Problemas com JPA (relacionamento N x M)

@Entity
@Table(name = "P_PERMISSOES")
public class Permissoes implements Serializable {

    @Id
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    @Column(name = "ID_PERMISSAO", nullable = false)
    private Integer idPermissao;

    @Column(name = "PAGINA", nullable = false)
    private String pagina;

    private String descricao;
    
    @ManyToMany
    @JoinTable(name="P_PADRAO_ACESSO_PERMISSAO", 
        inverseJoinColumns={
            @JoinColumn(name="ID_PADRAO_ACESSO")}, 
        joinColumns={
            @JoinColumn(name="ID_PERMISSAO")})
    private List<PadroesAcesso> padroesAcesso;

   //getters e setters omitidos
}

@Entity
@Table(name = "P_PADRAO_ACESSO")
public class PadraoAcesso implements Serializable {

    @Id
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    @Column(name = "ID_PADRAO_ACESSO", nullable = false)
    private Integer idPadraoAcesso;

    @Column(name = "DESCRICAO", unique=true)
    private String descricao;

    @ManyToMany(mappedBy="padroesAcesso", cascade= {CascadeType.PERSIST, CascadeType.MERGE, CascadeType.REFRESH})
    private List<Permissoes> permissoes;
    //getters e setters omitidos
}

@Stateless
@TransactionAttribute(TransactionAttributeType.REQUIRED)
public class PadraoAcessoBean implements PadraoAcessoLocal {
    
    @PersistenceContext(unitName="protocoloPU")
    private EntityManager em;

     ......

     public void insert(PadraoAcesso pa, Permissoes[] permissoes){
        
        List<Permissoes> p = new ArrayList<Permissoes>();
        
        for(int i =0; i < permissoes.length; i++){
            Permissoes permissao = em.find(Permissoes.class, permissoes[i].getIdPermissao());
            p.add(permissao);
        }
        
        pa.setPermissoes(p);
        
        em.persist(pa);
    }

     .....
}