a C(2) alkoxy radical, followed by chain scission to yield a C(2) methyl ketone and a C(1) ‘dangling chain end’ radical, in which the C(1) is attached to the polymer chain by only one CeC bond. Reaction of this C(1) radical with oxygen gives a peroxide that can lead to both C(1) aldehyde and peracid, which subsequently undergo loss of CO and of CO2, respectively, through further free-radical mediated reactions that are well known from small-molecule organic chemistry. The C(2) [tertiary] carbon atom of the polypropylene backbone contributes w33% of the CO2, and 5% of the CO, despite the fact that the C(2) position is well known to be the formation site of most of the oxidation compounds that occur when polypropylene undergoes degradation. (For example, in the macromolecular oxidation products in these same isotopically labeled polypropylene samples, as measured by NMR, w80e85% of all the degradation products are formed at the C(2) position). The fact that the C(2) contribution to the CO and CO2 products seems much less favored in comparison to the macromolecular oxidation products, can be understood in light of the structure of the C(2) carbon, which is connected to three other carbon atoms in the polypropylene; formation of either CO2 or CO requires cleavage of all three of these bonds. The fact that the relative contribution of C(2) to CO appears to be much smaller, compared with the C(2) contribution to CO2, may indicate a different mechanism for the C(2) CO2 production, possibly involving a perester intermediate. Despite the fact that the C(3) [methyl side chain] might at first seem an obvious candidate as a site for origin of CO and CO2, due to it being connected to the macromolecule by only one CeC bond, there is no indication of any measurable contribution of C(3) to the CO2, and any contribution to the CO is small at best. These observations reinforce the degree to which the C(3) methyl carbon is comparatively inert, with regards to participation in the free-radical chemistry underlying polypropylene oxidation. Acknowledgments Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the U.S. Department
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of Energy, National Nuclear Security Administration under Contract DE-AC04-94AL8500. The authors thank James Hochrein for assistance with the Quantra mass spectrometer. References [1] Chien JCW, Boss CR. Polymer reactions. VI. Inhibited autoxidation of polypropylene. Journal of Polymer Science: Part A-1 1967;5:1683e97. [2] Chien JCW, Vandenberg EJ, Jabloner H. Polymer reactions. III. Structure of polypropylene hydroperoxide. Journal of Polymer Science: Part A-1 1968;6:381e92. [3] Carlsson DJ, Wiles DM. The photodegradation of polypropylene films. III. Photolysis of polypropylene hydroperoxides. Macromolecules 1969;2:597e606. [4] Adams JH. Analysis of the nonvolatile oxidation products of polypropylene. I. Thermal oxidation. Journal of Polymer Science: Part A-1 1970;8: 1077e90. [5] Decker C, Mayo FR. Aging and degradation of polyolefins. II. g-Initiated oxidations of atactic polypropylene. Journal of Polymer Science: Polymer Chemistry Edition 1973;11:2847e77. [6] Niki E, Decker C, Mayo FR. Aging and degradation of polyolefins. I. Peroxide-initiated oxidations of atactic polypropylene. Journal of Polymer Science: Polymer Chemistry Edition 1973;11:2813e45. [7] Iring M, Tu¨do¨s F. Thermal oxidation of polyethylene and polypropylene: effects of chemical structure and reaction conditions on the oxidation process. Progress in Polymer Science 1990;15:217e62. [8] Lacoste J, Vaillant D, Carlsson DJ. Gamma-, Photo-, and thermally-initiated oxidation of isotactic polypropylene. Journal of Polymer Science: Part A: Polymer Chemistry 1993;31:715e22. [9] Gijsman P, Hennekens J, Vincent J. The mechanism of the low-temperature oxidation of polypropylene. Polymer Degradation and Stability 1993;42:95e105. [10] Gijsman P, Kroon M, Oorschot MV. The role of peroxides in the thermooxidative degradation of polypropylene. Polymer Degradation and Stability 1996;51:3e13. [11] Gugumus F. Thermooxidative degradation of polyolefins in the solid state e 6. Kinetics of thermal oxidation of polypropylene. Polymer Degradation and Stability 1998;62:235e43. [12] Carlsson DJ, Wiles DM. The photooxidative degradation of polypropylene. Part 1. Photooxidation and photoinitiation processes. Journal of Macromolecular Science e Reviews in Macromolecular Chemistry 1976;C14:65e106. [13] George GA, Celina M. In: Hamid SH, editor. Handbook of polymer degradation. 2nd ed. New York: Marcel Dekker, Inc; 2000. p. 277e313. [14] Carlsson DJ, Wiles DM. The photodegradation of polypropylene films. II. Photolysis of ketonic oxidation products. Macromolecules 1969;2:587e97. [15] Philippart JL, Posada F, Gardette JL. Mass spectroscopy analysis of volatile photoproducts in photooxidation of polypropylene. Polymer Degradation and Stability 1995;49:285e90.
