Groupon, the Chicago-based coupons-deal phenomenon, has raised $500 million of a planned $950-million investment — and is negotiating financial commitments from major institutional investors such as Fidelity Investments, T. Rowe Price and Morgan Stanley, as Groupon prepares to go public and remain independent, according to published reports. The move comes three weeks after Groupon reportedly rejected a bid as high as $6 billion from Google.

    T. Rowe Price has previously invested in Twitter, and both Fidelity and T. Rowe led a $50 million investment two years ago in Slide.com, the largest developer of third-party applications for Facebook. Google acquired Slide Inc. in August.

    Groupon’s existing shareholders have the chance to cash out big. That’s because the company plans to use up to $344.5 million of the proceeds of its fundraising campaign to buy back shares from its shareholders, including founder and CEO Andrew Mason; startup investors and serial Chicago entrepreneurs Brad Keywell and Eric Lefkofsky; Jason Fried, co-founder and president of West Town-based Web application developer 37signals, and Kevin Efrusy, a partner at venture capital fund Accel Partners, according to published reports.

    Lou Kerner, social media analyst at Wedbush Securities, said Thursday that Groupon must “run extraordinarily fast to innovate and marry the sales engine that the company has put in place with great technology,” especially since the existing business model is already being copied by so many potential rivals.

   “This kind of social commerce is so embryonic, the only thing we know for sure is that it will change dramatically in coming years. Groupon will need to evolve or adapt, or it will be surpassed,” Kerner said.

    Groupon’s previous funding — $135 million — came from Mail.ru Group, also known as Digital Sky Technologies, a Russian Internet investment firm that also holds a stake in Facebook.