RAPID COMMUNICATIONS IN MASS SPECTROMETRY

Rapid Commun. Mass Spectrom. 2004; 18: 3075–3078

Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/rcm.1715

Tandem (quadrupole-time-of-flight) electrospray mass

spectrometry of oligo(vinyl acetates) with isopropylol

or (1-hydroxyethyl)-2-oxyisopropanyl end groups

Steven Collins and Stephen Rimmer*The Polymer and Biomaterials Chemistry Laboratories, Dept of Chemistry, Polymer Centre, University of Sheffield, Dainton Building,

Brook Hill, Sheffield, S3 7HF, UK

Received 11 August 2004; Revised 8 October 2004; Accepted 8 October 2004

Poly(vinyl acetates) with either isopropylol or (1-hydroxyethyl)-2-oxyisopropanyl end groups were

analysed by tandem mass spectrometry using a quadrupole-time-of-flight (Q-TOF) instrument.

Random scission along the polymer backbone was not observed. Instead the initial scissions

were derived from the cleavage of end-group species. The resultant macrocations were subject to

further elimination reactions that eventually produced polyene macrocations. A smaller fraction of

the initial macrocations also reacted by loss of ketene and this reaction produced macrocations con-

taining vinyl alcohol units. Copyright # 2004 John Wiley & Sons, Ltd.

The mass spectrometry (MS) of synthetic polymers, using soft

ionisation techniques, is now well established and matrix-

assisted laser desorption/ionisation (MALDI) and electro-

spray ionisation (ESI) sources are both now in routine use.

Tandem MS (MS/MS) is much less used. Nonetheless, the lat-

ter method can be very useful in the identification of polymer

end groups.1–3 In general, fragmentation occurs along the

polymer backbone and results in fragments that are the ran-

dom scission products of degradation. For example, direct

cleavage of a poly(methyl methacrylate-co-butyl methacry-

late) by MALDI-collision-induced dissociation (CID) was

used to prove block copolymer architecture2 and cyclic and

linear polyglycols were both studied by fast-atom bombard-

ment tandem mass spectrometry (FAB-MS/MS).4–6 Simi-

larly, ESI with CID in an ion trap was used to provide

backbone fragmentation data on polyglycols.7 ESI has also

been used to provide MS/MS data on polyesters.3,8–11 How-

ever, not all polymers produce MS/MS spectra via backbone

fragmentation; poly(vinyl acetate) (PVA) in particular frag-

ments by loss of acetic acid to yield a polyene.12 In this latter

report, Giguere and Mayer studied PVAs with end groups

derived from the initiator, azobisisobutyronitrile (AIBN),

and showed that the first process was elimination of acetic

acid. These reactions were then followed by loss of the end

groups derived from the initiator. In this work we report

the observation of similar behaviour in PVAs with either iso-

propylol or (1-hydroxyethyl)-2-oxyisopropanyl end groups.

However, the predominant primary fragmentation is scis-

sion at the 2-propyl carbon. Fragmentations are also observed

that allow us to precisely distinguish the two end groups.

EXPERIMENTAL

MaterialsVinyl acetate (VA) was purified by passage through an

inhibitor removal column (Aldrich), followed by fractional

distillation. Isopropanol (Aldrich) was refluxed over

calcium hydride before being distilled. 2-Isopropoxyethanol

(Aldrich) was used as received. AIBN (Aldrich) was recrys-

tallised twice from diethyl ether.

SynthesisAIBN (0.9 g) was added to isopropanol (950 mL). When the

AIBN had dissolved, VA (71.5 mL) was added. The solution

was heated under nitrogen at 608C for 6 h. A polymerisation

of VA (3.9 mL) in 2-isopropoxyethanol (56.1 mL) was also car-

ried out using AIBN (50 mg) as the initiator. Size-exclusion

chromatography, in tetrahydrofuran (THF) using a polystyr-

ene calibration, gave number average molecular weights of

both products of 1600 g mol�1. The products gave 1H NMR

spectra that were identical to conventional high molecular

weight PVA except that resonances associated with the end

groups were seen, as indicated below.

1H NMR

End groups on polymer prepared in: (a) isopropanol CH3–

(isopropyl end groups) d& 1.1 (broad), –CH2– (of ultimate

VA repeat unit-o-chain end) d& 3.4–3.7 ppm; (b) 2,2-

isoproxyethanol CH3– (isopropyl end groups) d& 1.1

(broad), –CH2– (of ultimate VA repeat unit -o-chain end)

and HOCH2*CH2

* – (a-chain end) give a complex multiplet

d& 3.1–3.8 ppm.

Direct infusion electrosprayDirect infusion electrospray was performed on a Micromass

Ultima Global Q-TOF. Argon was used as the collision gas.

Copyright # 2004 John Wiley & Sons, Ltd.

*Correspondence to: S. Rimmer, The Polymer and BiomaterialsChemistry Laboratories, Department of Chemistry, PolymerCentre, University of Sheffield, Dainton Building, Brook Hill,Sheffield S3 7HF, UK.E-mail: S.Rimmer@Sheffield.ac.uk

The samples were dissolved in THF at a concentration of

2 mg mL�1. Ammonium acetate at a concentration of

10 mmol dm�3 was then added. In the first stage source the

capillary was set at 3 kV; the cone was set at 100 V; the RF

lens energy was 130.2; the source temperature was 1008C;

the desolvation temperature was 1508C; and the gas flow

was 300 L h�1. The collision energy was set at 20 eV and the

cone voltage in the second stage source was set at 35 kV.