[16] Philippart JL, Gardette JL. Thermo-oxidation of isotactic polypropylene in 32O2e36O2: comparison of the mechanisms of thermoand photo-oxidation. Polymer Degradation and Stability 2001;73: 185e7. [17] Commereuc S, Vaillant D, Philippart JL, Lacoste J, Lemaire J, Carlsson DJ. Photo and thermal decomposition of iPP hydroperoxides. Polymer Degradation and Stability 1997;57:175e82. [18] Barabas K, Iring M, Lazlo-Hedvig S, Kellen T, Tudos F. Study of the thermal oxidation of polyolefines. VIII. Volatile products of polypropylene thermal oxidation. European Polymer Journal 1978;14: 405e7. [19] Hoff A, Jacobsson S. Thermal oxidation of polypropylene close to industrial processing conditions. Journal of Applied Polymer Science 1982;27:2539e51. [20] Hoff A, Jacobsson S. Thermal oxidation of polypropylene in the temperature range of 120e280 C. Journal of Applied Polymer Science 1984;29:465e80. [21] Celina M, Clough RL, Jones GD. Initiation of polymer degradation via transfer of infectious species. Polymer Degradation and Stability 2006;91:1036e44. [22] Kato Y, Carlsson DJ, Wiles DM. Photo-oxidation of polypropylene: some effects of molecular order. Journal of Applied Polymer Science 1969;13:1447e58. [23] Celina M, George GA, Billingham NC. Physical spreading of oxidation in solid polypropylene as studied by chemiluminescence. Polymer Degradation and Stability 1993;42:335e44. [24] Mowery DM, Assink RA, Derzon DK, Klamo SB, Clough RL, Bernstein R. Solid-state 13C NMR investigation of the oxidative degradation of selectively labeled polypropylene by thermal aging and gammairradiation. Macromolecules 2005;38:5035e46. [25] Marshal AG, Hendrickson CL, Jackson GS. Fourier transform ion cyclotron resonance mass spectrometry: a primer. Mass Spectrometry Reviews 1998;17:1e35. [26] Mowery DM, Assink RA, Derzon DK, Klamo SB, Bernstein R, Clough RL. Radiation oxidation of polypropylene: a solid-state 13C NMR study using selective isotopic labeling. Radiation Physics and Chemistry, in press. [27] Pryor WA. Free radicals. New York: McGraw-Hill; 1966. [28] Breslow R. Organic reaction mechanisms an introduction. Menlo Park: W.A. Benjamin; 1969. [29] Walling C. Free radicals in solution. New York: John Wiley & Sons; 1957. [30] Walling C, Gibian MJ. The photosensitized decomposition of peroxides. Journal of American Chemical Society 1965;87:3413. [31] Geuskens G, Kabamba MS. Photo-oxidation of polymers e Part V: a new chain scission mechanism in polyolefins. Polymer Degradation and Stability 1982;4:69e76. [32] Clough RL. Isotopic exchange in gamma-irradiated mixtures of C24H50 and C24D50: evidence of free radical migration in the solid state. Journal of Chemical Physics 1987;87:1588e95.
The genesis of CO2 and CO in the thermooxidative degradation of polypropylene Steven M. Thornberg a, Robert Bernstein a, Adriane N. Irwin a, Dora K. Derzon a, Sara B. Klamo b,1, Roger L. Clough a,* a
b
Sandia National Laboratories, Albuquerque, NM 87185, USA Arnold and Mabel Beckman Laboratories of Chemical Synthesis, California Institute of Technology, Pasadena, CA 91125, USA Received 27 July 2006; accepted 13 August 2006 Available online 7 November 2006
Abstract Using a set of three isotactic polypropylene samples that had been individually labeled with carbon-13 at each of the three positions in the monomer unit, we conducted experiments to determine the position of origin of carbon monoxide and carbon dioxide that arise during thermal oxidation of this polymer. By GCemass-spectral analysis, we find that 2/3 of the CO2 derives from the C(1) [methylene] carbon and the remaining 1/3 comes from the C(2) [tertiary] carbon, with none coming from the C(3) [methyl group] carbon. The CO also comes mainly from the C(1) [methylene] carbon (80%). This is in contrast to the solid-phase oxidation products, which have been found (by C-13 NMR on these same labeled PP materials) to originate predominantly (80e85%) from oxidation at the C(2) [tertiary] carbon. These results can be understood in terms of the free-radical reactions that underlie the polypropylene oxidation chemistry. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Carbon dioxide; Isotopic labeling; Polypropylene degradation; Oxidation; Degradation mechanism
1. Introduction The partial oxidation of organic substances has long been known to be accompanied by the formation of CO2 and CO, as the most basic and often the major offgassing products. These most fundamental of oxidation products arise almost universally during the unwanted degradation of many organic items, including oxidation of foods or of small-molecule organic liquids, as well as under many other scenarios including industrial processes involving the intentional oxidation of organic compounds in petrochemical syntheses, in the oxygenmediated curing of certain resins and coatings, etc. CO2 and CO have been often discussed as useful indicators of degradation, and have sometimes been evaluated as * Corresponding author. Tel.: þ1 505 821 2729. E-mail address: [email protected] (R.L. Clough). 1 Current address: Dow Chemical Company, 1776 Building, Midland, MI 48674, USA. 0141-3910/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2006.08.020
potential means of quantitating the extent of degradation for materials in laboratory aging experiments or in applications involving sealed containers. Virtually all common commercial polymers indeed generate these simple gaseous molecules when undergoing oxidation under the influence of a wide range of environments, including elevated temperature, ambient-temperature autoxidation, UV light, ionizing radiation, high mechanical stress, etc. The degradation of polypropylene (PP) has been widely studied, owing to its extensive commercial importance and its high susceptibility to oxidative chemistry. Numerous prior studies have made important contributions to the ongoing effort by the polymer community to better understand the complex mechanisms of oxidation chemistry of polypropylene [1e11], including useful reviews [12,13]. A number of papers have focused on volatile oxidation products of PP [14e23], and have reported on the formation of CO and CO2. We report here studies directed toward utilizing these two oxidative outgassing products to gain insight into the degradation of PP, by
