Annick-Camille C Dumaret
email: dumaret@vjf.cnrs.fr

Pascale Donati

Monique Crost

After a long-term placement: Investigating behaviour, educational achievement and transition to independent living

Keywords: child welfare, placement, young people, leaving care, follow-up study, transition to adulthood

This study describes the transition towards independent living of 123 former fostered young people reared for long periods in a private French organisation, SOS Children's Villages. Three generations of care leavers were analysed through a postal survey and interviews. Their narratives show typical pathways after leaving care. Two thirds became independent without major problems by the age of 24-25. Analyses have shown that the absence of severe emotional and behavioural disorders, and diplomas obtained, improved their odds of becoming independent. Results suggest a vital need to implement appropriate therapeutic care during placement and to develop care leaver support well beyond the age of 18, in order to help them personally and academically and to ensure their successful transition to adulthood.

Introduction

Changes in French child protection have contributed to improvements in foster care.

However, debates have highlighted the lack of research on the quality of care services and on youth care leavers (Naves and others, 2003). A culture of evaluation has been gradually developing since the passing of a law on medical and social intervention in 2002 (Ministry 2006). Thus, a study on long-term outcomes of adults formerly reared in children's villages was conducted. The evolution of care services, and changes in the wider socio-economic context, have been taken into account, and an analysis was made of the period lasting from the end of care to the transition to independent living.

Child protection and leaving care in France

Child welfare services are under the responsibility of local authorities, and the Judicial Juvenile Protection department is under the responsibility of the state. These two sectors of child protection are independent but complementary. When educational assistance providing support to families on a voluntary basis (administrative measures) proves difficult to implement, children's judges can order judicial measures, which are revised every two years.

Placements end when the children's judge so decides (the child returns home, generally with a family assistance measure), or when young people reach the age of majority. Child protection services may support them, and other young people without adequate family support networks, through agreements ("contrats Jeunes Majeurs"), until their 21 st birthday. Jeunes Majeurs represent 0.9% of the young people in their age group. They receive different forms of assistance to pursue educational or job training projects, and may remain in placement. But there is a four-year financial gap before they become eligible for welfare allowances at the age of 25. Since the mid-1980s, different support systems have been developed by communities, private foundations, and local authorities to fight social exclusion, particularly for young people aged 16-25 from poor socio-economic backgrounds, and/or with low educational levels. Results and key findings are still missing on a national or regional level.

Transitions from care to adulthood and independent living

International comparisons show different pathways for entry into adulthood, which are structured by welfare state interventions, education and family, and by the individuals themselves ( Van der Velde, 2008). A recent book on research and practices in sixteen countries has shown the risk of social exclusion and marginalisation for former foster youth, despite significant disparities in policy and practices [START_REF] Stein | Young people's transitions from care to adulthood: international research and practice[END_REF]. Young care leavers undergo more accelerated transitions than their peers, who can generally count on dependable family support, and assistance from other networks [START_REF] Stein | Research Review: Young people leaving care[END_REF]. Furthermore, specific ethnic groups and isolated refugees face considerable challenges.

Many young care leavers have experienced multiple disruptions in their schooling, lack educational qualifications, and hold unskilled jobs [START_REF] Broad | Young people leaving care: Life after the Children Act[END_REF]Courtney and others, 2001;[START_REF] Pecora | Educational and employment outcomes of adult formerly placed in foster care: Results from the Northwest Foster Care Alumni[END_REF][START_REF] Wade | Making a home, finding a job: investigating early housing and employment outcomes for young people leaving care[END_REF]. In addition, several US and UK studies have highlighted care leavers' high rates of housing instability and youth homelessness, as well as of unplanned pregnancies and early parenthood (Carpenter and others, 2001;[START_REF] Pecora | Educational and employment outcomes of adult formerly placed in foster care: Results from the Northwest Foster Care Alumni[END_REF]. Mental health problems (depression, stress disorders, alcohol and drug dependency…) have also been shown to have an impact on their access to the labour market and individual housing [START_REF] Pinkerton | Responding to the needs of young people leaving state care: Law, practice and policy in England and Northern Ireland[END_REF]Lenz-Rachid, 2008). It was noted that the highest rates of poor social adjustment in early adulthood affect those who had multiplacements [START_REF] Andersson | Family relations, adjustment and well-being in longitudinal study of children in care[END_REF][START_REF] Wade | Making a home, finding a job: investigating early housing and employment outcomes for young people leaving care[END_REF]. Some French studies using multidisciplinary approaches also reveal that care leavers have difficulty entering the workforce, work unskilled jobs, have weak networks, and lack preparation for independent living (Corbillon and others, 1997;Gheorghiu and others, 2002).

Others focusing on individual pathways, noted the chain of transitions and have shown that high housing instability and mental health problems resulting in psychiatric hospitalizations largely decreased three years after leaving care [START_REF] Frechon | Stratégies féminines d'entrée dans la vie adulte après un placement à l'adolescence[END_REF].

Research with long follow-up periods highlights the pervasiveness of offending behaviour and/or continued use of social services, particularly before services for care leavers existed [START_REF] Festinger | No one ever asked us. A postscript to foster care[END_REF][START_REF] Coppel | Que sont-ils devenus ? Les enfants accueillis à l'Oeuvre Grancher. Analyse d'un placement familial spécialisé[END_REF][START_REF] Dumaret | Adult outcome of children reared for long term period in foster families[END_REF]). An analysis of 30 studies has shown a decrease in psychosocial difficulties and better social integration after the age of 25 [START_REF] Frechon | Bilan critique de 50 ans d'études sur le devenir des enfants placés[END_REF].

Evaluating the outcomes of care leavers requires taking into account experiences before entering care and events that occur during and after care. A large proportion of care leavers comes from troubled families, and has experienced neglect, abuse, as well as learning and behavioural difficulties. They have also experienced changes in placement and break-ups in their relationships, leading to insecure attachments. All these factors put them at greater risk of future difficulties. Consistent findings highlight the importance of placement stability, bonds with a key person, and societal and family support during the transition to adulthood (Biehal and others, 1995;[START_REF] Leathers | Placement disruption and negative placement outcomes among adolescents in long-term foster care: the role of behaviour problems[END_REF][START_REF] Courtney | Early outcomes for young adults transitioning from out-ofhome care in the USA[END_REF][START_REF] Wade | Making a home, finding a job: investigating early housing and employment outcomes for young people leaving care[END_REF].

Conditions of transition to adult independence have changed considerably during the last two decades. However, little attention has been paid to different generations of care leavers and their views of the transition period.

The SOS children's villages study

"SOS Children's Villages" (CVs) is an international non-governmental social development organization providing long-term care to children without parental care and to children from vulnerable families (100 CVs in Europe). In France today, 600 children and adolescents are being raised in 13 CVs by foster caregivers (the "SOS mothers") working with a director, psychologists, case workers, and family helpers. CV staff were gradually hired during the mid-1980s, at a time when families in France were getting smaller. Prior to that period, SOS mothers volunteered for "philanthropic work" and raised large sibling groups. From the mid-1980s on, the new SOS mother took on a more professional role in carrying out their duties.

Most of them began to rear two groups of siblings together. At the same time , services were created: a job training service, informal support (housing and job searches, administrative support), and transitional apartments and residential accommodation for adolescents or young adults. The objective was to prevent social exclusion, particularly for youth with mental health problems or disabilities, and those who had had run-ins with the law or disrupted careers, and more generally, to develop life training skills.

The CV study was designed to assess the social integration and quality of life during adulthood of former foster children and adolescents, as well as relationships with former foster caregivers and among siblings. A follow-up study in the past was conducted on an entire CV in the south of France from which 150 young people aged 15 and older left care between 1972 and 2000. Such a long period following discharge from care allowed us to examine the principal changes both in institutional practices and in the conditions of access to independence. The impact of parental pathologies and of maltreatment on behavioural problems was analysed in an earlier publication. This paper is focused on the transition period during which young people strove towards financial and housing independence. The purpose is twofold: a) to describe the evolution of forms of support provided to several generations of care leavers; and b) to analyze educational outcomes and access to independent living. The results will be highlighted through the former foster youths' perceptions of their first years after support ended.

