what's the mechanism of formation of succinimide ester?

I am forming succinimide ester:


[![enter image description here](https://i.stack.imgur.com/vyCfK.png)](https://i.stack.imgur.com/vyCfK.png)


Below is the experimental protocol I have adapted from ([*ACS Chem. Biol.* **2012,** *7*(2), 316–321](https://pubs.acs.org/doi/10.1021/cb200374e)) pg 7 in supporting information:



> 
> ### Formation of Activated Ester
> 
> 
> **N-Succinimidyl Trifluoroacetate** (15.0 mmol, 5 equiv) was placed in a flask. The flask was
> backfilled with Nitrogen, capped with a rubber septum, and maintained under an argon atmosphere. To the flask were
> added THF (15 mL, 0.2 M) and **2,2-dimethoxyacetic acid** (3.00 mmol, 1 equiv). Pyridine (1.69 mL, 21.0 mmol, 7 equiv) was added by syringe dropwise (over 3
> min, slightly exothermic). The mixture was stirred in the dark (3 h, producing a homogeneous solution), quenched with water (5 mL, 5 min), and diluted with DCM (45 mL) and water (50 mL). The organic phase was isolated, the aqueous phase was further extracted with a mixture of DCM and THF (3:1, 25 mL), and the combined organic phases were washed with sodium bisulfate (0.1 M aqueous, 40 mL) and dried with sodium sulfate. Volatiles were removed under reduced pressure
> 
> 
> 


1. Could somebody provide the mechanism of this reaction? My guess is that pyridine abstracts hydrogen in the carboxylic acid, and the lone pair on oxygen attacks the carbonyl centre in N-Succinimidyl Trifluoroacetate.. Is this correct? I could not rationalise why 5 equiv. would be needed.
2. The reaction did not quite work out and I think it may be attributed to the fact that I did not have pure N-Succinimidyl Trifluoroacetate to begin with. When forming N-Succinimidyl Trifluoroacetate, I saw two peaks in 19F NMR at $\delta \: \pu{-72.14 ppm}$ and $\delta \: \pu{-75.62 ppm}$. I believe a peak at $\delta \: \pu{-75.62 ppm}$ was unreacted Trifluoroacetic anhydride. The 1H NMR showed that there was some unreacted N-OH succinimide. I want to figure out what kinds of side products these unreacted materials have led, but to do that, I believe I must understand the mechanism of this reaction first.


Would appreciate any help or guidance