S.M. Thornberg et al. / Polymer Degradation and Stability 92 (2007) 94e102
identifying their position of origin from within the macromolecular structure of PP, with the intent of obtaining information pertinent to the chemical reaction pathways leading to their formation. We have prepared [24] three isotactic polypropylene samples having specific C-13 labeling at each of the three positions in the monomer unit. These samples have been subjected to elevated temperature under oxygen in enclosed containers, after which the gaseous species in the atmosphere above the samples were analyzed by mass spectroscopy (MS). This experiment provides the capability to determine the position(s) of origin of CO and CO2, from within the macromolecular framework of polypropylene, thus providing insight into the degradation chemistry through which these important products are formed. To our knowledge, this technique provides the first assessment of the genesis of CO2 and CO from an organic material. The isotopic labeling of the three samples used in this study (as illustrated below) was for the C(1) sample, C-13 in the secondary (methylene) carbon position at w97% enrichment; for the C(2) sample, C-13 in the tertiary carbon position at w99% enrichment; and for the C(1,3) sample, C-13 in both the secondary (methylene) carbon position and in the methyl group, at w68% enrichment and w31% enrichment, respectively. The C(1,3) sample had the dual labeling due to a partial scrambling of the C(1) and C(3) positions during polymerization of the C-13-labeled propene monomer. I CH2 CH
CH2 CH
CH3
CH3
n
n unlabeled
C(1)
II CH2 CH
I CH2 CH III CH3
CH3
n
n C(2)
C(1,3)
2. Experimental The polypropylene samples used in this study were prepared by two different polymerization techniques [24], starting with propene monomers which had been prepared with C-13 labeling in specific positions. The polypropylene specimens obtained were largely isotactic, with Mw in the range of 1.6e2.6 105. The isotopic labeling of the polypropylene samples, as measured by C-13 NMR, is summarized in Table 1. Table 1 Relative C-13 abundance of selectively labeled polypropylene samples Sample
CH
CH2
CH3
C(1) C(2) C(1,3)
1.0 98.5 0.9
96.7 0.8 68.3
2.3 0.8 30.8
95
The polypropylene polymers (50 mg), in the form of either powders or pressed film, with no added stabilizers, were subjected to 110 C under pure oxygen (at 1 atm pressure) for 84 h in enclosed stainless steel vials with gold-plated gaskets. Analysis of the headspace gas was performed on a Siemens Quantra Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR/MS) equipped with a 1 Tesla permanent magnet. The sample was introduced into the FT-ICR/MS via a voltage-regulated pulse valve and ions were formed by electron impact ionization. The fundamentals of FT-ICR/MS are described elsewhere [25]. The operating conditions were as follows: filament beam electron energy, 70 eV; filament beam current, 45e650 nA; trap voltage during beam, 1.2e 1.4 V; and trap voltage during excite, 1.0e1.2 V. Each spectrum was collected using 128 co-adds and either 250,000 or 500,000 data points. The typical mass spectral resolution realized was 8300 (m/Dm) and 14,200 (m/Dm) at m/z 44 with 250,000 and 500,000 data points, respectively. The scanned mass range was 12e1000 Da. 3. Mass spectral interpretation The objective of this study depended on a capability to measure quantitatively the relative amounts of 12CO2 versus 13 CO2, and of 12CO versus 13CO, coming from each of the isotopically labeled polypropylene samples following oxidation at elevated temperature. To achieve this, issues of the resolution of the mass spectrometer, the identification of peaks, and the quantification/repeatability of peak heights had to be addressed. The mass spectra obtained after admitting a small portion of the gaseous atmosphere from the vessel containing the oxidized polypropylene also showed peaks corresponding to the constituents of ambient air (N2, O2, Ar, H2O, and others). Carbon dioxide is also present in ambient air, but contamination of our samples is not a problem, because its relative abundance is too low to affect the 13CO2/12CO2 ratio measurements in our samples (for example, argon occurs at 0.93% by volume in air, while carbon dioxide occurs at about 0.033 volume percent). In addition, numerous peaks coming from the fragmentation within the spectrometer of trace organic volatiles produced in the course of the polymer degradation were seen. Several experiments were performed in which the sample container was held at lower temperature (down to 48 C) to ascertain that no significant interference at mass values of 44 or 45 was coming from the decomposition of volatile organic oxidation products, whose vapor pressure would be reduced at the lower temperature (for example, the melting points of acetic and formic acids are 16.5 C and 8.4 C, respectively). Experiments were also performed to confirm that under the mass spectroscopy conditions used in the measurements, carbon monoxide measurements were not significantly affected by fragmentation peaks coming from carbon dioxide. Also seen were numerous species characteristic of the mass spectroscopy technique (such as HCO2, HCO, N2H, etc.) which arise inside the spectrometer by ions picking up an H atom from trace amounts of water adsorbed on the inner walls.