Methods

The population study (n=123) met the following three criteria: 1) the subjects had been placed at least three years, 2) they had been discharged from care for over three years, and 3) they were at least 23 years old when the survey began. The research protocol was approved by the National Committee of Data-processing and Freedoms (Commission Nationale Informatique et Libertés).

Materials

Institutional files: Case records provided information on demographic data, individual situations at admission, care services, and annual scholastic and psychological appraisals.

Important events during placement and after discharge, when young adults came back to ask for assistance, were also identified. Emotional and behavioural (EB) disorders during placement and until late adolescence were classified as follows: no significant disorders mentioned, mild disorders, and severe disorders [START_REF] Dumaret | Placements à long terme en villages d'enfants depuis plus de 30 ans : troubles psychiques et prises en charge[END_REF].

Follow-up:

In 2004, a survey was mailed to the 123 subjects aged 23-50 (median age: 38) to determine both their current social integration and their living conditions after discharge from care, including diplomas and access to housing. They were asked to be interviewed, and in 2005 and 2006, face-to-face interviews were conducted by psychologists who were unaffiliated with the institution and could assure interviewees that their reports would remain anonymous. An interview guide was used, covering several areas of their past and present lives. All but one of their SOS mothers were interviewed in the same manner.

Data collection enabled us to specify main difficulties care leavers had to cope with during the transition to independent living at approximately 24-25 years of age. Independence is defined here as self-sufficiency on a financial and material level (stable job held for six months, and independent housing) and being independent of any institutional support.

Independence differs from the more subjective notion of "autonomy", also discussed in interviews, which refers to the ability to make one's choices alone [START_REF] Singly | Etre soi d'un âge à l'autre. Famille et individualisation[END_REF]. Thus, it has been possible to assess 114 transitional situations through the 123 institutional files, 99 questionnaires, and/or interviews with adults (73 adults formerly in care and 22 SOS mothers).

Analysis

To take into account changes in the educational system, in child welfare practices, and in the wider socio-economic context, three time periods were chosen (Note 1). Change took place gradually, but such classification allowed us to distinguish among generations who experienced vastly different care and social constraints. The three groups were 1) those from the earliest P1 period: 30 subjects (mean age at survey: 45.0; =2.2); 2) those from the middle P2 period: 65 subjects (mean age: 37.9; =2.5); and 3) those from the most recent P3 period: 28 subjects (mean age: 27.0; =3.7). The oldest generations left care before 1980, the intermediate ones during the 1980s, and the youngest generations during the 1990s.

Bivariate statistical tests were used to test significant differences between analyzed groups.

To take into account the effects of confounding factors, multivariate analyses by logistic regression were used to examine the specific role of decisive factors (SAS software, version 9.1). The variables tested in the multiple logistic regression are those showing a significant difference at the p<0.10 level in the bivariate analysis. Adjusted odds ratio estimates (OR a ) were considered to be significant at the p<0.05 level. After transcription of the audio-taped interviews, content analysis was used to study several themes; a comprehensive analysis of perceptions of leaving care and entry into adulthood allowed us to reconstruct personal pathways and typical life experiences.

Results

Characteristics of the young people

Although death or disappearance of one or both parents had long been the principal reason for CV placement, other parental risk factors existed (alcohol, antisocial and mental health problems). Two thirds had previously been subjected to intra-family violence, and an identical proportion had been neglected and abused. Four out of ten had developmental delays, impairment, and various health problems, and half had experienced school failure.

Three quarters of those admitted were older than six (m=8.8, =3.9); the majority left care at 18, and a third did so at 19 or older (m=18.6, =1.5). The youngest generations were admitted earlier and their CV placement was prolonged beyond the age of majority. The average placement was long (m=9.8, =4.1).

Table 1 The proportion of youths with EB disorders was stable throughout the three periods; half received regular educational and therapeutic support (minimum: one year) in the CV or in day-care centres (Table 1). During the P2 period, with the growing number of staff members, care services increased (psychological support, after-school activities), and support services for adolescents were diversified. Family support during the transition period (financial help, housing) was also significant (59%).

A third of those from the younger generations received a high school diploma or higher, while the older ones obtained more technical diplomas and apprenticeship certificates.

However, four out of ten of all the youths had no diploma. A quarter of the diplomas were obtained thanks to prolonged support after the age of the majority, particularly among the youngest generations of care leavers (43% during P3 vs. 20% during P1/ P2, p<0.02).

EB disorders had an impact on their schooling: two thirds of those with severe disorders and mental health problems had no diploma. Nevertheless, the disorders had an impact on the age of leaving care: a quarter of those with severe EB disorders left care earlier than the others. Nine adolescents returned to their biological families before the age of 17; the family reunifications failed, and the majority of these adolescents were placed elsewhere or requested child welfare services. The other departures involved young men with antisocial behaviour and/or those who performed their military service, and young women who left care to be closer to their families or to set up a home with a partner after running away.

Nearly four out of ten subjects did not live in individual housing in the first years following their discharge, living instead with their family, friends, or in residential lodging. Housing independence was attained more rapidly by young adults with diplomas (74% vs. 43% for those without diplomas, p<0.001) and by those without disorders (79% vs. 40% with severe problems, p<0.002). Across the three generations of care leavers, two thirds of young adults began living independently towards the age of 24-25 without encountering major problems; it was more difficult for a quarter of them, and one tenth was still not independent..

Factors associated with independent living towards the ages of 24-25

The most significant characteristics in determining access to independence were linked to gender (women were more successful than men), family and child risk factors before admission (domestic violence, developmental delay, abuse), EB disorders, and diplomas.

Table 3 Detailed analysis has shown that, of the 27 youths who had a difficult time becoming independent, one out of three had psychiatric problems (eight men and one woman), and a similar proportion obtained a diploma. The situation was worse for those who were not independent: 10 out of 13 (six men and four women) had psychiatric problems, and only one had a training diploma.

Multiple logistic regressions were carried out on the outcome variable: independent living without major problems. A first step was conducted by simultaneously taking into account the ex-care periods, parental risk factors, gender, and neglect and abuse before placement. Four factors were significantly associated with independence: absence of developmental delay at admission (OR a =8.27), sex (OR a =4.03), absence of abuse (OR a =3.48), and absence of parental psychiatric problems (OR a =3.25).

Table 4

In the second step, disorders and diplomas were added to the test (Table 4). After adjustment for the ex-care periods and developmental delay, only two factors significantly improved the odds of independence. The OR a were high for youths who showed no sign of severe EB disorders. Having a diploma also improved the odds of becoming independent; the higher the level, the greater the effect.

In the first years after leaving care, we estimated that 16-20% of young adults (the majority were men) experienced violent episodes, behavioural problems, and alcohol and/or drug dependence. As shown by the interviews, the majority of these problems were temporary, with the exception of those caused by severe mental health problems.

Life after care: young people's perceptions

Discussions about the period during which subjects had left the familiar CV environment led to ambivalent feelings: the satisfaction of being free, coupled with feelings of isolation.

Most remember having been motivated by projects and an eagerness to take on adult roles (working, starting a family). The majority of interviewees felt that they quickly became autonomous, approximately two years after their departure. For the older generations, feelings of autonomy were linked to their financial independence, while in younger generations they were tied to their ability to manage their emotional lives and to distance themselves from life in the CV family framework. Yet a few, especially men, claimed to have had no project or support; they presented their experiences as chaotic and said they became independent later, or only partially independent for those with a poor sense of mental well-being.

Family solidarity was a key point in the narratives; emotional and material support provided by their main caregiver, the SOS mother, was mentioned first. Those who became parents, even those who had broken off all contact after leaving care, expressed how they were able to count on her when their own children were born. For the oldest, most support came from their eldest brothers and sisters (large families). For all of them, job training support and transitional apartments were held up as examples of decisive support providing the first steps towards autonomy and entry into adulthood within a supervised setting.

