Propargyl radical addition to 1,3-butadiene seems to be a promising channel to form 5-, 6-, and 7-membered rings. These are important steps in the growth of polycyclic aromatic hydrocarbons and soot platelets. The reaction mechanism, involving 97 intermediates and 115 transition structures, was studied by CBS-QB3 method (reported here) and density functional theory. All these structures were included in the subsequent RRKM study at different combustion pressures (P=30-0.01 atm) and temperatures (T=1200-2100 K). At P=30 atm, open-chain products dominate in the whole range of temperatures. The importance of 5- and 6-membered rings rises with T, reaching a maximum in the T range 1500-1800 K. A more modest yield in 7-rings is present at T=1500 K. At P =I atm, in the range 1200-1500 K, the yield in 5- and 6-rings dominate. 5- and 6-rings yields are about 41% at 1200 K (CBS-QB3 data). At P =0.1 atm, 6-rings become the main products a 1000 K (35%), and then they decrease to 12% (2100 K) and 5-rings rise up to 44% (1200 K), and then decrease to 14% (2100 K). Open-chains are the main products at T < 1000 K and T > 1500 K. Then, at P=0.01 atm, open-chain products are important below 900 K and above 1500 K,6-rings are main contributors between 900 K and 1500K. 6-Rings reach a maximum yield of 47% (1200 K) and 5-rings 44% (1200K) and 7-rings 17% (900 K). The main products form through to H losses. (C) 2016 The Combustion Institute. Published by Elsevier Inc. All rights reserved.