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96
Table 2 Exact atomic mass values for some species of interest
C(1) Labeled PP
Nominal mass
Exact atomic mass
12 CO N2 13 CO 12 CHO HN2 13 CHO O2 Ar 12 CO2 13 CO2 12 CHO2
28 28 29 29 29 30 32 40 44 45 45
27.9949 28.0061 28.9983 29.0027 29.0139 30.0061 31.9898 39.9624 43.9898 44.9932 44.9977
4. C-13 labeling in CO2 Fig. 1a shows a portion of the mass spectrum obtained from analysis of the gaseous components in the vessel containing C(1) labeled polypropylene, following exposure under oxygen
CO2
CO2
Ar 39
40
41
42
43
44
45
46
47
m/z C(2) Labeled PP
Signal Intensity
CO2
13
CO2
Ar 39
40
41
42
43
44
45
46
47
m/z C(1,3) Labeled PP
CO2
Signal Intensity
A number of molecules relevant to this study share the same nominal mass value. For example, carbon monoxide and nitrogen both have a molecular mass of 28, 13CO2 and HCO2 each have a mass of 45, 13CO and HCO both have a mass of 44. All of the aforementioned species are seen (with varying peak height in different experiments), but peaks corresponding to all of these isomass pairs can be separated with baseline resolution by the instrument used in this study, as each member of the isomass pair in fact differs very slightly in atomic mass. Exact masses of some of the atomic and molecular species of interest are listed in Table 2. The mass value of a given species, as measured by the instrument, varied slightly from day to day, but the differences in mass value between peaks of relatively similar mass were quite reproducible. Thus, 12CO occurs at 0.0112 mass units below N2 and 3.9949 mass units below O2. 13CO occurs 1.0034 mass units above 12CO. 13CO2 falls 1.0034 mass units above 12 CO2, whereas 12CHO2 occurs 1.0079 mass units above 12 CO2. Utilizing the known values for molecular mass differences, confident assignments of the various peaks were thus possible by identifying the patterns which gave correct fits to the expected mass differences. Relative peak heights were assumed to be proportional to relative concentration, for isotopic variants of the same molecule (i.e., 13CO versus 12CO and 13CO2 versus 12CO2). For small peaks (particularly for the 12CO and 13CO peaks), the signal to noise ratio could be improved by selectively removing the ions corresponding to the larger peaks from the spectrometer (such as N2 or O2, depending on the particular sample run) by the technique of ion ejection. This technique was used in the measurements of 13CO and 12CO, for which the higher-concentration 13CO2 and 12CO2 ions were removed. Some variability (scatter) in relative peak height measurements was observed, and so data collected from a number of mass spectral runs, conducted during several different days, were averaged to minimize error in the measurement.
13
Signal Intensity
Formula
13
CO2
39
40
41
42
43
44
45
46
47
m/z Fig. 1. Mass spectra obtained from the gas atmosphere above C-13 labeled polypropylene samples, following exposure to 110 C under oxygen for 84 h. The mass region from 39 to 47 is shown. Top: spectrum obtained from the C(1) labeled polypropylene, center: spectrum from C(2) labeled polypropylene, bottom: spectrum from C(1,3) labeled polypropylene.
