End-Functionalized Poly(bisphenol A carbonate) Oligomers Part III

Downloaded by UNIV OF MARYLAND BALTIMORE COUNTY on February 2, 2015 | http://pubs.acs.org Publication Date: March 8, 2005 | doi: 10.1021/bk-2005-0898.ch006

Chapter 6

End-Functionalized Poly(hisphenol A carbonate) Oligomers Part III: Characterization via HPLC and Mass Spectrometry Michael R. K o r n Department of Chemistry and Biochemistry, Texas State University, San Marcos, T X 78666

The analysis of several end-functionalized P C oligomers by H P L C and various mass spectrometrical methods, including ESI, A P C I , and M A L D I will be discussed and their results exemplified for cyanobiphenyl-terminated P C oligomers compared.

Introduction This section describes the results obtained from characterizing endfunctionalized P C oligomers by analytical methods that identify individual species in contrast to analytical methods that determine overall averaged properties (see previous chapter). Several end-functionalized P C oligomers and chemical structures were introduced in part I and II (chapters 4 & 5), and for consistency sake, the numbers assigned there will be used here as well. The analytical data reported in this chapter are taken from the literature but also include some so far unpublished data (oligomers 20-22, and in particular 22b). Oligomers 22b were analyzed by all four methods, H P L C , ESI, APCI, and M A L D I as well as by G P C and N M R and thus allow a comparison of those results. 70

© 2005 American Chemical Society

In Advances in Polycarbonates; Brunelle, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.

71


High Performance Liquid Chromatography (HPLC) H P L C separates molecules by size and polarity, and thus is a very useful tool in shedding light on the molecular composition of an oligomeric sample. As H P L C does not only separate by size but also by polarity, an unambiguous assignment of the individual (envelope of) peaks is not always straight forward and can be facilitated by using individual authentic molecules. When coupled with a U V detector, oligomers with different end groups can be discriminated by certain ratios obtained from the intensities of H P L C traces recorded at different wavelengths. It must be noted though, that intensities not only depend on the amount of oligomers present but also on their individual extinction coefficient, and therefore quantitative conclusions can be made only after calibration. Applying the above principle, Mork and Priddy used H P L C traces recorded at 264 nm and 288 nm to determine the average amount of oligomers terminated by O H vs. phenyl groups (/). Brunette et al. distinguished linear O H terminated oligomers from cyclics by synthesizing authentic linear and cyclic oligomers and recording their H P L C traces at 254 nm and 285 nm. It could be demonstrated that the ratio 254/288 was less than 2.5 for linear oligomers 3 and about 50 for cyclics (2,3). Marks and Sekinger also applied H P L C for the analysis of oligomers 13 for purity purposes and to calculate M (4). n

The analysis of 22a and 22b by H P L C at a wavelength of 300 nm is shown in Figure 1. A cyano-terminated silica gel column was used for separation with the compounds being eluted from THF/hexanes (30/70v/v) with a 0.5%/min. gradient in T H F to 45/55v/v THF/hexanes. The starting diarylcarbonate 25 and the first member of the oligomeric envelope 25' were eluted also to facilitate peak assignments.



72

Figure 1. HPLC chromatogram at 300 nm on a cyano-modified column for 22a (A) and 22b (B); traces for 25 and 25 are included (Fig. IA is adaptedfrom reference 9. Copyright 1998 American Chemical Society.) 9

In both chromatograms, peaks are not fully resolved and both show two distinct envelopes, of which the more pronounced envelope belongs to the oligomeric series 22 ( M W = n*254 (BPA repeating unit) + 416 (endgroups + C 0 = 25). Following peak assignment based on 25 and 25', the maximum peak for sample 22a is the tetramer (n=4; M W = 1432 g/mol), and for oligomers 22b the trimer (n=3; M W = 1178 g/mol). Oligomers 22b were also eluted on an amino-bonded column from CH Cl /hexanes, which gave a slightly different chromatogram (see Fig. 4 below). 3