Typical experiences

Beyond individual experiences, three types of personal history patterns could be identified, revealing different processes used to reach independence.

Making do and dealing with "old-fashioned hardships"

Such patterns are typical of the older generations of care leavers. This period represented a break with the institution and/or their foster caregiver (the SOS mother), when they were easily able to get a job, even without a diploma: "I had set a goal for myself: find work, earn money, and find a wife. It wasn't easy, but I held in there […] I slept in a car for several months, and by working I was able to integrate myself into to the community." (A man in his forties).

Halting progress for the youngest generations

Like most people in their age group living through transitional situations, the youngest generations of care leavers made halting progress, alternating between dependence on the institution or their caregivers and partial autonomy, with high residential mobility. Their statements highlight their personal fragility and identity research. The separation from the SOS mother took place in a more gradual way, in which the attachment lasted longer: "I carry the trauma of the separation from her" (a 25 year-old man). Sometimes, this attachment was tied to a depressed state: "My mother was always there to house me, and the village was offering me work... There was a period when I was trying to find myself; I was a bit depressed and felt unhappy, so I decided to go see a neuropsychiatrist lady." (A 29 year-old man who transitioned slowly to independence).

Gradual social integration process

The most common pathway was a gradual or halting transition, without long-lasting or repeated hardships. There were fewer generational differences than gender differences in these experiences, with relationships marking the first step towards autonomy for women. However, a large majority of women who left care before the mid-1980s said that their professional activity was a more decisive factor in their experience than their relationships and family lives: "As soon as I obtained my training diploma, I looked for work everywhere!

[…] Afterwards, I had a boyfriend, but for two years there were highs and lows. I almost lost my job; I almost ended up on the street… I tested quite a few things out, in other words! We make mistakes and it allows us to move forward." (A 36 year-old woman) However, even during these transitions, all felt fairly powerless when confronted with the demands of daily life, as was the case with a 24 year-old man: "Fitting in after leaving the CV was hard: taking mass transit, budgeting, registering at the social security office…"

Discussion and Perspectives

SOS children's villages offer long-term placement through their family units and important care services to children and siblings' groups. This study is the first to describe the entry into adulthood of several generations of care leavers raised for a long time in foster care. The high rate of participation in this study reflects both their motivation to talk about their life experiences and their wish that their reflections on the strengths and weaknesses of this type of foster care be helpful to others.

For the older generations, the institutional services and support provided were minimal.

Nevertheless, they benefited from a favourable labour market, even for those with low qualifications. The middle generations witnessed changes in school policy; care services were developed and diversified, essentially from the mid-1980s. Support proved to be all the more necessary as the youngest generations were confronted with devalued lower-level diplomas and worsening labour market entry conditions.

The large majority underwent transitions and successfully began living independently by the age of 24-25. Their social integration processes ranged from chaotic to gradual, as mentioned by the interviewees. The high proportion can be linked to placement stability and to continuing care services, whatever the age of departure. Such prolonged support, sometimes beyond the age of 21, allowed a quarter of young people to obtain diplomas. High levels of support from their siblings and from their foster caregivers (the SOS mother, the educators) were reported. Such relationships and attachment may have been protective factors to cope with transitional problems and exclusion risks; supportive partners were also mentioned in their narratives [START_REF] Biehal | Moving on: Young people and leaving schemes[END_REF][START_REF] Coppel | Que sont-ils devenus ? Les enfants accueillis à l'Oeuvre Grancher. Analyse d'un placement familial spécialisé[END_REF][START_REF] Jahnukainen | High-risk youth transitions to adulthood: A longitudinal view of youth leaving the residential education in Finland[END_REF][START_REF] Wade | The ties that bind: Support from birth families and substitute families for young people leaving care[END_REF]. With the abrupt withdrawal of support for some troubled care leavers before 17-18, experiences were similar to those described by research on young runaways or on young people from child welfare services: antisocial behaviour, violence, admission to mother-child hostels [START_REF] Marpsat | Les ressources des jeunes sans domicile et en situation précaire[END_REF]).

Disparities exist among child welfare services, agencies, and populations in care. In spite of the fact that results cannot be applied to other placements, one may suppose that better experiences produce better outcomes. Young people from Judicial Juvenile Protection faced more difficulties in their transitions (Hubert and others 2006). Other limitations to this study should be noted. Regarding behavioural and mental health problems: without standardized measures, precautions in interpreting results are necessary; the collected data remain descriptive. As for these adults' memories of their journey to adulthood and independent living, some problems may have been forgotten by those who are satisfied with their current situation. Some concerns over these results are shared by both SOS children's villages and child welfare services. Two issues which need to be dealt with together are the percentage of children and adolescents placed in care that show signs of behavioural and mental health problems, and the proportion of young carers without diplomas:

-Today, half of these adults now regret having left school too early, which suggests that young people had not seized opportunities offered during placement, as shown in other studies (Trout and others, 2008;[START_REF] Vacca | Foster children need more help after they reach the age of eighteen[END_REF]. Children in care should be encouraged to finish school.

-We had noted in a previous publication that severe disorders were persistent, especially among children and adolescents who had been multiplaced before their admission in Children's Villages [START_REF] Dumaret | Placements à long terme en villages d'enfants depuis plus de 30 ans : troubles psychiques et prises en charge[END_REF]. Improvements in psychological assessment are being strongly urged by foster care specialists, particularly to identify young people at risk of severe difficulties, and to implement appropriate therapeutic care if necessary [START_REF] Berger | L'échec de la protection de l'enfance[END_REF].

Thus, prolonging all support after the age of 18, and even 21, is essential to help young people explore career options and obtain a diploma, particularly for those with little schooling and for adolescents with psychiatric problems.

Formerly, most of the SOS mothers were perceived as adoptive parents by the children or groups of siblings they had reared. The professionalisation of foster care has transformed their task (they officially became family educators in 2005) and has brought into question their long-term relationships with young people. While bonds will endure with some young people who need long-term placement, it remains to be seen whether these caregivers will be able to continue providing tangible support to the others.

Whatever the leaving care period, feelings of isolation were mentioned by many, even when transitions were free of major problems. All noted the necessity of developing life skills during placements. Some research has shown that, despite assistance in this area, a third of young care leavers reported feeling unprepared to live on their own (Courtney and others, 2001).

Important changes in child welfare policies took place over the course of a decade: the decreasing length of placement and greater instability in children's lives, the failure of many family reunifications, the increase in measures for home assistance and support services to help parents assume their responsibilities, greater attention given to users of child welfare services. These tendencies increase the need to re-think innovative frameworks for ongoing supportive relationships (referents or mentors). Such programs have been developed in other countries and in a very small number of French départements (Lemon and others, 2005;[START_REF] Goyette | Préparation à la vie autonome et l'insertion socioprofessionnelle des jeunes pris en charge par l'Etat : quelles interventions ? Revue Sociétés et jeunesses en difficulté[END_REF][START_REF] Reid | Building a future together: Issues and outcomes for transition-aged youth, Centres of excellence for children's well-being[END_REF][START_REF] Osterling | Mentoring adolescent foster youth: promoting resilience during developmental transitions[END_REF]. To be effective, all services and professionals must coordinate their actions through a coherent platform to prevent these young people from being "lost in transition". More research with quantitative and qualitative approaches is needed to improve standards of care and the lives of children and adolescents in care.

Note 1: Professional high school diplomas were created in 1985. Improving levels of instruction were accompanied by the devaluation of diplomas received. The age of majority went from 21 to 18 in 1974. Compulsory military service ended in 1998. Notes to table 1 (1) diplomas were classified as follows: a) high school diploma (baccalaureate for general education or vocational diploma of equivalent level) and/or years in university, b) youth training or less than 3 years of high school (equivalent to 4 years in a French secondary school), c) no diploma, including low-level certificates in general education or apprenticeships.