at 110 C for 3½ days. The 13CO2 and 12CO2 peaks are indicated. From the average ratio of these two peaks, it is found that 66 5% of the carbon dioxide has the C-13 isotopic label, and therefore 66 5% of the CO2 coming from the polypropylene material must originate from the C(1) carbon atom. Fig. 1b shows a mass spectrum for the gaseous components from the C(2) material, following oxidation as for the sample
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above. Based on the average ratio found for the 13CO2 and 12 CO2 peaks, 33 5% of the CO2 contains the C-13 label, indicating that 33 5% of the CO2 coming from the polypropylene material must originate from the C(2) carbon atom. Determination of the C(3) contribution to the total carbon dioxide generated is an inherently less accurate measurement, compared with C(1) and C(2), because the latter involve samples which are specifically labeled in the positions of interest, whereas for C(3), only the C(1,3) sample, which is labeled to the extent of 31%, in conjunction with 68% labeling in the C(1) position, is available. Determination of C(3) from this sample is inherently less sensitive, as it has labeling that is slightly less than 1/3 of the C(1) amount, and this difficulty is increased in the case of the C(3) CO2 being present in much lesser amount than C(1) CO2. The C(3) determination in any event requires a subtraction, with two possibilities available. The first possibility is to calculate C(3) based on the result of the C(1) and C(2) measurements:
97
5. C-13 labeling in CO Quantitation of the CO product was more difficult compared with CO2, primarily because the amount detected was lower by an order of magnitude or more. Fig. 2a shows data for the 12CO/13CO region of the mass spectrum obtained from the oxidized C(1) sample, which was measured under mass spectral conditions in which several of the larger peaks (12CO2, 13CO2, O2, and Ar) had been ejected to maximize signal to noise. The average value of the 13CO/12CO peak ratios yields a determination of the proportion of CO coming from the C(1) position as slightly over 80%, with larger error bars than for the CO2 measurements. For the C(2) samples, the weak CO peaks were difficult to measure, and in particular the 13CO peak was not visible above the baseline in a number of runs. Fig. 2b shows some of the best data for the 12CO/13CO region of the mass spectrum obtained from the oxidized C(2) sample, for which a 13CO peak is visible. Based on averaging several such runs,
Cð3Þ ¼ 100 ½Cð1Þ þ Cð2Þ
N2
C(1) Labeled PP 13
CO
Signal Intensity
Given the fact that the C(1) and C(2) experiments indicate that essentially all of the carbon dioxide can be accounted for as coming from these two positions in the macromolecule (66 þ 33%), it would be concluded that none of the carbon dioxide originates from the C(3) methyl carbon (or an insignificant amount). The second possibility for obtaining the C(3) value is by analyzing the C(1,3) sample, taking into account the result derived from the C(1) experiment, together with the relative amount of C(1) and C(3) labeling in the sample:
CO
fCð1; 3Þ ½Cð1Þ 0:68g 0:31
N2H
Given the previously discussed measurement indicating 66% of the CO2 coming from C(1), and assuming that C(3) contributes w0%, the equation above predicts that the percentage of C-13 label in the CO2 coming from the C(1,3) sample should be 45%. Fig. 1c shows the mass spectrum obtained from the C(1,3) sample, with the 13CO2 and 12CO2 peaks labeled. The average value for the C-13 labeling in this sample was measured as 46 5%, in excellent agreement with the above. The results on the origin of CO2 from the thermal oxidation of polypropylene are summarized in Table 3. The error bars are estimated as 5% absolute, based on the overall scatter in the measurements coming from repeated experiments for which the values were averaged.
28
28.2
28.4
28.6
28.8
N2
C(2) Labeled PP
CO
13
Table 3 Position of origin of CO2 from the thermal oxidation of polypropylene at 110 C Carbon atom within the polymer
% CO2 from this position
C(1) [methylene] C(2) [tertiary] C(3) [methyl group]
66% [5%] 33% [5%] 0% [<5%]
29
m/z
Signal Intensity
Cð3Þ ¼
28
28.2
28.4
28.6
28.8
CHO CO
N2H
29
m/z Fig. 2. Mass spectra obtained from the gas atmosphere above C-13 labeled polypropylene samples, following exposure to 110 C under oxygen for 84 h. The mass region from 29 to 30 is shown. Top: spectrum obtained from the C(1) labeled polypropylene, bottom: spectrum from C(2) labeled polypropylene.
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98
Table 4 Position of origin of CO from the thermal oxidation of polypropylene at 110 C Carbon atom within the polymer
% CO from this position
C(1) [methylene] C(2) [tertiary] C(3) [methyl group]
80% [10%] 5% [10%] Not determined
a measurement of the proportion of CO coming from the C(2) position was somewhat less than 5%. The C(1,3) spectrum suffered from extremely weak signal in the CO region (with CO signals often not clearly distinguishable from the baseline). Given the fact that a determination of C(3) from this sample is inherently prone to large error because of mixed isotopic labeling, with C(3) labeled at only 31%, we did not obtain a satisfactory measurement of the C(3) value. Given the combined C(1) and C(2) values of 85% with large error in the measurement, and given that we did not obtain any direct evidence of a C(3) CO contribution, we can only estimate C(3) as being quite possibly near zero, and very likely less than 15%. The CO results are summarized in Table 4. 6. Discussion of CO2 and CO formation from each chain position in polypropylene 6.1. CO2 and CO from the C(1) methylene carbon Most of the chemistry that takes place in polypropylene undergoing oxidation involves attack by free radicals at the C(2) [tertiary] carbon, and indeed most of the macromolecular oxidation products involve oxidation at the C(2) position [12,24,26]. This is because the tertiary hydrogen atom is much more susceptible to abstraction, compared with the hydrogen atoms at the C(1) and C(3) positions, due to the formation of a more stable tertiary radical. Probably the most important mechanism by which chemistry is initiated at the C(1) [methylene] carbon is in the course of a well known
III CH3 I I II CH2 CH CH2
III CH3 I II CH CH2
III III CH3 CH3 I I II I II CH2 C CH2 CH CH2
OH
I II III CH2 C CH3 + O
.
R
.
