July 28 (Reuters) - Robinhood Markets Inc, the owner of the trading app which emerged as the go-to destination for retail investors speculating on this year's "meme' stock trading frenzy, raised $2.1 billion in its initial public offering on Wednesday.

Some investors stayed on the sidelines, citing concerns over the frothy valuation, the risk of regulators cracking down on Robinhood's business, and even lingering anger with the company's imposition of trading curbs when the meme stock trading frenzy flared up at the end of January.

In an unusual move, Robinhood had said it would reserve between 20% and 35% of its shares for its users.

Its simple interface made it popular with young investors trading from home during the COVID-19 pandemic.

regulators looking into whether its employees traded shares of GameStop and AMC Entertainment Holdings, Inc (AMC.N) before the trading curbs were placed at the end of January.

In June, Robinhood agreed to pay nearly $70 million to settle an investigation by Wall Street's own regulator, the Financial Industry Regulatory Authority, for "systemic" failures, including systems outages, providing "false or misleading" information, and weak options trading controls.