(2) small numbers; but the youngest care leavers' generations obtained significantly more frequently high school or university diploma than the oldest ones (p<0.01). (1) also controlling for two others significant factors : leaving care period and developmental delay

Table 1 : Main characteristics and care leavers' generations

 1

	Leaving care period

Table 2 : Access to independent living by demographic, educational characteristics and EB Disorders

 2

		Independent living
		Without serious	With difficulty
	Total	problems	/not independent
	N=114	N=74	N=40
	n	%	%	p value

(1)

The comparison "abuse vs. no maltreatment or neglect" is significant (p<0.05)

Table 3 : Determinant factors in access to independence (logistic regression model)

 3

	Independent living

Acknowledgments

Research was supported by grants from the SOS Children's Villages organization, the board of the Ministry of Social Cohesion, Health and Solidarity, and the Wyeth Foundation. We would like to thank Marie Constantin-Kuntz and Catherine Mousseau, psychologists who conducted some of the interviews. We extend special thanks to Professor Mike Stein of the University of York, UK, for his comments and review of an earlier draft.


Julien Rigolini

Bruno Grassl
email: bruno.grassl@univ-pau.fr

Laurent Billon

Stephanie Reynaud

Olivier F X Donard

Microwave-Assisted Nitroxide-Mediated Radical Polymerization of Acrylamide in Aqueous Solution

Keywords: microwave, acrylamide, controlled/living polymerization, Nitroxide Mediated Polymerization

The present work describes a combination of microwave irradiation as a heating source and water as a solvent for carrying out a living/controlled polymerization of acrylamide.

Reasonable results were obtained for a nitroxide-mediated radical polymerization (NMP) with a combination of a conventional hydrosoluble radical initiator and a β-phosphonylated nitroxide. The microwave enhancement of the polymerization was found to depend on the mode of irradiation, i.e., either a DYN mode or an SPS mode. The former mode corresponded to a dynamic control of the temperature by way of a high initial microwave power, and in this case, no specific microwave effect was observed. On the other hand, in the SPS mode, which is a pulsed power mode, the result showed a strong acceleration of the polymerization process (> 50 times) without the loss of the living/controlled polymerization characteristics which is relevant with a re-initiation of the polyacrylamide macroinitiator even after 100% of conversion.

Introduction

Controlled radical polymerization (CRP) is a well-known route for preparing (co)polymers with controlled functionalities and topologies, as well as targeted molar masses with low polydispersities. The CRP process requires selecting conditions that allow, or create, a dynamic equilibrium between a low concentration of active propagating chains and a large number of dormant chains, which are unable to propagate or self-terminate. There exist several CRP processes based on this fundamental understanding and the techniques that have received the most attention include Atom Transfer Radical Polymerization, (ATRP); Stable Free Radical Polymerization (SFRP), e.g., Nitroxide-Mediated Polymerization (NMP) and OrganoMetallic Radical Polymerization (OMRP); Degenerative Transfer (DT) processes, e.g., Reversible Addition Fragmentation Transfer (RAFT), and Macromolecular Design via the Interchange of Xanthates (MADIX). 1 Microwave-assisted polymer chemistry is a rapidly growing field of research. In the last few years, various examples of reaction accelerations, selectivities, and higher yields have been reported. [2][3][4][5][6][7][8][9] Overviews on microwave-assisted polymerizations have recently been provided by Ritter 10,11 as well as Schubert and co-workers, [12][13][14] whereby special attention was given to microwave-assisted CRP. In resumé, the polymerization rates were found to be clearly faster under microwave (MW) irradiation than under classical thermal heating at identical polymerization temperatures and under controlled conditions. This was true for NMP of styrene in bulk 15 , and methyl acrylate and tert-butyl acrylate in a relatively polar solvent (1,4dioxane); 16 for RAFT polymerization of methyl methacrylate, 17 vinyl acetate, 17 and styrene, 17,18 and for ATRP of methyl methacrylate [19][20][21] and acrylonitrile in refluxing tetrachloromethane. 22 The effectiveness was seen to depend essentially on the method used for performing the MW-assisted polymerization. Even faster polymerization rates have mostly been obtained under MW irradiation. It is indeed difficult to compare the results due to the great variety of experimental conditions described within the studies. Results have been obtained with a domestic or monomode MW oven and various modes of irradiation.

Demonceau 19 showed that a microwave effect was noted during ATRP of methyl methacrylate; an effect that strikingly depended on both the temperature and the method used, e.g., conventional microwave synthesis (CMS) or enhanced microwave synthesis (EMS), in the monomode MW oven. Their results differed from those reported by Schubert and coworkers 20 for the microwave-assisted ATRP of MMA using a copper-based system, thereby demonstrating the absence of any 'microwave effect' in ATRP, in contrast to several literature reports 19,21 , but without a consistent comparison of the irradiation method in terms of power and energy supplied to the reaction medium.

Microwave effects are obviously the subject of considerable current debate and controversy, and extensive research efforts are necessary to truly understand these and related phenomena.

On the other hand, the combined use of microwave irradiation as a heating source and water as a solvent for synthetic organic transformations is a relatively new approach, as recently reported by Kappe et al. 23 Nevertheless, to the best of our knowledge, work involving microwave-assisted CRP in aqueous media has yet to be published. Recently, Erdmenger et al. have used water as solvent for the microwave-assisted free radical polymerization of styrene 24 .

Thus, the issue of microwave effects on the controlled radical polymerization in water is the primary focus of this work. For that, the choice was made to carry our microwave-assisted NMP of acrylamide at high concentrations (40 wt-%) in a purely aqueous solution, using a combination of a conventional hydrosoluble radical initiator (Vazo56) and a βphosphonylated nitroxide, SG1. Evidence of a living/controlled nature of the polymer is presented and the effects of the employed MW irradiation method are described. Unlike other polyacrylamide derivatives that are soluble in common organic solvents, poly(acrylamide) PAM is only soluble in water, dimethylsulfoxide and glycerol. Moreover, the monomer is a powder, which inhibits the bulk polymerization from being considered at a reasonable temperature. Under such constraints, the controlled radical polymerization, CRP, of acrylamide in aqueous solution seems to be a realistic route, as both monomer and polymer are highly soluble in water up to 40 wt%, and respect the environmental benefits associated with microwave-assisted aqueous polymerizations. 25,26

Experimental section

Materials

Acrylamide (AM) (purity >99%) was purchased from ABCR, and 2,2-azobis (2methylpropionamidine) dihydrochloride, denoted VAZO56, was obtained from Dupont Chemicals. N-tert-butyl-N-[1-diethylphosphono-(2,2-dimethylpropyl) nitroxide] (DEPN), also called SG1, was kindly provided by Arkema and used as the counter radical. All chemical products were used as received. Deionized water, with an electrical conductivity of 18.3 MΩ.cm at 25°C, was filtered through a 0.22 µm Millipore filter prior to use.

Polymerization

A bimolecular system, based on Vazo56 (initiator) and free SG1 (counter radical) was used to run polymerizations of AM at 40 wt%. The molar ratio [SG1] 0 /[Vazo56] 0 was kept constant at 1.8 according to previous optimizations carried out for acrylamide 27 and organosoluble monomer. 28,29 Monomer solutions, containing a constant amount of initiator ([AM] 0 /2[Vazo56] 0 =500) were purged with nitrogen for 5 min directly in a 15-mL Pyrex culture tube with a screw-cap, or in a standard 10-mL Pyrex microwave process vial before being heated in an oil bath or through microwave irradiation. A total amount of 4.5 mL and 3 mL were heated conventionally in an oil bath and by way of microwave experiments, respectively. The polymerization was terminated by cooling in an ice bath (for the case with conventional heating in an oil bath) or with compressed air (for the microwave experiments) and the reactive medium was sampled for size exclusion chromatography coupled with light scattering detector (SEC-MALS). For the kinetic experiments, the same reaction mixture has been used to prepare several tubes and each tube was withdrawn at different time intervals.