III CH3 I II I CH2 C CH2
.
reaction initiated by cleavage of a C(2) peroxide, leading to a mixture of C(2) [tertiary] alcohol and C(2) methyl ketone (Scheme 1). As an indication of the importance of macromolecular oxidation products that can be explained by Scheme 1, it was found in our NMR work [24] on these same isotopically labeled polypropylene samples that following oxidation at 109 C, the amount of these functionalities was as follows: C(2) [tertiary] peroxides w 45%, C(2) [tertiary] alcohols w 20%, and C(2) methyl ketones w 8%, for a total of w73% of all oxidation products found in the polymer. It is from the C(2) methyl ketone route (Scheme 1) that a free radical is simultaneously formed at a C(1) carbon atom, and it is through this ‘‘dangling chain end’’ radical that the formation of large amounts of C(1) CO2 and CO is understandable. This C(1) radical can undergo reaction with oxygen to form a peroxide, as shown in Scheme 2, which will lead to further oxidation products. Because of the structure of this C(1) radical, which is attached to the macromolecule by only one CeC bond, it is understandable that a substantial fraction of the products of further oxidation chemistry of this radical can in fact end up as CO2 and CO. Following thermal decomposition of the C(1) chain-end peroxide, one of the products expected is an aldehyde (Scheme 3). In an environment of ongoing free-radical chemistry, aldehydes are well known to be susceptible to further reaction by hydrogen atom abstraction. Mechanistic studies with small organic molecules indicate that the hydrogen atom on the aldehyde is in fact an order of magnitude more reactive toward abstraction than a tertiary hydrogen atom [27]. Indeed, in our NMR studies of the isotopically labeled polypropylene [26], we found no evidence of any aldehydes (despite finding significant quantities of ketones), which is consistent with high reactivity of the former, leading to other products. Abstraction of the aldehydic hydrogen atom (Scheme 4) gives a carbonyl radical. This radical can readily undergo decarbonylation (i.e., loss of CO), which is a reaction that is well documented from the literature on radical chemistry of small molecules [28]. As one extreme example of decarbonylation, trimethylacetaldehyde is found to decompose to CO and isobutane while sitting at room temperature, which proceeds
III CH3 I II CH CH2
O2
III CH3 I II I CH2 C CH2 OO
.
III CH3 I II CH CH2
R-H
R H
III CH3 I I II CH2 CH CH2
III III CH3 CH3 I I II I II CH2 C CH2 CH CH2
O
.
Scheme 1.
III CH3 I II I CH2 C CH2 OOH
III CH3 I II CH CH2
S.M. Thornberg et al. / Polymer Degradation and Stability 92 (2007) 94e102
.
III CH3 I I II CH2 CH CH2
III CH3 I I II CH2 CH CH2
O2
OO
.
R-H
99
III CH3 I I II CH2 CH CH2 OOH
Scheme 2.
through a free-radical process initiated by trace impurities of peroxides [28]. The carbonyl radical shown in Scheme 4 can also react with oxygen [28] to give a peroxy radical, which can further react to give a peracid. Subsequent cleavage of the peroxide bond will yield a carboxyl radical (Scheme 4), which can undergo hydrogen abstraction to give an acid, or it can undergo loss of CO2. These reactions of carboxyl radical are widely known from small molecule chemistry, and are very analogous to the widely used thermal decomposition of benzoyl peroxide, which gives both phenyl radicals and benzoic acid [28,29]. As a further example from the organic chemistry literature, it is known that the decomposition of acetylperoxide yields the radical CH3CO2 which undergoes loss of CO2 to produce methyl radicals in high yield [28,29]. As discussed above, the majority of the CO2 and CO emanating from polypropylene oxidative degradation likely comes from reaction of C(1) carbon atom in a mechanism which is initiated by a chain cleavage event. It may thus be reasonable to consider the measurement of these outgassing products as potentially effective indicators of the extent of degradation of macroscopic properties in this polymer. 6.2. CO2 and CO from the C(2) tertiary carbon As mentioned earlier, studies have consistently shown that the oxidation of polypropylene involves the formation of oxidation products primarily at the tertiary carbon atom, as seen by analysis of the partially degraded, solid polymer [12]. Our NMR studies of the same isotopically labeled polypropylene samples utilized in this study afforded a measurement indicating that approximately 80e85% of the macromolecular thermooxidative products are formed at the C(2) [tertiary] carbon atom. Given the knowledge about the preponderance of polymer oxidation products occurring at C(2), it might seem at first surprising that CO and CO2 come mainly from the C(1) carbon atom. However, upon reflection on the fact that the C(2) carbon in polypropylene is bonded to three other carbon atoms, and that all three of these CeC bonds must be broken in order to release a molecule of either CO or CO2, it becomes apparent that a rather extensive series of chemical reactions must be
required to generate either of these two degradation products from the C(2) position. From this point of view, it may seem somewhat remarkable that any significant amount of C(2) carbon proceeds all the way to CO or CO2. As noted in Section 6.1, the most abundant oxidation products at the C(2) position following oxidation at 109 C are tertiary peroxides (representing w45% of all of the solid-phase polypropylene oxidation products), and tertiary alcohols (w20%). In neither of these products has any CeC bond breakage occurred. The third largest C(2) oxidation product, representing w8% of total products found, is the methyl ketone, for which one of the three C(2) CeC bonds has been cleaved, which is at least one step in the right direction for formation of CO and CO2, and some amount of these products may arise through further oxidation of the C(2) methyl ketone. This could be envisioned as proceeding through free-radical abstraction of a hydrogen atom from the methyl group attached to the ketone (which is reasonable, since the resultant radical would be relatively favorable due to stabilization by conjugation with the adjacent carbonyl group). Further oxidation at this methyl group, via chemical routes similar to those shown in Schemes 2e4, could result in eventual loss of the methyl carbon as CO, CO2, or other small molecule product such as formaldehyde, leaving a carbonyl radical at the C(2) carbon, which could then undergo reactions analogous to those in Scheme 4, leading to C(2) CO and CO2. The problem with this mechanism is that one might likely expect to see C(3) CO and CO2 coming from the methyl group that is being oxidized away. However, the results found in this paper are not consistent with any significant amount of these products coming from C(3). An interesting aspect of the results for the C(2) carbon atom is the fact that very different (relative) amounts of CO versus CO2 are generated; C(2) may be viewed as contributing at least six or seven times less to the total amount of CO, compared with the C(2) contribution to CO2. One possibility suggested by this observation is a reaction that leads directly to CO2 but not to CO (unlike the sequential reactions in Schemes 2e4, which seem most suited for chain-end or side-chain reactive groups). One possibility that is intriguing in addressing the C(2) CO2 formation mechanism, would be a reaction analogous
III CH3 I I II CH2 CH CH2
III CH3 I I II CH2 CH CH2
OOH
O
.
