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Controlled Radical Homo- and Copolymerization (ATRP, NMP, RAFT) and Other Living Types (ROMP)



- Common concept: alternating activation-deactivation processes;

- Different mechanisms:

• Chain growth by removal for a short time of the end cap from the reversible protected chain growing ends (left: ATRP, NMP)

• Chain growth by reversible transfer of the end cap among different chains (right: RAFT)

It is believed that the use of ACOMP in the study of copolymerization reactions will lead to a better understanding of underlying kinetics and mechanisms and eventually lead to the ability to control the composition profile formation during the polymerization.



1. Nitroxide mediated polymerization (NMP):


The lower the value of K is: the more strongly the equilibrium is shifted toward formation of dormant chains, the better is the control, the lower is the rate of polymerization. If K value is too low, the persistent species becomes an inhibitor

1. First controlled polymerization reaction monitored by ACOMP: NMP of butyl acrylate (BA) using N-tertiobutyl-1-diethylphosphono-2,2-dimethylpropyl Nitroxide (SG1 )


  • ACOMP was used to follow conversion, Mw and hr during the reactions.

  • It has also been used for determination of detailed kinetics, and the evolution of polydispersity to be followed in some cases. No chromatographic separation columns are used.

  • Typical CRP behavior was observed in the increase of polymer mass and decrease in polydispersity with conversion.

Top: Mw vs. f for several PBA reactions with different. Also shown are Mw and polydispersity index Mw/Mn values from GPC measurements made on aliquots withdrawn from the reactor during polymerization. Bottom: Opposite trends in the evolution of Mw during NMP and free radical polymerization reactions.

(F. Chauvin, A. M. Alb, D. Bertin, P. Tordo, W. F. Reed,Macromol. Chem. Phys. 2002, 203, 2029–2041)



2. ACOMP quantifies deviations from ideal living polymerization due to chain transfer.

b) Polymerization of butyl acrylate in butyl acetate with MONAMS/SG1: By continuous monitoring and powerful analysis methods, ACOMP is able to observe and quantify CRP deviation in Mw due to chain transfer.


3. ACOMP provides means for direct determination of composition profile for gradient copolymers: Styrene/butyl acrylate copolymerization by NMP

By simultaneously monitoring and combining signals from the continuously diluted reactor stream, light scattering, viscosity, differential refractive index, and UV absorption were used, in a model-independent fashion, to follow:

- the weight-average molecular mass Mw, weight-average intrinsic viscosity [h]w, the concentrations of each comonomer, and hence the evolution of the average instantaneous and cumulative compositions along the chains as comonomers were consumed.

Invoking the Mayo-Lewis model allowed sequence length averages and reactivity ratios to be computed.

Styrene/butyl acrylate gradient copolymerization: ACOMP used to determine composition profile, Finst at any moment during the reaction (Finst,styrene vs. total conversion is shown in the figure for different initial comonomer compositions )

 

(E. Mignard, T. Leblanc, D. Bertin, O. Guerret, W. F. Reed, Macromolecules 2004, 37, 966-975).
 



4. Ring Opening Metathesis Polymerization (ROMP) - a recent expansion of ACOMP monitoring, ‘ROMP-ACOMP' - another case where deviations from ideal kinetics are quantified 

 

(A. M. Alb, P. Enohnyaket, J. F. Craymer, T. Eren, E. B. Coughlin, W. F. Reed, Macromolecules 2007, 40, 444-451)


ACOMP findings:

The kinetics and mechanisms involved in the ring-opening metathesis polymerization of 5-norbornene-2-yl acetate (NAc) and cyclooctadiene (COD) in dichloromethane (DCM) were studied.

An important goal was to establish a means of determining whether termination or transfer reactions occurs and whether direct measurements on intra- and/or intermolecular degradative mechanisms can be made.

The evolution of the molecular mass was generally consistent with a “living” mechanism in a rapid first phase, where expected target masses for p(NAc) were met, but often revealed a secondary, slight degradative phase. In contrast, p(COD) yielded molar masses far below target values and generally showed a more pronounced degradative phase. These latter two phenomena for p(COD) appear to be symptomatic of a mechanism that shortens chains with concomitant increase in polydispersity. Furthermore, through a combination of Mw, viscosity, and concentration dependencies it was deduced that the slow degradative phase is due almost entirely to cross-metathesis reactions.

Top: Different long-term trends in Mw and [h]w for COD and NAc reactions - ACOMP provides means to quantify deviations from ideal kinetics.    Bottom: ACOMP mass evolution vs. conversion f for several NAc reactions. Discrete, square points are from GPC on manually withdrawn aliquots. The inset shows final values of Mw. The line is the theoretical expectation for final Mn.


 

5. Reversible Addition Fragmentation Transfer (RAFT) polymerization - a new advance in ACOMP monitoring 


Key feature: equilibrium between polymeric radicals, intermediate radicals, and dormant species

 

ACOMP was used to follow kinetic trends in RAFT polymerization of butyl acrylate (BA) in butyl acetate, using 2 {[(dodecylsulfanyl)carbonothioyl] sulfanyl} propanoic acid (DoPAT) as the RAFT agent and AIBN as initiator.

(A. M. Alb, A. K. Serelis, W. F. Reed, Macromolecules 2008, 41, 332-338).

 

Goals

- Demonstrate the use of ACOMP for RAFT studies and examine the trend in conversion and evolution of weight-average molar mass Mw and weight-average intrinsic viscosity [h]w for a series of experiments in which the RAFT agent concentration was varied while all the other reaction conditions were held constant;

- The conversion kinetics were found to be essentially zeroeth order in DoPAT, and deviations from living behavior allowed estimates of radical efficiency.

- The evolution of Mw vs. monomer conversion varied dramatically from near-ideal living behavior at high [DoPAT]/[AIBN] to classical radical polymerization behavior at [DoPAT]=0.

Top: Weight average molecular mass, Mw as function of monomer conversion: The decrease in the concentration of the RAFT agent leads to loss of control.   Bottom: GPC - a valuable complement to ACOMP:


6. Reversible Addition Fragmentation Transfer (RAFT) copolymerization - ACOMP expanded to:


  • block copolymerization reactions by RAFT

Butyl acrylate/Styrene (10/90M/M)) block copolymerization in butyl acetate. Left: Signals from several detectors continuously furnish information on the reaction kinetics. Right: Weight average molecular mass, Mw and reduced viscosity, hr as functions of total conversion, f.

  • gradient copolymerization reactions by RAFT

Methyl acrylate/Vinyl acetate gradient copolymerization reactions in butyl acetate: Raw data from two reactions with different starting comonomer composition.

School of Science and Engineering, 201 Lindy Boggs Center, New Orleans, LA 70118 504-865-5764 sse@tulane.edu