Size exclusion chromatography coupled with multi-angle light scattering (SEC-MALS)

The number-average molar mass (M n ), polydispersity (Ip), and polymer conversion (p) of the polyacrylamide (PAM) samples were determined by size exclusion chromatography (SEC) using a Waters Alliance 2690 (USA) chromatograph equipped with two Shodex OHpak columns (SB-804HQ, and SB-802.5HQ) and two online detectors: a differential refractometer and a DAWN ® HELEOS™ II utilizing a 120-mW solid-state laser operating at 658 nm and fitted with a K5 cell. A 0.1-M solution of NaNO 3 was used as the eluent at a flow rate of 0.5 mL/min. Samples were prepared by diluting approximately 20 times the reaction mixture in 0.1M NaNO 3 and then filtering with a 0.45 µm filter (Millipore) after about 1 hour of stirring.

Samples of 25 to 75 µL of these solutions were injected. The number-average molar mass and polydispersity were obtained from data collected and analyzed using the ASTRA SECsoftware (version 5.3, Wyatt Technology Corp., USA). The calculation of molar mass was carried out according to the Zimm fit method. The differential molar mass distribution was carried out according to the common form: x(M)= dW(M)/dlog(M), where W(M) is the cumulative distribution (the weight fraction of sample having molar mass less than M). The refractive index increment, dn/dc, was determined to be respectively 0.184±0.001 and 0.144±0.001 mL/g for polyacrylamide and acrylamide according to the method described in the literature. 30,31 solution, the polymer conversion, p, could be computed from the surface areas of the PAM and AM peaks, respectively: p = S PAM /(dn/dc) PAM / (S PAM /(dn/dc) PAM +S AM /(dn/dc) AM )

Here, S PAM , S AM and (dn/dc) PAM , (dn/dc) AM are respectively the surface area and the refractive index increment of PAM and AM.

The coupling of SEC with multi-detectors, such as a refractometer (RI) or a multi-angle light scattering detector, was performed in order to determine both the polymer conversion and the molar masses of PAM with decent accuracy. It is worth noting that systematic errors occurred due to the incorrect values of dn/dc and to the refractometer calibration factor, k. The effects of RI calibration on the polymer conversion and molar mass were found to be proportional to k -1 ; the effects of dn/dc on p and Mw were respectively proportional to dn/dc and (dn/dc) -1 .

For the p and Mw values, these systematic errors could be prevented by using the same refractometer detector for determining dn/dc and p. The error limits of 4% and 8% for p and Mw, respectively, were based on the averages and standard deviations from multiple experiments. The random errors inherent to each experiment were due to baseline fluctuations of each detector.

Viscometric measurements of polymer solutions

The steady shear rheological properties of the polymer solutions were measured with a Bohlin CVOR150 rheometer in controlled stress mode with a cone-plate geometry (radius = 20.0 mm, angle=0.096 rad, gap=0.150 mm). Stepped shear stress values were selected in order to allow shear rates ranging from 0.01 to 150 s -1 . The systems were allowed to reach steady state at each shear stress prior to registering the measured values. The temperature was controlled with a Peltier plate system and was set to 20.0±0.1°C.

Microwave Methods (DYN and SPS)

The NMP of acrylamide in aqueous solution was performed either in a Discover single-mode microwave synthesizer (from CEM Corp.) using the DYN and SPS modes, or in an oil bath with conventional heating, denoted CH. The DYN mode corresponds to a dynamic control of the temperature by use of a high initial microwave power, and the SPS mode is a pulsed power mode. In each case, the volume ratio between the reaction mixture and the reactor was maintained unchanged to provide a constant pressure during the polymerization process.

Microwave-assisted polymerizations were carried out in standard Pyrex vessels (total capacity 10 mL) sealed with a Teflon septum cap. The temperature profiles of the polymerization (power control) were monitored using a calibrated infrared temperature control mounted beneath the reaction vessel. The reaction parameters (temperature, power, cooling, stirring…)

were set with a laptop and the SYNERGY software (version 1.35 from CEM Corp.). Each mode of irradiation corresponded to a specific set of parameters. For the pulse mode (SPS), these parameters included the power output (1W to 300W), the reaction time, the average temperature and the difference between this temperature and the lowest or highest bulk temperature (in our experiments ±2°C) and the maximum pressure allowed, i.e. the pressure at which the apparatus should stop the synthesis (17 bar in all experiments). The cooling system, based on a flow of compressed air at ambient temperature, and the stirring needed to be set at the beginning of the reaction (speed set to high). For the dynamic mode (DYN), the parameters were the bulk temperature, the maximum power output allowed (300W), the prereaction time of stirring (0 sec), the ramp time, i.e., the maximum time during which the apparatus was allowed to reach the desired bulk temperature (0 min), the reaction time, the stirring (speed set to high), the cooling system (Power Max off) and the maximum pressure allowed (17 bar in all experiments).

In the DYN mode, a dynamic control of the temperature was implemented by using a high initial microwave power, as shown in Figure 2a. The monitored temperature increased rapidly, i.e., in less than a few dozens of seconds, to the desired set point, T P . However, upon reaching this temperature, the microwave power decreased or was shut off completely in order to maintain the target temperature without exceeding it. Compared to a non-polar solvent or monomer such as styrene, the T p was reached much faster: more precisely, to reach T p = 105°C, the required time was 35 s for acrylamide in a water solution and 350 s in styrene.

The SPS mode corresponded to a pulsed power mode in which the reaction temperature ±2°C

was set, and attained by irradiation power. These experimental conditions were achieved by using a cooling feature consisting in compressed air. As shown in Figure 2b, in this mode, when the top limit of the targeted temperature was reached, the microwave power was shut off, the temperature decreased, and as soon as the lower instruction limit of the temperature was attained, the microwave power was reactivated. A pulsed MW power could thereby be obtained, providing an oscillation of the temperature around a set point. It is worth noticing that when the temperature instruction was T±2°C, the experimental conditions did not exceed T±4°C (Figure 2a), and this feature was mainly due to the inertia of the systems for one to three hours of irradiation, in the power range of 20-300W. On the other hand, the applied MW power could be perfectly controlled. In the SPS mode, the supply of microwave energy to the reaction mixture could thus be obtained by the following equation:

E MW =t MW . P (2)
where t MW is the time during which the nominal MW power, P, is applied. This corresponds to the surface area under the power fit, as shown in Figure 2b (integration is used for the DYN mode).

Results and discussions

Conventional heating (CH) versus dynamic mode (DYN)

Recently, Nicolay et al. 32 and Phan et al. 33 demonstrated that the polymerization in homogeneous aqueous solution was possible below the boiling point of water by using second-generation nitroxides, namely

N-tert-butyl-N-(1-diethylphosphono-2,2-
dimethylpropyl) nitroxide (SG1). SG1 exhibits a higher activation/deactivation equilibrium constant than TEMPO as well as a good solubility in concentrated solutions of water-soluble monomers (> 10 -2 mol/L in 40 wt-% of acrylamide, for example). Otherwise, very few watersoluble alkoxyamines have been described, and a majority of studies concerning NMPs in dispersed aqueous media have involved bicomponent or monocomponent initiating systems. [34][35][36][37] Recently, it was shown that reasonably controlled nitroxide-mediated polymerization of acrylamide could be achieved in a pure water solution at 120°C and high monomer concentration (40 wt%), using a bicomponent inititating system, i.e., a conventional hydrosoluble radical initiator (Vazo56) in combination with SG1. 27 The strategy adopted to prevent the loss of control of the polymerization and its lack of reproducibility involves adding an excess of scavenging nitroxide before triggering the polymerization. Usually in NMP, the amount of free nitroxide radicals present at the NMP equilibrium can be calculated from the equilibrium constant between active and dormant chains, K=k d /k c , the rate constant of self-termination, kt, and the rate constant of propagation, kp: 38

k t k d PAM + SG1 PAM-SG1 +AM k c
• PAM Since these constants were unknown, the experimental conditions were determined according to previous work: 27 the molar ratio r = [AM] 0 /2[Vazo56] = 500 for a constant acrylamide AM concentration of 40 wt% in water and a constant molar ratio [SG1] 0 / [Vazo56] 0 of 1.8. These conditions were far from optimized, but nevertheless, with such a system, a good compromise between kinetics and polymerization control was obtained, as can be seen from the discussion below.

