Lammot du Pont Copeland Sr., former chairman and chief executive officer of E.I. du Pont de Nemours & Company, the world's largest chemical concern, died of a heart attack in July 1 1983 at his home in Mount Cuba, Del. He was 78 years old. A great-great-grandson of Eleuthere Irenee du Pont, the founder of Du Pont, Mr. Copeland spent his 54-year career with the company. Mr. Copeland served as Du Pont's 11th president and chief executive officer from 1962 to 1967, then as chairman until 1971. He had been a director of the company since 1942 and was active on the board until failing health prompted his retirement. Known as ''Mots'' to those closest to him, he presided in the 1960's over one of the nation's largest corporate entities at a time of major growth, with sales exceeding $3 billion for the first time. For 14 years in the 1940's and 1950's, Mr. Copeland was also a director of the General Motors Corporation, representing Du Pont's 23 percent stock interest in the auto maker. As such, he was one of the principal defendants in Du Pont's 13-year antitrust battle with the Government over Du Pont's $3.5 billion stock interest in G.M. In compliance with a Federal court order forbidding the interlocking of Du Pont and G.M. directorates, Mr. Copeland resigned from the G.M. board in 1959. Mr. Copeland's resignation as Du Pont's chairman in 1971 was in part prompted by his efforts to straighten out the tangled financial affairs of a son, Lammot du Pont Copeland Jr., who filed the largest personal bankruptcy case in history in 1970, listing liabilities of $55 million. Lammot du Pont Copeland was born in Christiana Hundred, Del., on May 19, 1905, the son of Charles and Louisa d'Andelot du Pont Copeland. His mother was the sister of Pierre, Irenee and Lammot du Pont, who were presidents of the company from 1919 to 1940. His father was secretary and a director of Du Pont in the 1920's and 1930's. Mr. Copeland graduated from Wilmington Friends School and received a bachelor's degree in industrial chemistry from Harvard University in 1928. Joining Du Pont in 1929, he served in plant management, market analysis, general sales, development and other areas before moving up to the board room in 1942. He was named secretary in 1947, vice president and chairman of the finance committee in 1954, a member of the executive committee in 1959 and president three years later. An enthusiastic hunter and trout and salmon fisherman, Mr. Copeland collected early American antiques and served on the boards of various historical and educational institutions. He was also active in a wide range of philanthropic activities. His mother was the sister of Pierre, Irenee and Lammot du Pont, who were presidents of the company from 1919 to 1940. His father was secretary and a director of Du Pont in the 1920's and 1930's. Mr. Copeland is survived by his wife, the former Pamela Cunningham, whom he married in 1930; three children, Lammot Jr. and Gerret van Sweringen Copeland, both of Wilmington, and Mrs. Louisa Copeland Duemling, the wife of Robert W. Duemling, the United States Ambassador to Surinam, and 10 grandchildren.