III CH3 I II I H C CH CH2 O (Aldehyde)
Scheme 3.
S.M. Thornberg et al. / Polymer Degradation and Stability 92 (2007) 94e102
100
III CH3 I II I H C CH CH2
O
.
.
.
R
R H
O
I CO
III CH3 I II CH CH2
.
+
O
O2
III CH3 I II I OO C CH CH2
III CH3 I II I C CH CH2
III CH3 I II I HOO C CH CH2
.
O
III CH3 I II I O C CH CH2
O
I CO2
+
III CH3 I II I HO C CH CH2
R H
O
III CH3 I II CH CH2
.
Scheme 4.
to that discussed by a number of workers [17,30,31] for the formation of acetic acid from a methyl ketone. In this mechanism, a hydroxy radical ( OH) adds to the carbonyl carbon of the methyl ketone, creating an alkoxy radical at the oxygen that was formerly part of the ketone group. Chain cleavage, analogous to Scheme 1, directly gives acetic acid. Experimental evidence for this mechanism has been obtained by Walling and Gibian [30]. It is known that the tertiary peroxy radical of Scheme 1, which is formed in the first oxidation step of a C(2) carbon atom in polypropylene and is therefore the precursor to virtually all oxidation at the C(2) carbon, is a relatively long-lived radical. If, in overall similarity to the above-described mechanism involving hydroxyl radical, this peroxy radical (of Scheme 1) were to add to an existing C(2) methyl ketone (also in Scheme 1), the result would be an alkoxy radical which could then undergo chain cleavage (analogous to Scheme 1). The resultant product would be a C(2) perester, as shown in Scheme 5 (interestingly, our previous NMR study on the C-13 labeled polypropylene samples indeed found
III CH3 I II I CH2 C CH2 OO
.
evidence for what appears to be substantial quantities of C(2) peresters) [24,26]. Cleavage of the peroxide bond in the perester of Scheme 5 (or in a similar perester formed by any other mechanism) would give the radical CH3CO2 , which, as discussed in Section 6.1, is well known [28,29] to decompose into CO2 and methyl radical. The CO2 thus formed would be from the C(2) carbon. It is not known whether the peroxy radical of Scheme 5 would in fact be reactive enough to add to a ketone group; certainly this radical is far less reactive than the hydroxyl radical. At the same time, its long lifetime, and the fact that oxidation of a bulk polymeric material can be expected to consist of many chain reactions that are highly localized on a molecular scale, would give a high probability of the peroxy radical sitting for extended time periods in close proximity to methyl ketone groups formed in the course of the ongoing oxidation. A C(3) methyl radical is formed in this pathway, and while some fraction might undergo reaction with oxygen, much of this highly reactive species would likely terminate by hydrogen abstraction, and its mobility would also facilitate coupling with other radicals.
III CH3 I II I CH2 C CH2
III CH3 II I CH CH2
+
I II III CH2 C CH3
III CH3 I II CH CH2
O O I II III CH2 C CH3
O
.
O
.
II III O C CH3
+
O
III CH3 I II I CH2 C CH2 O
.
III CH3 II I CH CH2
III CH3 I II I CH2 C CH2 O O II III C CH3
II CO2 +
O
.IIICH3 Scheme 5.
III CH3 II I CH CH2
+
I CH2
.