Figure 3 displays the kinetic results and the living character of the polymerization for the CH and DYN modes, within a temperature range of 90 to 120°C (Table 1). The concentration of active species remained constant regardless of the experimental conditions as evidenced from the straight semi-logarithmic plots (Figure 3a). Moreover, neither an induction period nor a steepening with the increasing conversion could be observed. Control over the growth of polymer chains was furthermore shown by the straight line of the molar mass versus polymerization yield (Figure 3b) along with unimodal aqueous SEC chromatograms. The polydispersities followed the expected trend 39 and decreased with increasing conversion, with a final Mw/Mn < 1.1 for all studied polymerizations. The downward curvature that was observed at low conversion in Figure 3b could not be attributed to the SEC separation since the MALS detector provides absolute molar masses. A lack of linearity at the early stage of the polymerization may be discussed. One possible reason could be a slow initiation resulting in the formation of fewer chains at low conversion. In the case of the azo-initiator, the production rate of primary radicals was dominated by the thermal decomposition and the initiator efficiency can be defined as the fraction of radicals produced in the homolysis reaction initiating polymer chains. The disappearance rate of the initiator can be expressed in terms of the initiator half time (t 1/2 ), defining the time it takes for the concentration of the initiator to reach half of its original value. The initiator half time is temperature dependant and, for VAZO-56, it was calculated to a value of 9 minutes at 90°C and 109 seconds at 105°C. 40 However, this was not obvious from the kinetic plots as no temperature effect could be observed in Figure 3b and no upward curvature indicating an increase in [PAM ] occurring in case of slow initiation is observed in Figure 3a. As expected, the kinetics depended on the temperature (Figure 3a) and the polymerization rate was significantly higher at 120 °C than at 90 °C. Indeed, the cleavage rate constant, k d , depended on the dissociation energy of the alkoxyamine bond as well as the Arrhenius temperature, whereas the cross-combination constant, k c , demonstrated non-Arrhenius temperature dependencies. 41 The time dependence could be approximated as first-order over a 4 where they are also plotted versus 1/T according to the following Arrhenius relation,

k app = A exp (-E A /RT). (3)
Here, E A is the apparent activation energy of the polymerization and R is the universal gas constant.

Figure 4 clearly shows that both the CH and DYN modes presented similar E A values.

Moreover, the value E A = 140 KJ/mol, was of the same order of magnitude as that usually obtained in NMP. 38 This thus demonstrated the absence of specific MW effects in the case of the DYN mode.

While the plots at 90°C obtained by both the DYN and CH modes were in good agreement, a deviation was noticed at 105°C and was attributed to the experimental errors that were also present in Figure 3a. The absence of MW effects can be explained by the DYN parameters themselves, i.e., a high microwave power at the beginning of the polymerization increasing the bulk temperature to the set point as quickly as possible. Upon reaching the target temperature, the microwave power was decreased or completely shut off in order to maintain the desired bulk temperature, T p , without exceeding it (Figure 2a). In the absence of microwave irradiation, classical thermal chemistry takes over, and the full advantage of microwave accelerated synthesis is lost.

Pulsed mode (SPS)

To ensure that a high and constant level of microwave energy is applied during the polymerization, the previously described pulsed mode (SPS) was utilized. As opposed to the DYN mode, the reaction vessel is, in SPS mode, externally cooled with compressed air, while microwave irradiation is simultaneously administered. This way, more energy can be directly applied to the reaction mixture. With DYN, microwave irradiation is predominantly used to reach T P faster. SPS mode polymerizations were performed during one hour at 80, 90 and 105 °C and the MW power ranged from 10 to 300 W (Table 1). Consequently, the MW energy varied from 10 to 170 kJ for a constant volume of reaction mixture (3 mL). Figure 5 shows the polymer conversion, p, obtained after one hour according to E MW . At low E MW , the conversion increased gradually up to a point of MW energy where the conversion displayed a jump. After this specific point, the conversion once again increased gradually up to complete conversion of the monomer. It seemed that this specific microwave effect was independent of the temperature and occurred at an E MW of around 90 kJ. This series of experiments (carried out at a constant reaction time) was completed by two series performed at constant power (150 and 190W) and temperature (90°C). In these cases, the reaction time was the only variable parameter, i.e., the microwave energy. At 190W and 150W, the polymer conversion increased sharply between 50 and 120 kJ (Table 1). This could correspond to a lowering of the concentration of nitroxide radicals in the medium. As shown in Figure 6, the variation of ln([AM) 0 /[AM]) vs time was not linear, thereby suggesting a loss of the "living character" of the polymerization. However, the control over the growth of polymer chains was evidenced by the linear evolution of molar masses with conversion as well as an acceptable polydispersitiy index below 1.3 and unimodal aqueous SEC chromatograms. Figure 7 shows this linear evolution of molar masses, and as can be evidenced, there was a perfect match to the results previously obtained in DYN and CH modes. One of the differences between the DYN and SPS modes was the use of air cooling during the latter. This is known to give rise to a significant underestimation of the temperature readout of the IR sensor since the air cools mainly the exterior of the reaction vessel. The effect that the temperature may have on the auto-acceleration polymerization is discussed in the following section. The number-average molar mass (M n ), and the polydispersity index (Ip=Mw/Mn), on the monomer conversion, p, for NMP of acrylamide under pulsed microwave irradiation (SPS) at 105°C ( ), 90°C ( ) and 80°C ( ) for the runs presented in figure 5 and6 (the dashed lines provide the variation observed in Figure 3 for CH and DYN mode).

The living character of the PAM polymer chains produced with the vazo-SG1 system has been further confirmed by chain extension experiments with a second addition of the same AM monomer. The unpurified PAM formed at high conversion (p=99%) from aqueous NMP in SPS mode (90°C, 150W, 300 min, cf. Table 1) was used as a macroinitiator for further chain extension (Mn = 60400 g/mol and Ip=1.15). Two chain extension polymerizations, denoted E1 and E2, were carried out by a second addition of acrylamide to maintain the total concentration of monomer plus macroinitiator at 40 wt%. For E1 and E2, respectively 0.5 g and 1.16 g of macroinitiator solution were completed up to 3g with a 40 wt% of acrylamide solution. All other parameters and procedures were the same as in the NMP procedure in SPS mode. E1 and E2 were irradiated 120 min at 90°C under 100W and 60 min at 90°C under 190W, respectively. The conversions measured by CES were found to be 6% and 42% for respectively E1 and E2, which corresponded to the targeted PAM molar masses of 77 700 and 100 400 g/mol. The M n and Ip of the chain-extended polymer were increased up to 76 400 and 97 600 g mol -1 , with Ip = 1.17 and 1.25, respectively for E1 and E2. These results indicate that the PAM homopolymer, used as the macroinitiator, was effectively functionalized with nitroxide end groups. After the chain extension experiment, the molar mass distribution of the chain-extended product shifted to higher molar masses as compared to that of the initial PAM macroalkoxyamine (Figure 8). These experimental results exhibited a good control of PAM molar masses even at very high polymer conversion, i.e. even close to 100%, by Nitroxide Mediated polymerization under microwave irradiation in aqueous medium.

Specific microwave effect

Despite the relatively large number of consistent publications in the area of microwaveassisted organic/polymer chemistry, the exact reasons why microwave irradiation is able to enhance chemical processes remain unknown. 2,4 Since the start of microwave synthesis, rateaccelerations and occasional variations in product distributions as compared to experiments by conventional heating have led to speculations regarding the existence of so-called "nonthermal" or "specific" microwave effects. According to investigations by Kappe, 2

Loupy 4 and coworkers, it can be understood that the differences between "nonthermal" and "specific" microwave effects depend on the scale of the observation, which is either micro-or macroscopic. The existence of so-called "specific microwave effects" is in principle undisputed since they cannot be reproduced by conventional heating. They should thus be the result of the uniqueness of the dielectric phenomenon in microwave heating, i) the superheating effect of solvents at the atmospheric pressure; ii) the selective heating such as the strong microwave absorption of the heterogeneous catalysts or reagents in a less polar medium of reaction; and iii) the elimination of the wall effects caused by the inverted temperature gradients. 2 In contrast, "nonthermal microwave effects" represent a controversial subject and have been suggested to result from a direct interaction -which is unrelated to a macroscopic temperature effect -of the electric field with specific molecules in a reaction medium 4 . In our work, the intention was not to enhance the microwave-heated reaction with a high temperature reaction that can rapidly be attained when irradiating polar materials/reaction mixtures under closed-vessel conditions in a microwave field. The microwave-assisted polymerization in an aqueous solvent was carried out at a temperature lower than 105°C and at low pressure. It is worth noticing that the experimental conditions used within this study were far from the near-critical or supercritical water condition 23 which involves T> 150°C, pressure > 4 bar; T> 374°C, p> 200 bar.