2

2


73


Mass Spectrometry Mass spectrometry (MS) of polymers/oligomers has advanced greatly over the past decade and allows for the identification of individual oligomers. Especially matrix-assisted laser desorption/ionization (MALDI) coupled with a time-of-flight (TOF) analyzer and electrospray ionization (ESI) with various analyzers have become prominent over field ionization/desorption (FI/FD) mass spectrometry. The working range for F D - M S cuts off at about 10k Da (5), whereas M A L D I can detect masses of up to 1 M i o . Da. Sometimes, H P L C or gas chromatography (GC) is coupled with M S to preseparate oligomers and simplify the resulting spectrum; thus, Kim et al. (6) used G C - M S in conjunction with H P L C to identify and distinguish among oligomers 1-3. Brunelle et al. (3) employed F D - M S and thus confirmed dimer, trimer and tetramer of 3. For the analysis of cyanobiphenyl-terminated oligomers 22b three techniques will be compared: (ESI), atmospheric pressure chemical ionization (APCI), and (MALDI). In all cases, only molecules that will volatilize and be converted into charged molecules can be detected. In the ideal case, the energy applied to volatilize the molecule would not induce fragmentation unless desired for further experiments (e.g. for M S ) . A l l sets of oligomers contain multiples η (η = 0, 1, 2,...) of the B P A repeating unit and thus are designated as monomer (n = 1; equals structure 25'), dimer (n = 2), trimer (n = 3), etc. The B P A carbonate repeating unit has an exact mass of 254.09 amu, and two cyanobiphenyl endgroups plus one carbonate group has an exact mass of 416.12 amu. Thus, the exact masses for oligomers 22 are 416.1 + n*254.1, namely 670.2 (monomer), 924.3 (dimer), 1178.4 (trimer), 1422.5 (tetramer) etc. Sets of peaks belonging to the same oligomeric envelope should be 254.09 amu apart from each other (provided they are singly charged). 2

Electrospra} -Ionization (ESI) Figure 2 shows the ESI spectrum for 22b. The sample was electrosprayed in the presence of sodium cations ( M W = 23 g/mol) from a methanol/CH Cl solution and analyzed by an ion trap. Peak assignments and calculated (exact) masses are listed in Table I. The major peak occurs at m/z = 1201.5 and belongs to the set of peaks present as 693.6, 947.5, 1201.5, 1455.5, 1710.5, 1964.5. Subtracting the value for the sodium cation (23 amu), this set represents the values for oligomers 22, with its maximum peak at 1201.5 being the trimer. 2


2

74 The second set of peaks at m/z = 1080.3, 1334.2, 1588.2, 1842.3 matches with oligomers of structure 33 (+ Na ). 33 can stem from the interaction of the catalyst (potassium terf.-butoxide) when depolymerizing P C with 25 to obtain oligomers 22. A third set that can be identified matches B P A cyclics + N a at m/z = n* 254 +23. Many other small peaks are visible also. +


+

100 95 Î 90 I 85

™15

Î

80 I

m/z

Figure 2. ESI spectrum for 22b.

Table 1. Calculated and observed M W for 22b, 33, and cyclics by E S I Monomer Dimer Trimer Tetramer Ρentamer Hexamer 1,940.7 1,686.6 22 670.2 924.3 1,178.4 1,432.5 1,963.7 1,709.6 22+Na 693.2 1,201.4 1,455.5 947.3 1,964.5 Observed 693.6 1,710.5 947.5 1,201.5 1,455.5 +

33 33+Na Observed +

cyclics :yclics+Na Observed

+

549.2 572.2 -

803.3 826.3 -

1,057.4 1,080.4 1,080.3

1,311.5 1,334.5 1,334.2

1,565.6 1,588.6 1,588.2

1,819.7 1,842.7 1,842.3

n/a n/a n/a

508.2 531.2 -

762.3 785.3 785.6

1,016.4 1,039.4 1039.5

1,270.5 1,293.4 Yes

1,524.5 1,547.5 yes

33


75


Atmospheric Pressure Chemical Ionization (APCI) The same oligomers 22b were subjected to A P C I analysis. This method is energetically much harsher in its conditions as the temperature of the nozzle operates at about 400 °C. Figure 3 shows the corresponding spectrum and Table 2 lists observed and assigned peaks (as mono-protonated species). Five different sets of peaks are visible and are assigned to the following structures: 1) . oligomers 22 (major peaks); 2) . fragmented oligomers 34 (second prominent set of peaks); 3) . cyclics; 4) . fragmented oligomers 35; and 5) . fragmented oligomers 36.

