Tonight at 11:59 pm ET, passive index funds will be forced to buy shares of Solstice Advanced Materials and SiTime, creating immediate structural demand for those mid-cap stocks; The moves follow the March 2026 quarterly rebalancing that left vacancies following improvements tied to the S&P 500 and S&P 100.
Immediate buying pressure for Solstice Advanced Materials and SiTime
The S&P MidCap 400, which has more than $100 billion in assets behind it, added SiTime (SITM) and Solstice Advanced Materials (SOLS) tonight, triggering automatic buying by funds that track the index. Solstice has a market cap of $11. 4 billion after gaining 49% over the past year, but shares fell 5.93% today and were little changed after a tough week for AI supply chain names.
S&p 500 Upgrades by Lumentum and Coherent Opened MidCap Vacancies
Lumentum (LITE) and Coherent (COHR) moved into the S&P 500, creating the two vacancies in the S&P MidCap 400 that Solstice and SiTime filled. That chain (S&P 500 and S&P 100 upgrades leading to MidCap replacements) is the mechanical reason why passive funds will rebalance without deliberation, purchasing new inclusions proportional to market cap.
March 2026 Quarterly Rebalancing and Company-Level Details That Matter to Investors
The additions were announced as part of the March 2026 quarterly rebalancing in S&P Global’s official announcement. SiTime was trading at $327. 35 at today’s close and has a one-year return of +83. 82%; The stock fell 8% today and 15.4% over the past week. SiTime reported fourth-quarter revenue of $113. 28 million and non-GAAP EPS of $1. 53, and CEO Rajesh Vashist highlighted broad AI-driven growth for communications, enterprise and data center businesses.
Still, that company data goes hand in hand with a structural fact of the market: Passive funds that track the S&P MidCap 400, including funds that collectively manage more than $100 billion linked to the index, automatically buy inclusions. For a company that is already under short-term selling pressure, that forced buying can materially impact near-term stock movement.
That mechanical purchase also connects to the main passive vehicles that follow the MidCap benchmark; The index is the benchmark for large ETFs that manage substantial assets and will execute purchases in line with the new index weights.
More details are in the official announcement from S&P Global. If passive fund inflows materialize as a result of the rebalancing, forced purchases should increase demand for Solstice and SiTime next week.