A microwave enhancement of chemical reactions takes place only during the application of microwave energy. This energy source provides a direct activation of the molecules in a chemical reaction, for which reason it would be undesirable to suppress its application as in the DYN mode. SPS ensures the application of a high and constant level of microwave energy. It was previously demonstrated that the experiments conducted with the DYN and CH modes were in good agreement and revealed no specific MW effects. The polymerization performed in SPS mode, on the other hand, exhibited a sharp increase in polymer conversion within 1 hour (Figure 5). As a comparison, the conversion, p, was lower than 0.5 for one hour of reaction at 120°C in CH mode and below 0.3 at 105°C in the DYN and CH modes, i.e., at least 15°C above the temperature used in the SPS mode (extrapolated data of figure 3). The temperature control as measured by IR sensors (±4°C) was the same in both MW modes and could not explain the auto-acceleration polymerization. In fact, the use of IR sensors may be inappropriate for temperature measurements in microwave-heated reactions. The reason for this is that they can easily lead to a misinterpretation of results since the real reaction temperatures are unknown, 2 in particular for SPS modes where external cooling can underestimate the reaction temperature. To specify the origin of the auto-acceleration polymerization, a complementary polymerization series at 80°C was carried out only in SPS mode (P= 250W), during which the viscosity of each sample was measured and the pressure in the vessel was monitored.

In order for the results to be as reliable as possible, the same reaction vessels, reaction volumes and stirring (speed and magnets) were used in all experiments. Consequently, all parameters were identical apart from the time of irradiation, and therefore, a fair comparison based on the polymerization times and conversion could be carried out for the experiments performed solely in the SPS mode. Figure 9 presents a superposition of the kinetic plot results and the viscosity, and the inset displays the linear evolution of the molar masses according to the results previously discussed. As can be seen, the sharp rate enhancement of the acrylamide NMP appeared at 240 min (p = 0.5 and E MW >> 90 kJ) and at the same time, the viscosity displayed a significant increase without raising the pressure inside the vessel. It was possible that the underestimation of the temperature reaction due to the IR sensors was even stronger after gelation of the polymerization mixture since the heat transport to the vessel wall worsened. This could explain the observation of a sudden increase in conversion. However, the auto-accelaration polymerization was not followed by a significant increase in pressure within the vessel, according to the precision of the pressure sensor as shown in Figure 9. It could thereby be assumed that the reaction temperature did not increase excessively after the gel point. As shown in Figures 6 and9, the apparent constant rate of the auto-acceleration polymerization was estimated to be higher than 710 -4 s -1 , corresponding to a theoretical temperature above 130°C according to the Arrhenius relation established previously.

An underestimation of the reaction temperature medium at the gel point could thus not be neglected. However this cannot be the sole explanation behind the auto-acceleration polymerization. Unfortunately, the CEM microwave reactor in the present study did not allow the measurement of the real temperature inside the vessel during the SPS mode as a quantification of the reliability of the IR sensor. It should be mentioned that a polymer solution at concentrations higher than c** enters a socalled concentrated regime in which each segment of the polymer chain lacks sufficient space [START_REF] Teraoka | POLYMER SOLUTIONS: An Introduction to Physical Properties[END_REF] . The chains thus display a greatly reduced mobility as opposed to their counterparts in dilute (c < c*) or semi-dilute (c* < c <c**) solutions, and their viscosity increases very sharply with the concentration and molar mass of the polymer: η ∝ c x with x= 15/4 in a good solvent, and η ∝ M 3 (where c* is the overlap concentration). Typically, the volume fraction of the polymer at c** is between 0.2 and 0.3. For a polymer of a sufficiently high molar mass, there is a broad range of concentrations between c* and c**. In the present study, the monomer concentration was kept constant at 400 mg/mL and a polymer concentration higher than c**=200 mg/mL would correspond to a polymer conversion greater than p= 0.5. For all microwave experiments, it appeared that the auto-acceleration of the polymerization rate occurred at a conversion close to 0.5 ( > 0.4 in figure 5) and with a microwave energy higher than 30 kJ per gram of reaction mixture.

Thus, for the present work, it can be suggested that the mobility of the long-chain radical species was significantly reduced as the viscosity increased, i.e., the radicals started demonstrating difficulties moving. On the other hand, under the sufficiently high energy of the electric field induced by the pulsed microwave mode, the molecules of water, monomers and counter radicals in the reaction system rotated and oscillated at a high speed before becoming polarized. At this gel point, the reactivity of the reaction system seems to be enhanced, either by increasing the propagation constant of the monomer (k p ) and/or by rateenhancing reductions of the persistent radical concentration (e.g., hydroxylamine formation), [START_REF] Sciannamea | [END_REF] resulting in the considerable acceleration of the polymerization process without lack of living/controlled polymerization according to the prediction of the persistence radical effect theory, 44 i.e., the so called Fischer effect. 45 The explanation that a microwave effect was induced by a mobility contrast between the polymer chain and the dipolar monomer/counter radical from a sufficient microwave energy is not obvious. Probably, the increase of the temperature and the mobility contrast arising from the gelification of the reaction media are concomitant. Future objectives include clarifying the respective parts of the rate-enhancing propagation of monomer, the rateenhancing reductions of the persistent radical concentration and the temperature effect with the use of an internal sensor.

In addition, Kappe et al. 2 recently demonstrated that a multiple fiber-optic probe system was an accurate temperature measurement device in both microwave and conventionally heated reactors, and that an efficient stirring/agitation of microwave-heated reactions remains essential. In contrast to an oil bath experiment, even completely homogeneous solutions need to be stirred when single-mode microwave reactors are employed. If efficient stirring/agitation cannot be ensured, temperature gradients may develop as a consequence of inherent field inhomogeneities alongside a single-mode microwave cavity. In the case of homogeneous polymer synthesis in bulk or in solution, the medium can become extremely viscous, thus resembling a pseudo or "physical" gel. If the disruption of the stirring had given rise to inhomogeneities alongside the microwave vial with regard to the temperature, the distribution of the molar masses would have been prevented from reaching very high values.

Indeed, the kinetic chain length in radical polymerization is a function of the polymerization rate, which, in turn, depends on the temperature. A too high variation in temperature alongside the microwave vial would have provided a broad distribution in molar masses 46 which was not observed. Thus, in our experimental device, the temperature along the length of the reactive medium can be supposed to be homogeneous but this feature cannot be extrapolated to bigger volumes.

Conclusions

The present work has demonstrated that the combined use of microwave irradiation as a heating source and water as a solvent provides a reasonable living/controlled polymerization of acrylamide by NMP. For the reaction, a combination of a conventional hydrosoluble radical initiator (Vazo56) and a β-phosphonylated nitroxide (SG1) was employed. The microwave enhancement of the polymerization depended on the mode of irradiation. The DYN mode corresponded to a dynamic control of the temperature by use of a high initial microwave power. In this case, no specific microwave effect was observed. For the SPS mode, which is a pulsed power mode, a strong acceleration of the polymerization process (> 50 times) without any loss of the living/controlled polymerization characteristics has been observed. In this case, a relevant efficiency of the re-initiation of the PAM macroinitiator has been demonstrated even after 100% of conversion of the first block. Moreover, a gelification of the reaction media has been observed with such mode.

