I highly doubt this is in any way a thorough exposition of the topic but the Catholic Encyclopedia under Divorce had this to say (far down the page):
Christian marriage before consummation can be dissolved by solemn profession in a religious order, or by an act of papal authority.
- Dissolution by Solemn Profession
The fact that religious profession causes the dissolution of the marriage bond, provided the marriage has not been consummated, is distinctly taught in the Extrav. Joan. XXII(tit. VI, cap. unic.), and was solemnly defined by the Council of Trent (Sess, XXIV, can. vi). The reason why this dissolution takes place is a theological question. The definition reads: “If anyone shall say that a marriage contracted, but not consummated, is not dissolved by the solemn religious profession of either one of the parties to the marriage, let him be anathema.” The expression, by the solemn profession, is important. Neither the mere entrance into a religious order, nor life in the novitiate, nor the so-called profession of simple vows, even though they be for life, as is customary in modern congregations, is capable of dissolving a previous marriage. The simple vows which are pronounced in the Society of Jesus, either as vows of scholastic or as vows of formed coadjutors, do not dissolve a marriage which has been contracted and not yet consummated, though they cause a diriment impediment in regard to any future marriage. The question as to how and for what reason such marriage is dissolved by solemn religious profession is answered by some by pointing to an immediate Divine right, as if God himself had so ordained immediately. Others, however, ascribe it to the power which the Church has received from God, and to its ordinance. The first opinion is defended by Dominic Soto, Thomas Sanchez, Benedict XIV, Perrone, Rosset, Palmieri, and others; the second by Henry de Segusia (commonly called Hostiensis), Francisco Suárez, Laymann, Kugler, the Würzburg theologians, Wernz, Gasparri, Laurentius, fahrner, and others. The tradition of the Christian Church for centuries bears witness that Christian marriage before consummation has not the same indissolubility as a consummated marriage. Scholars, however, are not unanimous about the limits of its dissolubility. Many facts from the lives of the saints, of St. Thecla, St. Cecilia, St. Alexius, and others, such for example as are narrated by Gregory the Great (III Dialog., xiv, in P.L., XXXIII) and by the Venerable Bede (Hist. Angl., xix, in P.L., XCV, 201 sqq.), are proof of the universal Christian conviction that, even after marriage had been contracted, it was free for either of the married parties to separate from the other in order to choose a life of evangelical perfection. Now this would be a violation of the right of the other spouse if in such circumstances the marriage bond were not dissolved, or at least could not easily be dissolved under certain conditions, and thereby the right granted to the other to enter upon another marriage. The precise conditions under which this dissolution of the marriage bond actually took place, and stil takes place, can only be decided with certainty by the authentic declaration of the Church. Such a declaration was made by Alexander III, according to III Decretal., xxxii, 2: “After a lawfully accorded consent affecting the present, it is allowed to one of the parties, even against the will of the other, to choose a monastery (just as certain saints have been called from marriage), provided that carnal intercourse shall not have taken place between them; and it is allowed to the one who is left to proceed to a second marriage.” A similar declaration was made by Innocent III, op. cit., cap. xiv. From this latter declaration we learn that religious profession alone has this effect, and that therefore those who wished to practise a life of higher perfection in any other manner could be obliged by the other spouse either actually to choose the religious state or else to consummate the marriage. Under earlier ecclesiastical conditions, no long delay was imposed upon the other party before entering upon another marriage, because religious profession might be made without a long novitiate. The introduction of a novitiate of at least a year by the Council of Trent, and the time of three years prescribed by Pius IX and Leo XIII for simple vows before the solemn profession, and the general restriction of solemn profession by the establishment of simple profession, which does not dissolve the marriage bond, have rendered difficult the dissolution of unconsummated marriage by religious profession. So that now it seems practically necessary that if one of the married parties should choose the state of evangelical perfection before the consummation of the marriage, the marriage bond should be dissolved by papal authority.
- Dissolution by the Pope of Marriage not yet Consummated.
The pope’s authority as supreme head of the Church to dissolve Christian marriage not yet consummated is proved on the one hand from the words of Christ to Peter, Matthew 16:19 (see above, under B, 2), and on the other, from the dissolubility of such a marriage by religious profession, inasmuch as this profession must be solemn, for according to the declaration of Boniface VIII (III Sexti Decretal., xv, c. unic.), solemn vows as such depend entirely upon the ordinance of the Church – “voti solemnitas ex solâ constitutione Ecclesiæ est inventa”. Hence it follows without a doubt that the dissolution of a marriage by solemn profession could never take place without the exercise of the Church’s authority. Now if the Church can cause such a dissoltuion according to a general law, a fortiori she can do this in single cases – not indeed arbitrarily, but for grave reasons – because this power has been granted by God to dispense in matters of Divine right, and a delegated authority may not be exercised without a sufficient reason (cf. Wernz, “Just decretal.”, IV, n. 698, not. 39). The actual exercise of this power on the part of the popes, which has become constant and general, is a further proof of its propriety and its actual existence. Clear instances occur during the pontificates of Martin V (1417-31) and Eugene IV (1431-47). St Antoninus tells us that he had seen several Bulls of the popes which granted such a dispensation of a dissolution of a marriage that had not been consummated, so that thereafter they might proceed to a new marriage. (Summa theol., III, tit. i, c. xxi). We can find traces of such a practice even in much earlier times. A decretal of Alexander III, namely, IV Decretal., xiii, 2, seems, according to a probable interpretation, to refer to a possible concession of such a dissolution. Perhaps the decision of Gregory II to St. Boniface, in 726 (see above under A. 4) might possibly be explained in the same sense, though it is very uncertain, for it seems to refer neither to the dissolution of a consummated marriage, as some supposed, nor to the dissolution of a real marriage that had not been consumated, but rather to a declaration of invalidity. For several centuries the exercise of this power of dissolving such marriages has belonged to the ordinary functions of the Holy See, and is exlusively papal, for the work of the Roman Congregations in such cases is only preparatory. However, exceptional instances occur when it has been delegated to bishops (Wernz, op. cit., n. 698, not. 41). The judicial procedure in such cases was exactly prescribed by Benedict XIV in his Bull of judicial procedure (“Dei miseratione”, 3 November, 1741 (section 15), obligatory on the whole Latin Church. Any uncertainty about this ecclesiastical power (cf. Fahrner;Geshichte des Unauflöslichkeitsprincips, p. 170 sqq.) was removed bythis Bull; for if this power did not belong to the Church, then the Bull in question would have approved and originated an institution against all good morals. It is, however, inconceivable that the pope could issue an attack on morality and could formally sanction bigamy in certain cases. Several of the older canonists, especially those of Bologna, brought forward some special reasons which are supposed to justify the dissolution of a marriage before consummation. If thereby they wish to assert the right of dissolution by private authority, then they erred. If they intended to speak of a dissolution that could be granted by the Church, that is, by its supreme head, and the permission for a new marriage, then they had merely collected the cases in which such a dissolution might take place in virtue of the papal authority just spoken of, but they had not given a new title to such dissolution. Some held the erroneous opinion of private dissolubility, because they regarded such a union as no real marriage, but simply as betrothal, and therefore they treated it according to the juridical principles in regard to betrothal. This theory of marriage, however, was not often defended, and has long dissapeared from theological schools; neither does it deserve any consideration at present, because it is in conflict with established Catholic dogmas.