34



76

Table 2. Calculated M W of oligomers 22,34, cyclics, 35, and 36 and observed values by APCI (all mono-protonated) Resid.

0-mer

+

22+H Obsv.

34+ΙΓ Obsv. Cyc+H Obsv,

3

+

+

35+H Obsv.

356.1 356.1

196. l 193.1

b

450.2 447.1

C

1mer 671.2 -

2mer 925.3 925.1

3-mer

4-mer

5-mer

6-mer

1,179.4 1,179.0

1,433.5 1,432.9

1,687.6 1,687.9

1,941.7 1,942.0

610.2 610.2

864.3 864.1

1,118.4 1,118.1

n/a n/a

509.2 -

763.3 762.0

1,017.4 1,017.1

1,271.5 1,271.1

1,525.5 1,525.1

704.3 702.9

+

36+H 688.3 942.4 1,196.4 Obsv. 942.1 1,196.1 a: 34 with n=0; b: residue is 4-hydroxy-4'-cyanobiphenyl; c: 35 with η = 0. In both cases, ESI and A P C I , oligomers 22 represent the major constituents, though the trimer is predominant in ESI whereas the dimer is in APCI. Cyclics are detected by both methods. Oligomers 33 which contain the thermally labile teri.-butoxy group are visible in the soft ionization method (ESI) but not observed by APCI. Also, several fragmented oligomers 34, 35 and 36 are visible. The fifth set of peaks observed in Figure 3 at m/z = 942.1 and 1196.1 is 17 amu higher than the peak observed for the corresponding dimer and trimer of 22. Support for 36 being the correct assignment is based on M A L D I analysis (see below).


77

LC/APCI-MS Oligomers 22b were subjected to H P L C separation coupled with APCI. Using a different column and different eluents than reported above (HPLC section), 22b was eluted on an aminopropyl bonded column from dichloromethane/hexanes (10/90 v/v) linearly increasing to 80% C H C 1 over 23 min. A U V detector (254 nm) was employed for H P L C detection. Figure 4 shows the obtained chromatogram and detected masses by APCI. Downloaded by UNIV OF MARYLAND BALTIMORE COUNTY on February 2, 2015 | http://pubs.acs.org Publication Date: March 8, 2005 | doi: 10.1021/bk-2005-0898.ch006

2

2

Figure 4. Chromatogram of 22b at 254 nm (eluted on an aminopropyl modified column) with peaks characterized by APCI (x-axis is elution time, min) The chromatogram shows a set of prominent peaks between 17 and 19 min corresponding to oligomers 22. The trimer of 22 being the most prominent one, as seen also in the H P L C chromatogram performed on a cyano-silica column (Fig. 1). In addition, the 17-19 min. region shows the presence of another compound or set of compounds. At earlier elution times, two peaks appear at 1017.3 and 1271.2 amu which were correlated to cyclics. Because cyclics lack the cyano end group their interactions with the column is reduced and earlier elution times are expected. Compounds eluted between 16.95 min and 18.21 min were selectively analyzed by APCI and gave the chromatogram below (Figure 5). Four distinct


78 sets of oligomers could be identified, and their observed values are listed in Table 3.

95Î


OK

! 6S\ 73 6ÙÎ § 55Î

>

1179.2

864.3

M150.1

50Ê 45j 1-611.3 354

3

End-Functionalized Poly(bisphenol A carbonate) Oligomers Part III

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