The SPS mode seemed to represent a superior means of assuring microwave-assisted nitroxide mediated (co)-polymerization in water to achieve high conversion and low temperatures, i.e. below 90 °C, as measured with external IR sensors.

Graphical Abstract:

Microwave-Assisted Nitroxide-Mediated Radical Polymerization of Acrylamide in Aqueous Solution

Julien Rigolini, Bruno Grassl, Laurent Billon, Stephanie Reynaud and Olivier F.X. Donard

The SPS modes which is a pulsed power mode, represents a superior mean of assuring microwave-assisted nitroxide mediated (co)-polymerization in water at high conversion and low temperatures, i.e., below 90 °C, as measured with external IR sensors.

Captions to table and figures

Figure 1 :

 1 Figure 1: SEC-MALS results for a typical experiment: CH run, 120°C, t = 20, 40 and 50 min (see Table 1). a) Chromatograms (top), and b) molar mass distribution (bottom). The three peaks on the left of the top figure correspond to the three peaks in the bottom figure.

  Figure 1: SEC-MALS results for a typical experiment: CH run, 120°C, t = 20, 40 and 50 min (see Table 1). a) Chromatograms (top), and b) molar mass distribution (bottom). The three peaks on the left of the top figure correspond to the three peaks in the bottom figure.

Figure 1

 1 Figure 1 present a typical trace from SEC experiments on reactive media, displaying two characteristic peaks of the PAM polymer and acrylamide (AM) monomer. With knowledge of the refractive index increment of acrylamide and polyacrylamide in 0.1 mol.L -1 NaNO 3

Figure 2 :

 2 Figure 2: Profiles for temperature and power during microwave irradiation in DYN (top) and SPS (bottom) modes.

Figure 3 :a)

 3 Figure 3: a) Kinetic plots of ln([AM] 0 /[AM]) vs. reaction time for the NMP polymerization of acrylamide under conventional heating (CH) and dynamic microwave irradiation (DYN) at various temperatures: CH (full symbols), 120 °C ( ), 115°C ( ), 105°C ( ), 90°C ( ); DYN (open symbols), 105°C ( ), 97°C ( ) and 90°C ( ).b)The dependence of the number-average molar mass (M n ) and the polydispersity index (Ip = Mw/Mn ), on the monomer conversion, p. Same symbols as in (a). The dashed lines are guides for the eyes.

Table 1 :

 1 Temperature (T), polymer conversion (p), reaction time (t), number-average molar mass (M n ) and polydispersity index (Ip) of the NMP of acrylamide in aqueous solution using conventional heating (CH) and two microwave modes (DYN and SPS): r = [AM]/2[Vazo56] = 500 for a constant acrylamide (AM) concentration (40 wt%) and a constant molar ratio [SG1] 0 / [Vazo56] 0 = 1.8. a) used for chain extension polymerization

Figure 4 :

 4 Figure 4: An Arrhenius plot of the NMP of acrylamide under conventional heating (CH, ) and dynamic microwave irradiation (DYN, ).

Figure 5 :

 5 Figure 5: The polymer conversion as a function of the microwave energy during one hour of the NMP of acrylamide under SPS microwave irradiation at various temperatures: 105°C ( ), 90°C ( ) and 80°C ( ).

Figure 6 :

 6 Figure 6: Kinetic plots of ln([AM] 0 /[AM]) (full symbols) and polymer conversion, p (open symbols) vs. the reaction time for NMP of acrylamide under pulsed microwave irradiation (SPS) at 90°C and varying power: 150 W (ring) and 190W (square).

Figure 7 :

 7 Figure 7:The number-average molar mass (M n ), and the polydispersity index (Ip=Mw/Mn), on the monomer conversion, p, for NMP of acrylamide under pulsed microwave irradiation (SPS) at 105°C ( ), 90°C ( ) and 80°C ( ) for the runs presented in figure5 and 6(the dashed lines provide the variation observed in Figure3for CH and DYN mode).

Figure 8 :

 8 Figure 8: Molar mass distributions, differential (dashed line) and cumulative (continuous line), for chain extension polymerization: macroinitiator, E1 and E2 (from left to right).

Figure 9 :

 9 Figure 9: Kinetic plots of ln([AM] 0 /[AM]) vs. reaction time for NMP of acrylamide ( ) and the corresponding viscosity of the reaction mixture ( ), under pulsed microwave irradiation at 80°C and 250W. Inset: the dependence of the number-average molar mass, Mn, on the monomer conversion, p ( ). The lines are guides for the eyes. Below: superposition of measured pressures for each experiment.

Table 1 :

 1 Temperature (T), polymer conversion (p), reaction time (t), number-average molar mass (M n ) and polydispersity index (Ip) of the NMP of acrylamide in aqueous solution using conventional heating (CH) and two microwave modes (DYN and SPS): r = [AM]/2[Vazo56] = 500 for a constant acrylamide (AM) concentration (40 wt%) and a constant molar ratio [SG1] 0 / [Vazo56] 0 = 1.8.

Figure 1 :

 1 Figure 1: SEC-MALS results for a typical experiment: CH run, 120°C, t = 20, 40 and 50 min (see Table 1). a) Chromatograms (top), and b) molar mass distribution (bottom). The three peaks on the left of the top figure correspond to the three peaks in the bottom figure.

  Figure 1: SEC-MALS results for a typical experiment: CH run, 120°C, t = 20, 40 and 50 min (see Table 1). a) Chromatograms (top), and b) molar mass distribution (bottom). The three peaks on the left of the top figure correspond to the three peaks in the bottom figure.

Figure 2 :

 2 Figure 2: Profiles for temperature and power during microwave irradiation in DYN (top) and SPS (bottom) modes.

Figure 3 :a)

 3 Figure 3: a) Kinetic plots of ln([AM] 0 /[AM]) vs. reaction time for the NMP polymerization of acrylamide under conventional heating (CH) and dynamic microwave irradiation (DYN) at various temperatures: CH (full symbols), 120 °C ( ), 115°C ( ), 105°C ( ), 90°C ( ); DYN (open symbols), 105°C ( ), 97°C ( ) and 90°C ( ).

Figure 4 :

 4 Figure 4: An Arrhenius plot of the NMP of acrylamide under conventional heating (CH, ) and dynamic microwave irradiation (DYN, ).

Figure 5 :

 5 Figure 5: The polymer conversion as a function of the microwave energy during one hour of the NMP of acrylamide under SPS microwave irradiation at various temperatures: 105°C ( ), 90°C ( ) and 80°C ( ).

Figure 6 :

 6 Figure 6: Kinetic plots of ln([AM] 0 /[AM]) (full symbols) and polymer conversion, p (open symbols) vs. the reaction time for NMP of acrylamide under pulsed microwave irradiation (SPS) at 90°C and varying power: 150 W (ring) and 190W (square).

Figure 7 :

 7 Figure 7:The number-average molar mass (M n ), and the polydispersity index (Ip=Mw/Mn), on the monomer conversion, p, for NMP of acrylamide under pulsed microwave irradiation (SPS) at 105°C ( ), 90°C ( ) and 80°C ( ) for the runs presented in figure5 and 6(the dashed lines provide the variation observed in Figure3for CH and DYN mode).

Figure 8 :

 8 Figure 8: Molar mass distributions, differential for chain extension polymerization: macroinitiator, E1 and E2 (from left to right).

Figure 9 :

 9 Figure 9: Kinetic plots of ln([AM] 0 /[AM]) vs. reaction time for NMP of acrylamide ( ) and the corresponding viscosity of the reaction mixture ( ), under pulsed microwave irradiation at 80°C and 250W. Inset: the dependence of the number-average molar mass, Mn, on the monomer conversion, p ( ). The lines are guides for the eyes. Below: superposition of measured pressures for each experiment.

Acknowledgments

The "Conseil Géneral d'Aquitaine" is gratefully acknowledged for the PhD grant of J.

Rigolini and many thanks are expressed to Arkema for providing the SG1 counter radical. We also thank Gérald Clisson for his technical assistance during the SEC experiments and CEM for the loan of the Discover unit.

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