S.M. Thornberg et al. / Polymer Degradation and Stability 92 (2007) 94e102
6.3. CO2, CO, and the C(3) methyl side chain The results obtained in this study find that all of the CO2 evolved from polypropylene undergoing thermal oxidation can be accounted for by contributions from the C(1) and C(2) carbon atoms, and we find no evidence of any appreciable amount of C(3) CO2. It is also found that the preponderance of CO comes from C(1), with a small amount from C(2). The potential contribution of C(3) CO could not be obtained independently, but if it is present, it is at best small compared with the C(1) contribution. The lack of C(3) contribution to the CO2, and little if any to the CO, may seem, on first thought, somewhat surprising, given that the C(3) exists as pendant side groups all along the polypropylene. As such it is bonded to the macromolecule by only one CeC bond, so that oxidation at this site, once initiated, might be expected to proceed with relatively high yield to CO2 and CO, through reactions analogous to those in Schemes 2e4. The findings for C(3) can be understood as reflecting the low reactivity of the methyl site. As is well known, the ordering of reactivity of hydrogen atoms to free-radical abstraction is tertiary > secondary > primary. The primary hydrogens of the methyl group are thus the least reactive within the polypropylene structure. Moreover, to the extent that some small quantity of radicals may be formed by abstraction at C(3), they have the opportunity within the bulk matrix to abstract a tertiary hydrogen atom from a C(2) site on an adjacent chain, thus converting the primary radical to a tertiary radical, before O2 diffuses to the site. Intrachain H-atom abstraction (i.e., ‘‘hydrogen hopping’’) has been demonstrated to occur rather efficiently within hydrocarbons in the solid phase [32]. One other potential mechanism for activation of C(3) that should be mentioned here is the ejection of a C(3) methyl rad ical, CH3 , as an alternative path to the chain cleavage reaction coming from the C(2) alkoxy radical of Scheme 1. Actually, the formation of a CH3 radical could be expected to be less favored compared with the chain cleavage reaction, because the latter generates a primary radical, which is known to be significantly more stable compared with CH3 . Nevertheless, this reaction forming CH3 has been mentioned many times in earlier publications about polypropylene oxidation. Another possible mechanism for formation of C(3) CH3 was described in a preceding section of this article. If significant quantities of C(3) CH3 are indeed being formed, the products from this radical apparently must come largely through hydrogen abstraction and/or radicaleradical coupling. Previous studies have reported on the lack of participation of the C(3) methyl in the formation of macromolecular oxidation products [8]. Indeed, the NMR studies conducted previously [24,26] on these same C-13 labeled polypropylene materials found no measurable amount of solid-phase oxidation products at the C(3) methyl carbon. If C(3) oxidation products were to be found, one could have imagined that their most likely occurrence might be among single-carbon-atom volatile compounds, given the position of C(3) as a methyl side chain in the polymer. The lack of a significant contribution of C(3) to the CO2 and CO is a further indication of the
101
relative inertness of the C(3) methyl group in an environment of thermal oxidation. 7. Summary and conclusions Samples of polypropylene having carbon-13 isotopic labeling in each of the three positions along the chain were subjected to elevated temperature (110 C) in the presence of oxygen in enclosed vials, after which the gaseous atmosphere over the samples was analyzed by mass spectroscopy. This technique has provided the first ever determination of the positions of origin of the oxidation products CO and CO2, from within a macromolecular framework. The majority of both these gasses is found to originate from the C(1) [methylene] carbon atom in the polypropylene backbone (w66% of the CO2, and >80% of the CO). This result can be understood in terms of chemical reactions initiated by the thermal decomposition of a C(2) [tertiary] peroxide to give a C(2) alkoxy radical, followed by chain scission to yield a C(2) methyl ketone and a C(1) ‘dangling chain end’ radical, in which the C(1) is attached to the polymer chain by only one CeC bond. Reaction of this C(1) radical with oxygen gives a peroxide that can lead to both C(1) aldehyde and peracid, which subsequently undergo loss of CO and of CO2, respectively, through further free-radical mediated reactions that are well known from small-molecule organic chemistry. The C(2) [tertiary] carbon atom of the polypropylene backbone contributes w33% of the CO2, and 5% of the CO, despite the fact that the C(2) position is well known to be the formation site of most of the oxidation compounds that occur when polypropylene undergoes degradation. (For example, in the macromolecular oxidation products in these same isotopically labeled polypropylene samples, as measured by NMR, w80e85% of all the degradation products are formed at the C(2) position). The fact that the C(2) contribution to the CO and CO2 products seems much less favored in comparison to the macromolecular oxidation products, can be understood in light of the structure of the C(2) carbon, which is connected to three other carbon atoms in the polypropylene; formation of either CO2 or CO requires cleavage of all three of these bonds. The fact that the relative contribution of C(2) to CO appears to be much smaller, compared with the C(2) contribution to CO2, may indicate a different mechanism for the C(2) CO2 production, possibly involving a perester intermediate. Despite the fact that the C(3) [methyl side chain] might at first seem an obvious candidate as a site for origin of CO and CO2, due to it being connected to the macromolecule by only one CeC bond, there is no indication of any measurable contribution of C(3) to the CO2, and any contribution to the CO is small at best. These observations reinforce the degree to which the C(3) methyl carbon is comparatively inert, with regards to participation in the free-radical chemistry underlying polypropylene oxidation. Acknowledgments Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the U.S. Department
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