The phrase "a series of impromptu" is correct and usable in written English.
It can be used to describe a sequence of spontaneous or unplanned events or performances.
Example: "The event featured a series of impromptu speeches from various attendees, showcasing their thoughts and ideas on the topic."
Alternatives: "a sequence of spontaneous" or "a collection of unplanned".
I grew up in Astoria, and World Cups were always a series of impromptu sidewalk parades and car-honking caravans.
Soon she was holding a series of impromptu whispered discussions, speaking quietly to the economy minister and vice chancellor, Philipp Rösler, and Mr. Schäuble, her powerful finance minister.
En route, the plan is also for a series of impromptu gigs in city squares and workplaces, for pop-up workshops and more.
But in a series of impromptu interviews, Manhattan defense lawyers and judges contended that the lawyering on both sides had at times been mediocre.
Hodler, who died in 2006, wrote his own epitaph in a series of impromptu briefings in which he confirmed suspicions of a culture of bribery and collusion in the Olympic movement.
Beneath the chandeliers in the Los Angeles ballroom, and to strains of patriotic music, they held a series of impromptu press briefings, explaining their plans for a Reagan administration and forecasting the scale of victory.
The candidate's usually buttoned-up and bullet-point-laden stump speech here on Thursday afternoon was punctuated by a series of playful, impromptu moments as Mr. Romney reacted to a driving rain, contemplated someday moving to Florida and stopped his speech to retrieve a campaign check.
The spontaneity lives on: in the past month, the closet-size stage has hosted a comically varied series of impromptu performances, from a sing-along with Paul McCartney to a dance routine by the cast of "Cats".
I try a series of uncertain steps down this impromptu catwalk.
After a series of direct actions – storming council offices, holding impromptu parties on the premises of their housing association – all 29 managed to stay in Newham.
On every floor, there are glass-enclosed conference rooms, couches for impromptu meetings, and even a series of small, glass-walled rooms for private one-on-ones — but you are always visible.