RESULTS

We have previously used ESI-MS to analyse PVAs prepared

in the presence of isopropanol.13 Also, Giguere and Mayer

recently reported the utility of MS/MS in the analysis of

low molecular weight PVA.12 They reported that CID pro-

duced cleavage of successive acetate groups. Prompted by

this work we have therefore employed quadrupole-time-of-

flight (Q-TOF) MS/MS to materials produced with reactive

end groups derived from transfer to solvent (isopropanol or

isopropoxyethanol) and here we report significant differ-

ences in the fragmentations observed during MS/MS

between these species and the previously studied PVAs.

Figure 1 shows the results of the CID experiment carried

out on the precursor ion atm/z 852, selected following ESI of a

PVA prepared using isopropanol as solvent. The ion can be

assigned to a nonomer of vinyl acetate (9� 86 Da) with

isopropylol (60 Da) at the a-chain end and H at the o-chain

end and with ionisation by the ammonium cation. The first

major product ion we observe is at m/z 817, which is

indicative of the isopropylol end group in that it can be

rationalised to the process shown in Scheme 1. According to

Giguere and Mayer, PVA without these isopropylol end

groups initially undergoes sequential deacetylation followed

by loss of a cyanopropyl radical (from AIBN). However, the

data reported here indicate that in these PVAs loss of the

hydroxyl end group (probably as water) occurs first and only

after this cleavage has occurred is a series of ions derived

from sequential deactylation observed; these are indicated in

Fig. 1 by bold annotation of the m/z values. Deactylation

occurs, as shown in Scheme 2, by elimination of acetic acid to

give polyene cations. Ions derived from each deactylation can

be observed up to the maximum of 9 VA units. Another series

of ions can also be identified from Fig. 1. These ions are

indicated by annotation with the symbol, {. They differ from

their high mass neighbours by 42 mass units and can be

assigned to the loss of acetyl in the form of a ketene molecule,

as shown in Scheme 3. Similar results are observed in the

spectra of the PVAs produced in the presence of 2-

isopropoxyethanol (Fig. 2). In this example, we fragmented

the precursor ion atm/z 896, which corresponds to nine repeat

units of VA with (1-hydroxyethyl)-2-oxyisopropanyl at the a-

chain end and H at the o-chain end. As described above for

the PVAs with isopropyl end groups, the product ion at

Figure 1. CIDmass spectrum from fragmentation of the precursor ionm/z 852 derived from

a PVA prepared in isopropanol.

Scheme 1. Cleavage of the hydroxyl group at the chain end

of PVA prepared in isopropanol.

Scheme 2. Elimination of acetyl to produce macrocation

polyenes.

Copyright # 2004 John Wiley & Sons, Ltd. Rapid Commun. Mass Spectrom. 2004; 18: 3075–3078

3076 S. Collins and S. Rimmer

m/z 817 is due to fragmentation a to the isopropyl group and,

in a process analogous to that shown in Scheme 2, the

hydroxyethoxyl group is lost to leave the same macrocation

as shown in Scheme 2. The processes shown in Scheme 3 then

proceeded to yield the polyenes that are assigned to the ions

with the m/z values that are annotated in bold in Fig. 2.

However, an ion is also observed atm/z 861. This corresponds

to loss of the terminal hydroxyl group, as shown in Scheme 4.

As in the previous example, the loss of successive ketene

molecules to yield vinyl alcohol repeat units is also observed.

This process generates an ion series that is identical to that

observed in the previous example, and in Fig. 2 it is again

identified with the symbol, {. In contrast to the previous

example, there is also another series of ions (denoted by the

symbol {), in which m/z values of the ions increase

incrementally by 60 mass units. The repetition of the mass

loss of 60 Da, as in the previous series, results from successive

losses of acetic acid. The series begins with an ion at m/z 731

and the only reasonable rationalisation for this ion is cleavage

at the o-chain end of a vinyl acetate unit followed by loss at

the a-chain end of the hydroxyethoxy group. The process is

shown in Scheme 5.

SUMMARY

Comparison of the fragmentation patterns reported here and

those reported by Giguere and Mayer shows a strong effect of

the end-group structure on the progress of the CID. In the

Figure 2. CID mass spectrum from fragmentation of the precursor ionm/z 896 derived from a PVA

prepared in 2-isopropoxyethanol.

Scheme 3. Loss of a ketene from a PVA macrocation.

Scheme 4. Cleavage of the hydroxyl group at the chain end

of PVA prepared in 2-isopropoxyethanol.

Scheme 5. Elimination of the ultimate VA group followed by

loss of the hydroxyethyloxy group; from the a- and o-chainends of a PVA prepared in 2-isopropoxyethanol.

ESI-MS/MS study of oligo(vinyl acetates) 3077

Copyright # 2004 John Wiley & Sons, Ltd. Rapid Commun. Mass Spectrom. 2004; 18: 3075–3078

PVAs with isopropylol end groups the main event is cleavage

at the isopropyl carbon to yield an oligomer with intact VA

units and an isopropyl cation at the a-chain end. This cation

then loses acetic acid to yield a polyene cation. On the other

hand, the PVA with 2-cyanopropyl end groups, studied by

Giguere and Mayer, loses acetic acid initially and this is fol-

lowed by the loss of the end group. The presence of the oligo-

mer with intact VA units and an isopropyl cation at the a-

chain implies the loss of the hydroxyl group (probably as

water following protonation of OH, as shown in Scheme 1).

This process is driven by the relative stabilities of the isopro-

pyl cation and water. Thus, whilst fragmentation of PVA with

a (1-hydroxyethyl)-2-oxyisopropanyl end group also pro-

gresses by loss of hydroxyl, the dominant scission process

is fragmentation at the isopropyl ether and this material

also undergoes main chain fragmentation of the ultimate

VA unit.

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Copyright # 2004 John Wiley & Sons, Ltd. Rapid Commun. Mass Spectrom. 2004; 18: 3075–3078