( This ( ( is ( because ( ( pi bonds ) ( ( are not ) ( as ( strong ( as ( sigma bonds ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT This)) (VP (VBZ is) (SBAR (IN because) (S (NP (NN pi) (NNS bonds)) (VP (VBP are) (RB not) (ADJP (RB as) (JJ strong) (PP (IN as) (NP (NN sigma) (NNS bonds)))))))) (. .)))
This is because pi bonds are not as strong as sigma bonds.
(ROOT (S (NP (DT A) (JJ double) (NN bond)) (VP (VBZ consists) (PP (IN of) (NP (NP (CD one) (NN sigma) (NN bond)) (CC and) (NP (CD one) (NN pi) (NN bond))))) (. .)))